Knee Joint Response to Mechanical Loading: Bounding Mechanotransduction with Rehabilitation

Keywords

Abstract

The knee joint is a weight-bearing structure that endures varied mechanical stresses in daily and athletic activities. Its cells convert these stresses into biochemical signals through mechanotransduction, prompting changes essential for joint health, repair, and adaptation. Understanding these processes is pivotal for developing rehabilitation strategies that address injuries and degenerative conditions like osteoarthritis. Different loading modalities—compression, tension, shear, and hydrostatic pressure—impact knee tissues (cartilage, synovium, ligaments, and tendons) and their resident cells (chondrocytes, synoviocytes, and fibroblasts). Chondrocytes adjust extracellular matrix synthesis to maintain cartilage integrity, while synoviocytes regulate synovial fluid components crucial for lubrication. Fibroblasts modulate collagen production, preserving ligament and tendon strength. Underlying these activities are key signaling pathways (e.g., MAPK, NF-κB, and Wnt) that regulate gene expression and cellular metabolism in response to mechanical stimuli. By linking basic mechanobiology insights to clinical practice, clinicians can tailor therapeutic interventions—such as controlled loading, exercise regimens, manual therapy, and orthotic devices—to optimize tissue repair, restore function, and prevent further degeneration. This mechanotransduction-focused approach offers a comprehensive framework for improving knee joint health and enhancing rehabilitation outcomes.

Introduction

This review aims to examine how mechanical loading affects the knee joint at the molecular and cellular levels, with particular emphasis on the pathways and factors regulating cartilage maintenance, synovial fluid composition, and structural integrity. By analyzing these mechanisms, the study seeks to establish a scientific foundation for developing precise rehabilitation programs that tailor loading conditions to individual patient needs. Consequently, the objective of this work extends beyond advancing knee joint biomechanics to translating research findings into clinical practice, accelerating treatment processes, preventing overuse injuries, and improving patient outcomes.

Review is based on an analysis of mechanotransduction mechanisms in various knee joint tissues, including cartilage, synovium, ligaments, and tendons. It explores different types of mechanical loading—compression, tension, shear, and hydrostatic pressure—and their structural and metabolic effects on joint tissues. The role of key mechanotransduction cells, such as chondrocytes in cartilage, synoviocytes in the synovium, and fibroblasts in ligaments and tendons, is discussed, highlighting their response to mechanical forces through receptors like integrins and ion channels. Furthermore, the study examines major signaling pathways, including MAPK, NF-κB, and Wnt, which regulate gene expression and cellular metabolism in response to mechanical stimuli.

The knee joint comprises the femur, tibia, and patella, along with cartilage, ligaments, tendons, and synovial fluid, each essential for movement, shock absorption, and weight bearing [1–3]. Articular cartilage coats the bone surfaces, reducing friction and distributing loads, while key ligaments (ACL, PCL, MCL, LCL) prevent excessive motion [4–5]. Tendons, such as the quadriceps and patellar, facilitate extension and flexion [6]. Meanwhile, synovial fluid—produced by the synovium—lubricates the joint, nourishes cartilage, and absorbs shock [7–8].

Mechanical loading involves compression, tension, shear, and hydrostatic forces acting on the knee during daily activities [9]. These forces stimulate tissue repair and regeneration, but excessive or abnormal loading may trigger damage, inflammation, and conditions like osteoarthritis [10–11]. Determining optimal loading conditions is thus critical for preserving knee function.

Mechanotransduction underlies how knee cells convert mechanical cues into biochemical responses [12]. Chondrocytes, synovial fibroblasts, and osteoblasts detect forces through mechanoreceptors such as integrins, primary cilia, and ion channels. These stimuli activate intracellular signaling cascades—including MAPK, NF-κB, and Wnt pathways—that regulate transcription factors like AP-1 and β-catenin [13–14]. In turn, this modulates genes for collagen, proteoglycans, and inflammatory mediators, orchestrating the remodeling of the extracellular matrix, controlling synovial fluid composition, and maintaining cartilage resilience. Conversely, aberrant loading escalates catabolic enzymes (e.g., matrix metalloproteinases), fueling cartilage breakdown and inflammation.

Harnessing mechanotransduction insights enables targeted rehabilitation to optimize tissue repair, minimize inflammation, and restore function [15–17]. Controlled loading exercises fine-tune mechanical stimuli, enhancing extracellular matrix synthesis without overloading the joint [18-20].

Incorporating these molecular and cellular principles into clinical practice supports personalized rehabilitation protocols that align with each patient’s unique mechanical environment [21–25]. Improved understanding of mechanotransduction can accelerate recovery, reduce chronic knee issues, and ultimately enhance quality of life for individuals with knee joint injuries or degenerative conditions.

A key focus of this work is bridging mechanobiology with clinical applications. The findings provide a basis for tailoring therapeutic interventions, such as controlled loading, exercise programs, manual therapy, and rehabilitation devices, to optimize tissue repair, restore function, and prevent further degeneration. By integrating cellular biology with biomechanics, this review establishes a comprehensive framework for rehabilitation strategies that enhance knee joint health and improve therapeutic outcomes.

Mechanotransduction in the Knee Joint

Mechanotransduction in the knee joint involves mechanoreceptors, ion channels, and signaling pathways [26, 27]. This process converts mechanical stimuli into biochemical signals essential for joint health, tissue repair, and load adaptation (Fig. 1) [28]. The primary cells involved are chondrocytes (in cartilage), synoviocytes (in the synovium), and fibroblasts (in ligaments and tendons). These cells detect mechanical cues largely through integrins and stretch-activated ion channels, which couple extracellular forces to intracellular cascades.

Fig. 1: The Fig. illustrates the process of mechanotransduction, depicting how mechanical loading leads to changes in the extracellular matrix (ECM) and ultimately results in sustained or improved function. The sequence begins with mechanical loading (1) due to mechanical loading in rehabilitation process. This mechanical load propagates stress through the ECM from macro to micro scales. The ECM then interacts with cells through mechanotransduction (2), converting the mechanical signals into cellular responses. These signals induce new matrix synthesis and the degradation of damaged matrix components. The ECM undergoes incorporation and remodeling of new composition and structure (3), leading to sustained or improved function of the tissue. The diagram highlights the dynamic interplay between mechanical forces and cellular responses in maintaining tissue health and function).

Fig. 1

Fig. 1: The Fig. illustrates the process of mechanotransduction, depicting how mechanical loading leads to changes in the extracellular matrix (ECM) and ultimately results in sustained or improved function. The sequence begins with mechanical loading (1) due to mechanical loading in rehabilitation process. This mechanical load propagates stress through the ECM from macro to micro scales. The ECM then interacts with cells through mechanotransduction (2), converting the mechanical signals into cellular responses. These signals induce new matrix synthesis and the degradation of damaged matrix components. The ECM undergoes incorporation and remodeling of new composition and structure (3), leading to sustained or improved function of the tissue. The diagram highlights the dynamic interplay between mechanical forces and cellular responses in maintaining tissue health and function).

Chondrocytes reside in the avascular cartilage, where they depend on mechanical loading to facilitate nutrient diffusion and waste removal [29, 30]. Integrins on the chondrocyte surface bind ECM components (e.g., collagen, fibronectin), transmitting mechanical signals to the cytoskeleton and triggering mechanosensitive ion channel activation. This engagement launches several key intracellular pathways—most notably Mitogen-Activated Protein Kinase (MAPK), Nuclear Factor kappa-light-chain-enhancer of activated B cells (NF-κB), and Wnt signaling [31, 32]. MAPK includes the extracellular signal-regulated kinases (ERKs), c-Jun N-terminal kinases (JNKs), and p38, each controlling distinct aspects of gene expression and protein synthesis tied to cartilage maintenance [33].

On a molecular level, integrin clustering under load activates focal adhesion kinases (FAKs), which can phosphorylate MAPK components, thus relaying mechanical signals to the nucleus [34]. ERK often promotes anabolic functions, such as collagen II or aggrecan synthesis, whereas p38 and JNK can accelerate catabolic processes, including matrix metalloproteinase (MMP) expression. NF-κB, central to inflammation and cell survival, is typically held inactive by IκB proteins that sequester it in the cytoplasm; mechanical stress can activate IκB kinase (IKK), allowing NF-κB to translocate to the nucleus and regulate cytokine or MMP transcription [36]. Simultaneously, canonical Wnt signaling involves the stabilization and nuclear translocation of β-catenin, which promotes genes critical for cartilage repair [37]. These three pathways show significant crosstalk: for example, p38 or JNK activity can enhance IKK-mediated NF-κB activation, while moderate ERK signaling can cooperate with Wnt/β-catenin to drive anabolic gene programs [38, 39]. Balancing these signals under physiologic loading maintains tissue homeostasis; excessive or abnormal forces push the system toward degenerative outcomes.
Synoviocytes line the knee’s synovial membrane, producing synovial fluid that lubricates the joint, reduces friction, and supplies nutrients to chondrocytes [40]. Mechanical loading activates integrins and stretch-sensitive ion channels on synoviocytes, leading to increased synthesis of hyaluronic acid and lubricin, two critical components for joint lubrication [41]. On a molecular scale, hyaluronan synthase catalyzes hyaluronic acid production and is upregulated by mechanically induced MAPK phosphorylation events. NF-κB modulates the balance between pro- and anti-inflammatory signals; in mild or moderate activation states, synoviocytes secrete anti-inflammatory cytokines that protect joint tissues, whereas excessive NF-κB stimulation drives inflammatory cascades [42].

Wnt/β-catenin signaling also influences synoviocyte behavior, potentially regulating cell proliferation and cytokine profiles. Dysregulation of Wnt may promote synovial hyperplasia or exacerbate inflammation. Another relevant pathway is the mechanistic target of rapamycin (mTOR), which can interact with Wnt and NF-κB to fine-tune lubricin production and immune modulation. Proper mechanical cues thus ensure adequate synovial fluid properties, preventing cartilage wear while minimizing pathological inflammation [43, 44].

Fibroblasts populate ligaments and tendons, providing structural support and transmitting muscular forces to bones. Mechanotransduction in fibroblasts involves integrin clustering at focal adhesions, actin cytoskeleton remodeling, and activation of MAPK and Transforming Growth Factor-beta (TGF-β) signaling [45]. Under normal loads, fibroblasts maintain collagen fiber alignment and ECM turnover, conferring the tensile strength and elasticity required for joint stability [46][47].

Research from Feng R. et al., [48] investigates how mechanical loading affects subchondral bone remodeling and its impact on cartilage degradation in knee osteoarthritis (OA). The authors identify RANKL (Receptor Activator of Nuclear Factor Kappa-Β Ligand) as a key mechanotransduction mediator that influences osteoclast activity under compression forces. The research reveals that hydrostatic pressure modulates Wnt/β-catenin signaling, which regulates bone metabolism and cartilage integrity. The study suggests that targeted modulation of RANKL signaling via controlled mechanical loading could serve as a novel therapy for knee OA.

Another study from Nims R. et al., [49] explores how mechanosensitive ion channels TRPV4 and PIEZO1 mediate chondrocyte mechanotransduction in the knee joint. The authors demonstrate that mechanical stimulation increases Ca²⁺ influx through TRPV4, which activates the MAPK/ERK1/2 pathway, leading to collagen type II synthesis—a crucial factor in cartilage maintenance. Meanwhile, PIEZO1 signaling triggers downstream YAP/TAZ activation, which influences chondrocyte proliferation and differentiation. The findings suggest that altering PIEZO1 and TRPV4 activity can enhance chondrocyte survival and cartilage regeneration, providing potential therapeutic targets for knee injuries.

Collectively, MAPK, NF-κB, and Wnt signaling converge at multiple checkpoints in chondrocytes, synoviocytes, and fibroblasts, shaping anabolic or catabolic responses based on the magnitude and duration of mechanical input. In moderate loading regimes, ERK and β-catenin support ECM synthesis, lubricin production, and balanced inflammatory responses. Under high or aberrant loads, p38/JNK and NF-κB activities predominate, enhancing inflammatory mediators and MMP-driven breakdown.

1. Chondrocytes and Cartilage
Cartilage is an avascular tissue, meaning it depends on mechanical loading to facilitate nutrient diffusion and waste removal [50]. Chondrocytes, the sole cellular component of healthy cartilage, detect mechanical cues via integrins and mechanosensitive ion channels, which connect extracellular forces to cytoskeletal changes and intracellular signaling cascades [51, 52]. Once activated, these receptors stimulate pathways including MAPK (Mitogen-Activated Protein Kinase), NF-κB (Nuclear Factor kappa-light-chain-enhancer of activated B cells), and Wnt, coordinating gene transcription that governs cell survival, differentiation, and matrix homeostasis [53].
The MAPK pathway transmits signals from the cell surface to the nucleus through phosphorylation cascades involving ERK, JNK, and p38 MAPKs, each responding to specific stress stimuli [54, 55]. ERK generally supports chondrocyte proliferation and differentiation, while JNK and p38 mediate stress and inflammatory responses that can trigger apoptosis if overactivated [56]. In chondrocytes, MAPK signaling enhances synthesis of extracellular matrix (ECM) proteins such as type II collagen and proteoglycans, including aggrecan, which confers compressive strength by binding water molecules [57–59].
On a deeper molecular level, integrin engagement activates focal adhesion kinases (FAKs), which phosphorylate intermediates like MEK (MAPK/ERK kinase). MEK then phosphorylates ERK, driving nuclear translocation of transcription factors that upregulate cartilage-specific genes [34, 55]. In contrast, p38 and JNK often increase levels of matrix metalloproteinases (MMPs) or inflammatory mediators, tipping the balance toward catabolism when stress is excessive.
NF-κB orchestrates inflammatory and stress responses, regulating genes tied to matrix remodeling, cell survival, and apoptosis [60]. In chondrocytes, NF-κB activation boosts production of MMPs and aggrecanases that degrade the cartilage matrix, countered by tissue inhibitors of metalloproteinases (TIMPs) [61, 62]. An imbalance favoring catabolic enzymes facilitates cartilage breakdown, a hallmark of osteoarthritis [63]. Mechanistically, signals from integrins or toll-like receptors activate IκB kinase (IKK), phosphorylating IκB to liberate NF-κB, which then translocates to the nucleus to upregulate pro-inflammatory genes.
The Wnt pathway is another major regulator of chondrocyte function, modulating proliferation, differentiation, and ECM synthesis [64, 65]. Wnt ligands bind Frizzled receptors and LRP5/6 co-receptors, stabilizing β-catenin and promoting its nuclear accumulation. Once inside the nucleus, β-catenin forms transcriptional complexes that control anabolic gene expression [66]. Proper Wnt activity prevents premature chondrocyte hypertrophy, which can lead to calcification if unregulated [67]. TGF-β and BMP signaling often converge with Wnt, adding further layers of control over cartilage growth and remodeling.
Type II collagen forms the tensile framework of cartilage, while large proteoglycans such as aggrecan confer resistance to compression by retaining water [68–70]. Minor collagens (e.g., types IX and XI) and non-collagenous proteins (e.g., COMP) integrate into this network, ensuring biomechanical integrity [71, 72]. Balanced synthesis and degradation of these ECM components is key for homeostasis and repair [73, 74]. Excessive or insufficient loading perturbs this equilibrium, driving degenerative changes characteristic of osteoarthritis [75, 76].
Besides MAPK, NF-κB, and Wnt, calcium signaling also underlies mechanotransduction in chondrocytes [77, 78]. Mechanical stretch or compression opens mechanosensitive channels, increasing intracellular Ca²⁺ levels that activate kinases (e.g., CaMKII) or phosphatases, further modulating transcription factor activity [79].
Cartilage’s low-oxygen milieu makes hypoxia-inducible factors (HIFs) pivotal for energy metabolism and ECM production, particularly HIF-1α, which supports the chondrocyte phenotype under reduced oxygen tension [78]. Growth factors within the ECM—such as TGF-β, BMPs, IGF-1, and FGFs—bind chondrocyte receptors to drive collagen and proteoglycan synthesis, maintaining cartilage stability.
Study from Matheson D. et al., [80] explores how PIEZO1, a mechanosensitive ion channel, mediates chondrocyte responses to mechanical loading in the human knee joint. Increased mechanical stress activates PIEZO1, leading to elevated Ca²⁺ influx in chondrocytes, which affects intracellular signaling cascades. This study finds that OA-associated PIEZO1 genetic variants exhibit altered conductance properties, resulting in hyperactivation under normal loading conditions. Overactivation of PIEZO1 leads to excessive calcium signaling, triggering downstream pathways such as YAP/TAZ, MAPK, and NF-κB, which promote cartilage matrix degradation and inflammation. The findings suggest that targeting PIEZO1 activity could be a potential strategy to modulate chondrocyte mechanotransduction and slow osteoarthritis progression.
Mechanical loading is crucial for cartilage health [74]. Low-magnitude cyclic loading promotes ECM synthesis and chondrocyte activity, whereas excessive loading induces inflammation and degradation. Satic loading can lead to matrix breakdown by disrupting cellular homeostasis. Optimized rehabilitation strategies incorporating controlled mechanical stimuli can enhance cartilage repair and prevent degenerative joint diseases (Table 1).

Table 1: The table shows that under-loading of articular cartilage induces catabolic signalling and early degeneration, optimal physiological loading engages coordinated Ca²⁺-integrin-TRPV4 pathways that foster anabolic matrix renewal, while over-loading hyperactivates Piezo1 and inflammatory NF-κB/MAPK cascades leading to tissue breakdown, with specific strength-training, plyometrics, deceleration and rehabilitation guidelines prescribed for each condition

Table 1

Table 1: The table shows that under-loading of articular cartilage induces catabolic signalling and early degeneration, optimal physiological loading engages coordinated Ca²⁺-integrin-TRPV4 pathways that foster anabolic matrix renewal, while over-loading hyperactivates Piezo1 and inflammatory NF-κB/MAPK cascades leading to tissue breakdown, with specific strength-training, plyometrics, deceleration and rehabilitation guidelines prescribed for each condition

2. Synoviocytes and Synovial Fluid
Synoviocytes are specialized cells in the synovium, a membrane lining the joint capsule that produces synovial fluid [81]. This fluid lubricates the joint, reduces friction, and provides nutrients to avascular cartilage [82]. Mechanical loading activates synoviocytes via mechanosensitive receptors like integrins and ion channels, stimulating the production of synovial fluid components, particularly hyaluronic acid and lubricin [83].[84]
Hyaluronic acid, a high molecular weight glycosaminoglycan, enhances synovial fluid viscosity and forms a viscoelastic network that absorbs mechanical shocks [85].[86] Its synthesis is regulated by cytokines and growth factors such as TGF-β and PDGF, while pro-inflammatory cytokines like IL-1 and TNF-α inhibit its production, reducing joint lubrication [87].[88] Lubricin, also known as PRG4, minimizes friction between cartilage surfaces by forming a slippery layer on the articular cartilage [89].[90] Its expression is upregulated by mechanical stimuli and biochemical signals, including TGF-β and IL-4 [91].[92]
Synoviocytes include fibroblast-like synoviocytes (FLS), responsible for synovial fluid production, and macrophage-like synoviocytes (MLS), which regulate inflammation and tissue repair [93].[94] Mechanical loading activates intracellular pathways such as MAPK, NF-κB, and PI3K/Akt, governing cellular responses [95]. The MAPK pathway, through ERK, JNK, and p38 kinases, regulates synovial fluid component synthesis, while NF-κB modulates inflammatory responses, and PI3K/Akt influences cell survival and metabolism [96].[97]
Synoviocytes interact with joint microenvironment signals, including cytokines and extracellular matrix (ECM) components like collagen, fibronectin, and laminin [98]. Integrins mediate cell-ECM attachment, transducing signals for adhesion, migration, and differentiation. Disrupting these interactions alters synoviocyte function and contributes to joint pathology [99]. Epigenetic mechanisms, including DNA methylation, histone modifications, and miRNAs, regulate genes involved in synovial fluid production and inflammation [100]. Extracellular vesicles (EVs) from synoviocytes transport bioactive molecules, modulating inflammatory responses and cartilage metabolism, making them potential therapeutic targets [101].
MLS play a key role in immune response, producing cytokines that recruit immune cells. While crucial for infection defense and inflammation resolution, dysregulated responses contribute to chronic inflammation and joint damage in autoimmune conditions like rheumatoid arthritis [93]. Understanding synoviocyte signaling and molecular interactions provides insight into joint disease pathophysiology and therapeutic targets for improving lubrication, reducing inflammation, and promoting cartilage repair [94].
Study from Schröder A. et al., [102] investigates how mechanical loading influences synoviocyte behavior and synovial fluid composition in the knee joint. Synovial fibroblasts (SFs), a key component of the synovium, respond to mechanical stress by activating mechanotransduction pathways, notably YAP/TAZ and NF-κB, leading to inflammatory signaling. This study found that excessive mechanical stress upregulates pro-inflammatory cytokines (IL-6, IL-8, TNF-α) in SFs, leading to synovial inflammation and cartilage degradation. In contrast, moderate mechanical loading promotes the secretion of lubricin (PRG4) and hyaluronic acid, enhancing synovial fluid viscosity and joint lubrication. The results suggest that targeting SF activation through mechanical modulation could help balance synovial homeostasis and prevent osteoarthritis progression.
Mechanical loading regimes significantly affect synoviocyte activity. Cyclic compressive loading enhances anabolic responses, stimulating synovial fluid production, while excessive static or shear loading promotes catabolic pathways, contributing to joint degradation. Optimizing mechanical loading strategies is essential for maintaining joint health and informing rehabilitation protocols (Table 2).

Table 2: The table shows that in the synovial membrane (Hoffa’s fat pad) low shear suppresses lubricin (PRG4) and HAS2, physiological cyclic shear restores these lubricating and anti-inflammatory pathways, while excessive stretch triggers Piezo1-driven Ca²⁺ influx and NF-κB/NLRP3 inflammation leading to synovitis, fibrosis and pain, with corresponding progressive guidelines for strength work, jumps/decelerations and rehabilitation at each loading level

Table 2

Table 2: The table shows that in the synovial membrane (Hoffa’s fat pad) low shear suppresses lubricin (PRG4) and HAS2, physiological cyclic shear restores these lubricating and anti-inflammatory pathways, while excessive stretch triggers Piezo1-driven Ca²⁺ influx and NF-κB/NLRP3 inflammation leading to synovitis, fibrosis and pain, with corresponding progressive guidelines for strength work, jumps/decelerations and rehabilitation at each loading level

3. Fibrochondrocytes and Meniscus
The menisci are fibrocartilaginous structures within the knee joint, essential for load distribution, shock absorption, and joint stability [102]. They consist of a dense extracellular matrix (ECM) primarily composed of collagen and proteoglycans, ensuring both strength and flexibility [103]. Fibrochondrocytes within the meniscus regulate ECM composition through mechanotransduction, responding to mechanical stimuli by modulating ECM synthesis to adapt to varying mechanical stresses [104].[105]
The ECM mainly consists of type I collagen for tensile strength and type II collagen for compressive resistance [106]. Proteoglycans, particularly aggrecan, help retain water, enhancing shock absorption [107]. The ECM exhibits an anisotropic organization, with collagen fibers arranged to resist multidirectional loads, reflecting the knee joint's complex mechanical environment [108].
Mechanotransduction in fibrochondrocytes relies on mechanoreceptors such as integrins and mechanosensitive ion channels [109]. These receptors activate intracellular signaling pathways, including MAPK, NF-κB, and Wnt, influencing gene expression and ECM synthesis [110].[111] MAPK pathways—ERK, JNK, and p38—mediate responses to mechanical stimuli; ERK promotes cell proliferation, while JNK and p38 regulate stress responses and apoptosis [112].[113] In the meniscus, MAPK signaling modulates collagen and proteoglycan synthesis, maintaining biomechanical integrity [114].
NF-κB signaling is primarily associated with inflammatory regulation in fibrochondrocytes [115]. Mechanical loading modulates NF-κB activity, affecting cytokine and matrix metalloproteinase (MMP) expression, which governs ECM remodeling [116].[117] A balanced NF-κB response ensures ECM homeostasis, crucial for meniscal maintenance and repair [118].
Wnt signaling regulates fibrochondrocyte proliferation, differentiation, and ECM production [119].[120] It interacts with TGF-β and BMP pathways to coordinate cellular responses to mechanical stimuli [121]. This cross-talk ensures fibrochondrocytes adapt to mechanical stress, preserving meniscal functionality [122].
Intracellular calcium (Ca²⁺) signaling plays a vital role in mechanotransduction [123]. Mechanosensitive ion channels mediate Ca²⁺ influx, acting as secondary messengers that activate kinases and phosphatases, further influencing transcription factors involved in ECM organization [124].[125]
Meniscal vascularization impacts its healing capacity [126-128]. Peripheral regions contain blood vessels and nerves for nutrient supply and sensory feedback, whereas central regions are avascular, relying on synovial fluid diffusion [129].[130] Consequently, peripheral tears have better healing potential than central ones [131].
Additional ECM components like fibronectin, elastin, and decorin contribute to meniscal biomechanics. Fibronectin aids in cell adhesion and tissue repair, elastin enables shape recovery post-deformation, and decorin regulates collagen fibrillogenesis and ECM assembly.
Meniscal degeneration, as seen in osteoarthritis, results from disrupted ECM synthesis and degradation balance [124]. Elevated catabolic enzyme activity, such as MMPs and aggrecanases, accelerates collagen and proteoglycan breakdown. Pro-inflammatory cytokines like IL-1β and TNF-α further enhance catabolic pathways while suppressing anabolic pathways, worsening degeneration [129].
Study from Ma Z. et al., [132] examines how fibrochondrocytes in the human meniscus respond to altered mechanical loading conditions, particularly simulated microgravity, which mimics unloading stress similar to prolonged bed rest or space travel. The researchers found that mechanical unloading leads to downregulation of mechanotransduction pathways, including FAK (Focal Adhesion Kinase) and YAP/TAZ, which are essential for maintaining meniscus homeostasis. A reduction in mechanical stimulation suppresses extracellular matrix (ECM) synthesis, particularly collagen type I, collagen type II, and aggrecan, which are critical for meniscus integrity [131]. Unloading also increases oxidative stress and apoptosis in fibrochondrocytes, mediated by the activation of ROS (reactive oxygen species) and caspase-3/7 pathways, leading to cellular senescence and tissue degeneration [129]. The study highlights the importance of maintaining optimal mechanical loading to prevent meniscus degeneration, which is crucial for knee joint health.
Mechanical loading regimes significantly influence meniscal cell behavior and ECM maintenance. Cyclic compressive loading promotes ECM synthesis and chondroprotective responses, whereas excessive static or shear loading induces catabolic activity and accelerates degeneration. Optimal loading strategies are crucial for rehabilitation and tissue engineering applications, aiming to balance anabolic and catabolic processes for sustained meniscal health and functionality (Table 3).

Table 3: Under-loading weakens integrin-TRPV4 signalling, suppresses matrix genes and loosens the meniscus, optimal 5–10 % cyclic compression re-engages TRPV4/α5β1 pathways for anabolic repair, while > 12 % compression overstimulates Piezo1 and NF-κB/MAPK inflammation that drives vascular ingrowth and calcification—each state paired with tailored strength, plyometric, deceleration and rehabilitation guidelines

Table 3

Table 3: Under-loading weakens integrin-TRPV4 signalling, suppresses matrix genes and loosens the meniscus, optimal 5–10 % cyclic compression re-engages TRPV4/α5β1 pathways for anabolic repair, while > 12 % compression overstimulates Piezo1 and NF-κB/MAPK inflammation that drives vascular ingrowth and calcification—each state paired with tailored strength, plyometric, deceleration and rehabilitation guidelines

4. Fibroblasts and Ligaments/Tendons
Fibroblasts are the primary cells in ligaments and tendons, tissues that connect bones and muscles, playing essential roles in knee joint stability and movement [133]. These cells regulate collagen synthesis and matrix remodeling in response to mechanical loading via mechanotransduction mechanisms involving integrins, focal adhesion complexes, and mechanosensitive ion channels [134]. Activation of these structures initiates intracellular signaling pathways such as MAPK and TGF-β, crucial for collagen fiber production and organization, ensuring tissue strength and elasticity [135].
Type I collagen, produced by fibroblasts, dominates the extracellular matrix (ECM) of ligaments and tendons, forming parallel bundles that provide tensile strength and resistance to mechanical forces [136]. Its synthesis is tightly regulated by mechanical stimuli, with integrins acting as transmembrane receptors that sense stress and initiate intracellular signaling cascades [137]. When fibroblasts undergo mechanical loading, integrins cluster into focal adhesions, sites for biochemical signal transduction involving focal adhesion kinase (FAK) and Src family kinases [138].
The MAPK pathway, comprising ERK, JNK, and p38 MAPK, plays key roles in fibroblast responses to mechanical stimuli [139]. ERK promotes proliferation and collagen synthesis, while JNK and p38 MAPK mediate stress responses and inflammation [140]. These pathways regulate transcription factors that control ECM production and remodeling [141]. TGF-β signaling further modulates fibroblast function by enhancing collagen synthesis and regulating matrix metalloproteinases (MMPs) and tissue inhibitors of metalloproteinases (TIMPs), maintaining ECM homeostasis [142].
Mechanosensitive ion channels, such as Piezo1 and Piezo2, contribute to mechanotransduction by mediating calcium (Ca²⁺) influx, which acts as a secondary messenger in kinase activation, including CaMK and PKC [143]. These kinases influence transcription factors involved in collagen synthesis and fibroblast proliferation, fine-tuning cellular responses to mechanical stress [144-145].
Mechanical loading is essential for maintaining ligament and tendon function, ensuring joint stability and efficient force transmission [146-148]. However, excessive or abnormal loading can lead to microtears, inflammation, and tendinopathy, conditions associated with disrupted ECM balance [149]. Hypoxia-inducible factors (HIFs), particularly HIF-1α, help fibroblasts adapt to the relatively avascular environment of ligaments and tendons by upregulating angiogenesis, ECM production, and metabolic adaptation genes [150].
ECM stiffness significantly influences fibroblast behavior, impacting differentiation, proliferation, and apoptosis through integrin-mediated signaling cascades, including RhoA/ROCK and YAP/TAZ [151]. Fibroblasts also produce and respond to growth factors such as FGF, PDGF, and interleukins, which modulate cell proliferation, migration, and matrix synthesis, crucial for tissue repair and regeneration [152].[153]
Study from Stańczak M. et al., [154] explores how fibroblasts within knee ligaments and tendons respond to mechanical loading through mechanotransduction pathways. Mechanical strain activates integrin-mediated FAK (Focal Adhesion Kinase) signaling, leading to downstream activation of the MAPK/ERK and PI3K/Akt pathways, which regulate cell proliferation and extracellular matrix remodeling.The research identifies that cyclic mechanical loading enhances fibroblast alignment and increases collagen type I and III synthesis, which is crucial for tendon and ligament remodeling. However, excessive mechanical loading leads to an upregulation of MMP-1 and MMP-13 (matrix metalloproteinases), promoting collagen degradation and increasing the risk of injury. The findings emphasize that controlled mechanical loading is essential for maintaining the balance between fibroblast-mediated collagen synthesis and degradation, supporting tendon and ligament repair.
Mechanical loading regimes are crucial for optimizing tissue repair and regeneration. Low-magnitude cyclic loading enhances fibroblast proliferation and collagen deposition, while excessive or static loading can induce matrix degradation and inflammation [155]. Regulating loading parameters, including frequency, amplitude, and duration, is essential for promoting tissue adaptation and minimizing injury risk [156]. These insights contribute to developing targeted rehabilitation strategies and regenerative therapies for ligament and tendon injuries [157] (Table 4) (Table 5).

Table 4: Under-loading of the patellar tendon suppresses collagen-building genes and increases matrix breakdown, optimal 4 % cyclic stretch triggers αVβ3–FAK–ERK signalling that boosts anabolic collagen cross-linking, while > 8 % stretch or high-frequency loading hyperactivates Piezo1–NF-κB–COX-2 pathways causing inflammatory tendinopathy—each state matched to specific strength, jump, deceleration and rehab guidelines

Table 4

Table 4: Under-loading of the patellar tendon suppresses collagen-building genes and increases matrix breakdown, optimal 4 % cyclic stretch triggers αVβ3–FAK–ERK signalling that boosts anabolic collagen cross-linking, while > 8 % stretch or high-frequency loading hyperactivates Piezo1–NF-κB–COX-2 pathways causing inflammatory tendinopathy—each state matched to specific strength, jump, deceleration and rehab guidelines

Table 5: Under-loading of ACL/MCL ligaments suppresses collagen synthesis and cell activity, optimal 3–5 % cyclic stretch activates α5β1-integrin FAK/ERK-Smad signalling that densifies collagen and increases stiffness, while over-loading triggers Piezo1-driven Ca²⁺ influx and NF-κB/p38/STAT3 inflammation that accelerates matrix breakdown—each state matched to distinct strength, plyometric, deceleration and rehab guidelines

Table 5

Table 5: Under-loading of ACL/MCL ligaments suppresses collagen synthesis and cell activity, optimal 3–5 % cyclic stretch activates α5β1-integrin FAK/ERK-Smad signalling that densifies collagen and increases stiffness, while over-loading triggers Piezo1-driven Ca²⁺ influx and NF-κB/p38/STAT3 inflammation that accelerates matrix breakdown—each state matched to distinct strength, plyometric, deceleration and rehab guidelines

Mechanical Loading Modalities and Their Effects in Molecular Biology Context

Different mechanical loading types—compression, tension, shear, and hydrostatic pressure—affect knee joint tissues at macroscopic and molecular levels [183]. Understanding these effects is essential for optimizing rehabilitation protocols.

On a molecular level, mechanical loading regulates gene expression, protein synthesis, and signaling pathways within knee joint tissues [184]. Compression loading stimulates chondrocytes to produce extracellular matrix components such as collagen and proteoglycans, vital for cartilage integrity [185]. This loading activates mechanotransduction pathways involving integrins and the cytoskeleton, leading to transcriptional regulation by NF-κB and AP-1 [186]. Additionally, compression enhances anabolic factors like IGF-1 and TGF-β, promoting cartilage repair [187].

Tension loading, prevalent in tendons and ligaments, enhances tensile strength by increasing collagen synthesis [188]. It activates mechanosensitive ion channels and MAPK signaling, upregulating structural proteins and ECM remodeling enzymes [189]. Tension also modulates MMPs and TIMPs, balancing matrix turnover [190].

Shear stress, occurring during knee joint movement, affects endothelial cells and nitric oxide production, influencing vascular tone and inflammatory responses [191]. Shear-responsive genes such as eNOS and COX-2 regulate inflammation and angiogenesis through VEGF expression, supporting blood supply to joint tissues [192].

Hydrostatic pressure regulates synoviocyte behavior and synovial fluid composition, affecting joint lubrication and nutrient supply [193]. This pressure modulates ion channels, aquaporins, and fluid homeostasis genes, maintaining osmotic balance in the joint cavity [194, 195].

Mechanical loading also influences inflammatory pathways [196]. Compression suppresses pro-inflammatory cytokines like IL-1β and TNF-α while upregulating anti-inflammatory cytokines such as IL-10, fostering a regenerative environment [197, 198]. Additionally, it regulates MMPs and TIMPs, ensuring ECM integrity and preventing excessive matrix degradation [199, 200].

Understanding these molecular mechanisms enables the development of targeted rehabilitation protocols to optimize tissue repair, enhance joint function, and reduce injury risk. Tailoring rehabilitation strategies to modulate specific signaling pathways and cellular responses improves recovery and long-term knee joint health (Table 6).

Table 6: The table summarizes the impact of mechanical forces on cartilage health and repair, detailing key components, functions, signaling pathways, molecular mechanisms, and clinical relevance for compression, tension, shear, and hydrostatic pressure. Each force type influences chondrocytes, ECM, and related pathways to promote tissue synthesis, reduce inflammation, and maintain cartilage elasticity, with clinical applications in osteoarthritis prevention, tendon and ligament repair, and regenerative medicine strategies.)

Table 6

Table 6: The table summarizes the impact of mechanical forces on cartilage health and repair, detailing key components, functions, signaling pathways, molecular mechanisms, and clinical relevance for compression, tension, shear, and hydrostatic pressure. Each force type influences chondrocytes, ECM, and related pathways to promote tissue synthesis, reduce inflammation, and maintain cartilage elasticity, with clinical applications in osteoarthritis prevention, tendon and ligament repair, and regenerative medicine strategies.)

1. Compression
Compression loading is critical for cartilage, ligament, and tendon integrity, despite distinct structural and molecular demands [201]. In cartilage, moderate compression activates chondrocytes to synthesize proteoglycans like aggrecan while suppressing catabolic enzymes degrading the extracellular matrix (ECM) [202]. Chondrocytes sense compression via integrins and mechanosensitive ion channels (PIEZO1, TRPV4), triggering intracellular signaling cascades such as MAPK (ERK1/2, p38, JNK) and NF-κB pathways [203]. Mechanical deformation induces Ca²⁺ influx, activating enzymes like CaMKII and calcineurin, modulating transcription factors that regulate ECM synthesis and stress responses [204, 205].
Integrin clustering at focal adhesions recruits adaptor proteins (talin, vinculin, paxillin) and activates focal adhesion kinase (FAK), which triggers downstream kinases (Src, Ras/Raf), amplifying MAPK and NF-κB pathways. ERK1/2 enhances anabolic gene expression, whereas overactivated p38/JNK drives MMP and aggrecanase production, leading to ECM degradation [206, 207]. NF-κB fine-tunes homeostasis but, under excessive compression, promotes cytokine and protease expression, exacerbating tissue damage [208].
Ligaments and tendons, despite primarily experiencing tensile loading, also respond to compression via integrins (α5β1, αvβ3) and mechanosensitive ion channels [209]. PIEZO1-mediated Ca²⁺ influx fosters collagen I fibril organization and proteoglycan synthesis, enhancing shock absorption in fibrocartilaginous regions [210]. Growth factors like TGF-β and IGF-1 elevate under mild compression, activating SMAD and PI3K/Akt cascades to regulate matrix formation and cytoskeletal dynamics, ensuring resilience [211, 212].
Aggrecan in cartilage assembles with hyaluronic acid to form water-retentive aggregates that resist compressive cycles, stabilized by type II collagen [213]. Ligaments and tendons rely on type I collagen but incorporate type II in fibrocartilaginous zones under compressive forces. Moderate compression sustains ECM turnover, but excessive loads induce deleterious pathways—p38/JNK overactivation and prolonged NF-κB signaling upregulate MMPs and aggrecanases, dismantling ECM [214, 215]. Reactive oxygen species (ROS) accumulate, impairing proteins and disrupting ion channel function, further amplifying NF-κB/MAPK-driven inflammation [216, 217].
Chronic overloading predisposes cartilage to osteoarthritis, marked by cartilage erosion, bone sclerosis, and inflammation [218]. In ligaments, excessive compression weakens collagen, increasing tear risk and joint instability [219]. In tendons, maladaptive loading at entheses fosters tendinopathy [220]. Inflammatory cascades spread via synovial fluid, exacerbating joint dysfunction.
Understanding these molecular mechanisms enables targeted interventions to prevent compressive overuse injuries [221]. Biomechanical strategies like orthotics and exercise protocols optimize loading thresholds [222]. Pharmacological inhibitors of MMPs and ROS scavengers protect ECM integrity [223]. Regenerative approaches, including stem cells and growth factor therapies, balance beneficial compression-induced signaling while mitigating destructive pathways [224]. Controlled mechanical loading can harness anabolic responses, sustaining tissue function and reducing inflammation across cartilage, ligaments, and tendons.

2. Tension
Tensile loading affects ligaments, tendons, and fibrocartilage, driving collagen synthesis and alignment while also influencing cartilage regions subjected to tensile forces, such as menisci and entheses [225]. Fibroblasts (in ligaments) and tenocytes (in tendons) detect mechanical tension via integrin-mediated signaling [226]. Integrins link the ECM to the cytoskeleton, clustering upon stretch and recruiting focal adhesion proteins like vinculin and paxillin [227]. This activates focal adhesion kinase (FAK), which phosphorylates downstream targets, initiating MAPK signaling cascades (ERK1/2, p38, JNK) that regulate ECM production, cell survival, and remodeling [228]. Mechanosensitive ion channels (PIEZO1, TREK-1) also permit Ca²⁺ influx, triggering CaMKII and calcineurin pathways to control collagen and proteoglycan synthesis [229, 230].
Proper tensile loading upregulates type I collagen synthesis, the dominant structural protein in ligaments and tendons, with alignment facilitated by lysyl oxidase cross-linking collagen fibrils for improved tensile strength [231]. In fibrocartilage, tensile cues drive type II collagen production in menisci and mixed type I/II collagen expression in entheses, ensuring structural adaptation [232]. TGF-β and CTGF increase under tension, activating SMAD and PI3K/Akt pathways to enhance ECM assembly [233]. These responses support tissue resilience against multi-directional forces [234].
Mechanotransduction also involves nuclear translocation of transcription factors like YAP/TAZ via the Hippo pathway [235]. Under tension, YAP/TAZ enter the nucleus and interact with TEAD to regulate ECM remodeling and cytoskeletal organization [236]. While moderate tensile loading promotes adaptation, excessive tension induces microtears, releasing damage-associated molecular patterns (DAMPs) that upregulate cytokines IL-1β and TNF-α [237, 238]. These cytokines activate NF-κB, leading to increased MMP-1 and MMP-13 expression, degrading collagen and weakening tissue integrity, which contributes to tendinopathy and ligament laxity [239].
Excessive tension also disrupts homeostasis by increasing reactive oxygen species (ROS), exacerbating oxidative stress and activating JNK/p38 MAPK, which promote apoptosis and ECM degradation [240, 241]. ROS-driven NF-κB signaling intensifies inflammation, perpetuating tissue breakdown [242]. However, controlled tensile loading is integral to rehabilitation, as it modulates integrin signaling and growth factor release to reinforce ECM integrity [243, 244]. Proper activation of mechanosensitive ion channels fine-tunes intracellular Ca²⁺ signaling, balancing matrix turnover and preventing catabolic shifts [245].
Therapeutic strategies leverage tensile loading’s anabolic effects while mitigating overstimulation risks. Pharmacological approaches targeting MMP inhibition and ROS scavenging protect ECM integrity [246]. Regenerative techniques, such as mesenchymal stem cell (MSC) therapy, exploit mechanosensitive differentiation to enhance tenocyte and fibroblast ECM production under controlled tension [247]. Gene therapy holds potential for modifying transcription factors and growth factor expression to optimize tissue repair.
In summary, tensile loading drives molecular adaptations in ligaments, tendons, and fibrocartilage via integrin signaling, MAPK activation, and growth factor-mediated ECM regulation [248]. Mechanosensitive ion channels modulate calcium-dependent gene transcription, directing matrix synthesis. While physiological tension aligns collagen fibers and maintains tissue strength, excessive tension triggers inflammatory and degradative pathways. Understanding the balance between adaptive and pathological tensile stimuli is critical for rehabilitation, pharmacological interventions, and regenerative medicine aimed at preserving and restoring load-bearing tissue function.

3. Shear
Shear Shear stress influences synovial fluid dynamics, cartilage health, and the behavior of ligaments and tendons, adapting their responses through distinct molecular pathways [249]. In cartilage, moderate shear stress enhances proteoglycan and type II collagen synthesis via integrin-mediated signaling, activating Wnt and MAPK pathways (ERK1/2, p38, JNK) that regulate transcription factors and ECM remodeling [250]. Integrins anchor to the actin cytoskeleton, forming focal adhesions under shear force, recruiting focal adhesion kinase (FAK), and linking mechanical cues to biochemical signals for chondrocyte proliferation and matrix remodeling [251]. Shear forces also activate mechanosensitive ion channels (TRPV4, PIEZO1), inducing Ca²⁺ influx, which modulates metabolism, gene expression, and ECM composition. Additionally, shear regulates nitric oxide (NO) and prostaglandin E2 (PGE2) synthesis [252]. Moderate NO and basal PGE2 support ECM integrity, while excessive shear upregulates inducible nitric oxide synthase (iNOS) and amplifies COX-mediated PGE2 production, leading to inflammation, matrix degradation, and apoptosis through NF-κB and ROS accumulation [253].
Ligaments and tendons, while primarily experiencing tensile forces, endure localized shear at entheses and bony prominences [254]. Fibroblasts and tenocytes transduce shear via integrins, activating FAK and MAPK cascades. Mechanosensitive ion channels (TREK-1, TRPV4) allow Ca²⁺ influx, regulating collagen fiber organization and ECM turnover [255]. Moderate shear aligns fibers and sustains ECM integrity, but excessive shear induces DAMP release, upregulating IL-1β, TNF-α, and PGE2, which promote MMP-1 and MMP-13 expression, degrading collagen and weakening tissue structure [256]. ROS accumulation exacerbates matrix breakdown, further activating NF-κB signaling [257].
Across cartilage, ligaments, and tendons, shear stress modulates metabolism via AMPK and mTOR pathways, enhancing glucose and amino acid uptake for ECM synthesis [258]. When excessive, metabolic dysfunction reduces nutrient availability, lowers TIMP/MMP ratios, and increases ECM degradation [259]. Elevated shear alters synovial fluid composition, decreasing hyaluronic acid synthesis and increasing friction, accelerating cartilage wear [260]. In ligaments and tendons, shear-induced remodeling at entheses can weaken structural integrity and cause pain.
Shear stress also influences extracellular vesicle (EV) release, mediating molecular communication between tissues [261]. Cartilage-derived EVs under moderate shear propagate anabolic signals, while excessive shear releases inflammatory EVs that drive catabolic responses [262]. Similar processes in ligaments and tendons may dictate tissue adaptation or degeneration, depending on shear intensity [263].
Ultimately, shear stress regulates ECM turnover, inflammation, and cell survival via integrin and ion channel-mediated pathways [264]. Controlled shear fosters tissue adaptation, while excessive shear triggers inflammatory cascades, ROS production, and matrix degradation through MMP activation and cytokine upregulation [265]. Understanding these mechanisms aids in developing therapeutic strategies, including biomechanical adjustments to reduce abnormal shear, pharmacological inhibitors targeting inflammatory pathways, and regenerative approaches such as tissue-engineered scaffolds or stem cell therapies to optimize cellular responses [266-268].

4. Hydrostatic Pressure
Hydrostatic pressure, as experienced in aquatic therapy, influences cartilage, ligaments, and tendons through distinct molecular mechanisms [269]. In cartilage, chondrocytes respond by increasing proteoglycan and type II collagen synthesis via mechanoreceptors such as integrins, which recruit focal adhesion kinase (FAK) and activate PI3K/Akt and ERK pathways [270, 271]. PI3K/Akt signaling promotes cell survival and ECM synthesis by phosphorylating BAD and caspase-9, while ERK upregulates genes encoding collagen and aggrecan [272, 273].
Mechanosensitive ion channels (TRPV4, PIEZO1) mediate Ca²⁺ influx, activating CaMKII and other enzymes that regulate gene transcription and protein synthesis, further supporting ECM stability and chondrocyte viability [274, 275]. Hydrostatic pressure also mitigates inflammation by reducing NF-κB activity and downregulating IL-1β and TNF-α expression, while simultaneously enhancing antioxidant enzymes like superoxide dismutase (SOD) and catalase, protecting against ROS-induced damage [276, 277]. This anti-inflammatory effect extends to synoviocytes, promoting hyaluronic acid and lubricin synthesis, improving synovial fluid viscosity, and reducing cartilage wear [278, 279].
In ligaments and tendons, fibroblasts and tenocytes primarily adapt to tensile forces but can benefit from controlled hydrostatic pressure in therapeutic settings [280]. Immersion in an aquatic environment reduces gravitational forces and applies mild hydrostatic pressure, subtly activating integrin-based signaling and MAPK pathways [281, 282]. FAK phosphorylation, PI3K/Akt and ERK signaling, and mechanosensitive ion channels (TRPV4, PIEZO1) contribute to modest increases in collagen (type I) and proteoglycan synthesis, aiding tissue resilience and repair [283, 284]. Hydrostatic pressure also enhances nutrient diffusion, optimizing the metabolic environment for tendon and ligament healing.
Inflammatory responses in tendons and ligaments are modulated via hydrostatic pressure by suppressing NF-κB and activating the Nrf2 pathway, which governs antioxidant defenses and cytoprotective gene expression [285]. Reduced oxidative stress prevents the ROS-driven breakdown of collagen fibers, limiting tendinopathy and ligament degeneration [286]. Additionally, growth factors such as TGF-β and IGF-1 are upregulated, supporting ECM synthesis and collagen cross-linking, crucial for structural integrity and repair [287].

Fig. 2: Knee joints a remarkable capacity to adapt to different types of mechanical loads, with the most well-documented changes occurring in response to tensile and compressive stresses. The musculoskeletal system experiences three primary types of mechanical loads: tension (cells make more type I collagen and lysyl oxidase, resulting in a stiff aligned collagen matrix), compression (the same cells induce the expression of large proteoglycans that contain a protein-like hyaluronic acid and gylcosaminoglycans, such as chondroitin and keratin sulfate), and shear (leads to the production of proteoglycans, hyaluronic acid, superficial zone protein, and lubricin at the edge of the tissue, resulting in a collagen matrix that holds fluid at the edge of the tissue to lubricate movement). Knee soft tissues developing under tensile load show a dense, aligned matrix predominantly composed of type I collagen fibers. In contrast, musculoskeletal tissues subjected to compressive forces display a fibrocartilaginous phenotype characterized by sparsely connected, unaligned, and smaller type I collagen fibers along with larger proteoglycans. Knee joint tissues exposed to shear stress develop a partially aligned matrix and produce high levels of surface lubricating proteins such as lubricin, proteoglycan 4, and hyaluronic acid. Adapted from Kenneth Tam and Keith Baar, 2025).

Fig. 2

Fig. 2: Knee joints a remarkable capacity to adapt to different types of mechanical loads, with the most well-documented changes occurring in response to tensile and compressive stresses. The musculoskeletal system experiences three primary types of mechanical loads: tension (cells make more type I collagen and lysyl oxidase, resulting in a stiff aligned collagen matrix), compression (the same cells induce the expression of large proteoglycans that contain a protein-like hyaluronic acid and gylcosaminoglycans, such as chondroitin and keratin sulfate), and shear (leads to the production of proteoglycans, hyaluronic acid, superficial zone protein, and lubricin at the edge of the tissue, resulting in a collagen matrix that holds fluid at the edge of the tissue to lubricate movement). Knee soft tissues developing under tensile load show a dense, aligned matrix predominantly composed of type I collagen fibers. In contrast, musculoskeletal tissues subjected to compressive forces display a fibrocartilaginous phenotype characterized by sparsely connected, unaligned, and smaller type I collagen fibers along with larger proteoglycans. Knee joint tissues exposed to shear stress develop a partially aligned matrix and produce high levels of surface lubricating proteins such as lubricin, proteoglycan 4, and hyaluronic acid. Adapted from Kenneth Tam and Keith Baar, 2025).

Hydrostatic pressure also influences extracellular vesicle (EV) release, modulating intercellular communication among chondrocytes, tenocytes, ligament fibroblasts, and synoviocytes [288]. EVs generated under controlled pressure conditions can carry anabolic signals that enhance regeneration, while those under excessive pressure may propagate inflammatory mediators [289, 290].

Clinically, hydrostatic pressure reduces joint load while promoting beneficial cellular responses, making aquatic therapy and Blood Flow Restriction (BFR) a valuable intervention for osteoarthritis and ligament or tendon injuries [291]. By optimizing PI3K/Akt, ERK, ion channel activity, and Ca²⁺ signaling, hydrostatic pressure enhances ECM integrity, downregulates inflammation, and improves tissue recovery [292]. As research advances, therapeutic strategies will further refine aquatic therapy, pharmacological approaches, and regenerative techniques like stem cell therapy and gene modulation to maximize the protective and reparative benefits of hydrostatic pressure [293].

Rehabilitation and Mechanotransduction

Effective knee rehabilitation strategies leverage mechanotransduction principles to optimize tissue repair and functional recovery [294]. Mechanotransduction converts mechanical stimuli into biochemical signals, regulating gene expression, protein synthesis, and ECM remodeling, which are critical for musculoskeletal tissue adaptation after injury or surgery [295]. Properly controlled loading enhances healing, while excessive or improper loading disrupts repair, triggering inflammation, tissue breakdown, or re-injury [296] (Fig. 3).

Fig. 3: This Fig. illustrates the integration of the natural healing process of musculoskeletal tissues with a progressively increasing mechanical load during rehabilitation. It is divided into three phases: inflammation, proliferation, and remodeling. The physiological processes in each phase guide the rehabilitation strategies, beginning with isometric exercises and blood flow restriction (BFR) to maintain neuromuscular activity while minimizing stress. In the proliferation phase, controlled mechanical loading through dynamic constant external resistance (DCER) and accentuated eccentric training enhances collagen organization and tissue resilience. The remodeling phase introduces plyometric training, high-speed decelerations, and fast eccentric loading (FEL) to reinforce neuromuscular efficiency, optimize tendon resilience, and restore functional capacity. The flow of recovery is visually represented by a transition from injury to functional movement, emphasizing the synergy between biological healing and progressive loading.

Fig. 3

Fig. 3: This Fig. illustrates the integration of the natural healing process of musculoskeletal tissues with a progressively increasing mechanical load during rehabilitation. It is divided into three phases: inflammation, proliferation, and remodeling. The physiological processes in each phase guide the rehabilitation strategies, beginning with isometric exercises and blood flow restriction (BFR) to maintain neuromuscular activity while minimizing stress. In the proliferation phase, controlled mechanical loading through dynamic constant external resistance (DCER) and accentuated eccentric training enhances collagen organization and tissue resilience. The remodeling phase introduces plyometric training, high-speed decelerations, and fast eccentric loading (FEL) to reinforce neuromuscular efficiency, optimize tendon resilience, and restore functional capacity. The flow of recovery is visually represented by a transition from injury to functional movement, emphasizing the synergy between biological healing and progressive loading.

Mechanical forces deform cells, activating mechanoreceptors such as integrins and mechanosensitive ion channels, which initiate intracellular signaling cascades [297]. Integrins cluster at focal adhesions, recruiting focal adhesion kinase (FAK), triggering MAPK (ERK, JNK, p38) and PI3K/Akt pathways [298]. ERK signaling supports ECM synthesis and cell proliferation, while excessive JNK/p38 activation promotes inflammation and catabolic responses, accelerating tissue degradation [299]. Mechanosensitive ion channels (PIEZO, TRPV4) regulate calcium influx, activating calmodulin and calcineurin, modulating transcription factors that influence ECM dynamics and cellular survival [300].

Proper rehabilitation applies progressive mechanical loading to stimulate anabolic pathways without overstressing tissues [301]. Chondrocytes, fibroblasts, and tenocytes respond to controlled stress by producing collagen and proteoglycans, strengthening tissue integrity [302]. However, excessive loading induces NF-κB activation, increasing MMPs, cytokine release (TNF-α, IL-1), and oxidative stress, which hinder healing and may lead to osteoarthritis or tendinopathy [303].

Optimizing loading parameters—type, magnitude, frequency, and duration—is crucial [304]. Underloading leads to muscle atrophy and inadequate collagen deposition, whereas excessive loading upregulates catabolic genes, increasing ECM degradation [305]. The timing of loading is also critical; early moderate mechanical stress activates growth factors like TGF-β and IGF-1, which drive ECM remodeling and cell proliferation, enhancing tissue regeneration [306]. TGF-β strengthens collagen networks, while IGF-1 promotes muscle and tendon repair, reducing injury recurrence [307]. Conversely, excessive early loading hyperactivates JNK/p38 and NF-κB, increasing inflammation and delaying healing [308].

Clinical studies reinforce the benefits of structured loading [309]. Progressive eccentric loading improved collagen organization and pain reduction in Achilles tendinopathy, demonstrating its effectiveness in knee rehabilitation [310]. A controlled loading protocol post-ACL reconstruction accelerated functional recovery and reduced re-injury rates, linking systematic load progression to enhanced ECM remodeling [311]. Similarly, early mild loading in acute ankle sprains expedited swelling reduction and improved tissue organization, reinforcing the role of mechanotransduction in recovery [312].

Khan K.M et al., [313] explores the role of mechanotransduction in musculoskeletal rehabilitation. The authors highlight how targeted exercise can stimulate cellular repair mechanisms in the knee joint, leading to improved rehabilitation outcomes. Mechanotransduction underlies the effectiveness of rehabilitation protocols for chronic knee pain, particularly in conditions like patellar tendinopathy.

Another study from Longerstedt et al., [314] examines how mechanical loading affects knee rehabilitation by influencing structural tissue adaptation. This research emphasizes the importance of monitoring training loads to optimize knee rehabilitation. The findings suggest that different mechanical stimuli can either enhance or hinder tissue healing, depending on intensity and duration.

In conclusion, mechanotransduction-based rehabilitation optimizes tissue repair by modulating MAPK, NF-κB, and ion channel signaling, enhancing ECM integrity while preventing inflammatory degeneration [315-317]. Adjusting exercise intensity and timing activates TGF-β and IGF-1, reinforcing musculoskeletal strength while minimizing fibrosis and chronic inflammation [318]. Understanding these molecular processes allows clinicians to develop tailored rehabilitation protocols, integrating biomechanics with cellular biology to maximize recovery, minimize re-injury, and improve long-term joint function [319-325].

1. Controlled Loading
Gradual and controlled mechanical loading is essential for tissue repair and strength, as it stimulates mechanotransductive pathways without causing further damage [326]. Mechanoreceptors like integrins sense these forces, triggering intracellular signaling cascades that regulate ECM synthesis and remodeling [327].
To optimize physiological adaptations in tissue following regenerative medicine, precise loading strategies must be applied progressively in magnitude, direction, and rate, targeting specific tissues at appropriate healing stages [327]. Different phases of tissue repair necessitate varied loading applications, as mechanical stimuli influence the composition, structure, and function of musculoskeletal tissue through mechanotransduction [328].
In knee rehabilitation, controlled loading promotes collagen fibrillogenesis, enhancing structural integrity [328]. Fibroblasts synthesize procollagen, which assembles into mature fibers, cross-linking to increase tensile strength [329]. Growth factors such as TGF-β and VEGF facilitate ECM deposition and angiogenesis, ensuring oxygen and nutrient supply to healing tissues [330, 331].
Primary cilia and stretch-activated ion channels contribute to mechanotransduction. Primary cilia detect mechanical changes, influencing cell division and differentiation, while PIEZO channels mediate ion flux, activating downstream repair pathways [332, 333].
One study from Jin et al., [334] explores a novel wearable A-mode ultrasound system designed to measure joint torque in real-time, providing critical insights into mechanical loading patterns during rehabilitation exercises. The researchers examined how controlled mechanical loading affects knee joint torque during dynamic movements, offering real-time biofeedback to optimize rehabilitation strategies [335]. At the molecular level, findings suggest that mechanotransduction through integrin signaling and TGF-β activation enhances collagen fiber alignment and fibroblast proliferation, which are essential for ligament and cartilage healing [336]. By leveraging ultrasound imaging to fine-tune mechanical loading, this study introduces a non-invasive method for personalizing rehabilitation protocols, potentially reducing reinjury risks and improving long-term joint function.
Another study from Sharma et al., 2024 [337] evaluates the biomechanical effects of controlled mechanical loading via carbon fiber dynamic orthoses in patients recovering from lower limb traumatic injuries, including ACL and meniscal tears. Using gait analysis, the researchers demonstrated that custom dynamic orthoses improve joint loading symmetry, reducing excessive shear stress on cartilage and ligaments [338]. The study also highlights how controlled loading influences proteoglycan turnover and chondrocyte mechanosensation, preventing cartilage degradation. Molecularly, YAP/TAZ and FAK signaling pathways were implicated in the cellular response to controlled loading, promoting tissue adaptation and repair [339]. These findings support the integration of adaptive external supports in rehabilitation programs to optimize knee joint mechanics and reduce secondary injury risks.
The last randomized controlled trial from Jacobs et al., 2024 [340] investigates the impact of controlled mechanical loading combined with Vascular Occlusion Training (VOT) well known as Blood Flow Restriction Training (BFRT) in patients with knee osteoarthritis. The study found that low-intensity mechanical loading with intermittent vascular occlusion enhances muscle hypertrophy, joint stabilization, and cartilage integrity compared to traditional rehabilitation approaches. Mechanistically, VOT was shown to stimulate hypoxia-inducible factor (HIF-1α) and VEGF expression, promoting angiogenesis and enhancing chondrocyte survival. Additionally, controlled loading modulated MMP-13 and ADAMTS5 activity, reducing excessive cartilage catabolism [341]. These findings provide compelling evidence that vascular occlusion training, when combined with controlled mechanical loading, may optimize knee joint function and slow OA progression.
Glasgow et al [342]. reported that variable loading may enhance mechanotransductive effects by introducing controlled micro-stresses that facilitate adaptation while preventing repetitive strain injury and delayed healing. Variability in tensile, compressive, and torsional forces may promote the deposition of a structurally resilient extracellular matrix (ECM), strengthening the biological scaffold and improving tissue load tolerance [343]. Mechanosensitive ion channels, such as PIEZO1 and TRPV4, respond to these dynamic forces by modulating intracellular calcium signaling, which activates downstream pathways like MAPK and PI3K/Akt, enhancing ECM remodeling and cellular proliferation [339].
Molecular mechanisms involved in controlled loading include the Wnt/β-catenin and Hippo pathways. Wnt/β-catenin signaling regulates cell proliferation and differentiation in response to mechanical stress, while the Hippo pathway modulates cell growth and apoptosis, impacting tissue remodeling [342, 343].
Controlled loading also regulates MMP activity, balancing ECM degradation and synthesis for optimal tissue remodeling [344, 345]. Proper MMP control prevents excessive breakdown while ensuring new ECM deposition.
In conclusion, controlled loading optimizes knee rehabilitation by leveraging mechanotransduction to stimulate tissue repair and restore function [346]. Tailoring protocols based on molecular insights enhances recovery, structural integrity, and long-term joint function. Integrating molecular biology into rehabilitation strategies enables targeted interventions, ensuring effective tissue regeneration and improved patient outcomes.

2. Exercise Therapy
It is well-recognized that resistance exercise stimulates an increase in skeletal muscle protein synthesis and promotes hypertrophy [347]. When skeletal muscle fibers adapt to resistance training, they do so through incremental protein accretion, necessitating enhanced ribosomal function and protein translation. These two processes are strictly regulated by the mTOR signaling pathway [348]. Increasing evidence also indicates that the mTOR pathway intersects with MAPKs at multiple points, contributing to hypertrophic outcomes [349, 350]. Notably, resistance exercise strongly activates MAPKs; however, a sufficient intensity threshold is required to trigger ERK1/2 and p38, both part of the MAPK family [351, 352]. Another study highlighted that JNK, also a MAPK, is particularly sensitive to mechanical load, with its activation correlating to increases in exercise intensity [353]. Overall, MAPK activation is heavily influenced by exercise parameters. For instance, high-intensity, low-repetition resistance protocols elicit more robust ERK1/2 and p38 activation compared to low-intensity, high-repetition regimens [354]. Despite the wealth of data on acute MAPK responses following resistance exercise, there remains a gap in understanding MAPK contributions to long-term exercise adaptations in humans. While MAPKs are clearly integral to mechanotransduction, additional research is needed to clarify their roles in sustained resistance training adaptations in human skeletal muscle [355, 356].
Several factors drive satellite cell activation, thereby influencing the hypertrophic response to resistance exercise [357]. Each nucleus in a multinucleated fiber governs only a fixed volume of cytoplasm—the myonuclear domain—so substantial muscle fiber hypertrophy beyond that domain limit requires adding new nuclei. These additional nuclei are thought to come from satellite cells that differentiate and fuse with existing fibers [358]. Previous human studies have shown a marked rise in satellite cell numbers within 24 hours after acute lower-body resistance exercise, remaining elevated for 72–96 hours and then tapering off, with intensity serving as a key determinant of the acute response [359, 360]. This immediate response is minimal when exercise intensity is under 40% of one-repetition maximum (1 RM), but increases two- to three-fold at intensities exceeding 60% of 1 RM [361]. Likewise, long-term studies involving resistance training (comparing high-intensity to lower-intensity protocols) reported a notable increase in satellite cell proliferation over training periods of 9–16 weeks [362-372]. These findings collectively support the idea that satellite cells are activated during hypertrophy, supplying additional nuclei to accommodate the enlarged cytoplasmic volume in growing muscle fibers.
Contrary to the notion of continuous myonuclear addition, some investigations have observed muscle fiber hypertrophy without a clear increase in satellite cell–mediated myonuclear content [373–382]. More recent evidence, however, emphasizes that the hypertrophic response to mechanical overload largely depends on satellite cell activity [383-389]. Taken together, mechanical loading stands out as a major stimulus for muscle hypertrophy in resistance exercise. Hypertrophy is initially facilitated by protein accretion—regulated by the mTOR pathway alongside MAPKs (ERK1/2, p38, and JNK)—and is sustained by ongoing myonuclear addition via satellite cell activation. Although early muscle fiber enlargement may rely primarily on protein accretion, continuous hypertrophy over time likely requires additional myonuclei contributed by satellite cells as the muscle remains subject to mechanical loading through resistance exercise [390-395].
Study drom Du J. et al., [396] investigates how eccentric training impacts muscle and tendon remodeling at the molecular level in human knee rehabilitation. Eccentric loading activates the Akt/mTOR pathway, which enhances protein synthesis and muscle hypertrophy, leading to improved tendon resilience in the knee joint. The study identifies YAP/TAZ signaling activation, which is crucial for tendon mechanotransduction and fibroblast proliferation, promoting collagen type I and III synthesis. Eccentric training also triggers mechanosensitive ion channels like PIEZO1, which influence calcium influx and ECM remodeling, helping in ligament adaptation. Additionally, the study highlights a protective effect against oxidative stress, mediated by NRF2/KEAP1 signaling, reducing tissue degradation and inflammation.
Another study from Cheng L. et al., [397] investigates how isometric quadriceps training influences chondrocyte activity and cartilage regeneration in knee osteoarthritis (KOA). Isometric contractions activate the PI3K/Akt/mTOR pathway, promoting chondrocyte survival and cartilage matrix synthesis. This research highlights that mechanical stress from isometric exercises enhances autophagy in chondrocytes, a process crucial for cartilage homeostasis and degradation prevention. Increased autophagic flux protects chondrocytes from apoptosis, reducing oxidative stress and inflammation through the NRF2/KEAP1 pathway. This findings suggest that controlled isometric exercise could be an effective non-pharmacological strategy to slow cartilage degradation and enhance knee joint rehabilitation.

3. Manual Therapy
Manual therapy, including joint mobilization and manipulation, modulates mechanical stimuli to enhance mechanotransduction and tissue repair. By applying pressure and movement, these techniques alleviate pain, improve joint mobility, and activate cellular signaling pathways essential for regeneration [398]. Integrins, crucial mechanoreceptors, link the ECM to the cytoskeleton, clustering upon mechanical stimulation and initiating MAPK and PI3K-Akt pathways, promoting protein synthesis and cellular repair [399].
At the molecular level, mechanical forces activate integrins, triggering conformational changes that facilitate ECM protein binding (e.g., fibronectin, collagen, laminin) [400]. This recruits focal adhesion kinase (FAK), leading to phosphorylation cascades activating Ras-Raf-MEK-ERK and PI3K-Akt pathways [401]. These cascades regulate protein synthesis, cell proliferation, and survival, vital for tissue regeneration.
Gentle mobilization techniques enhance synovial fluid production, improving joint lubrication and cartilage health [402]. Synoviocytes increase hyaluronic acid and lubricin secretion, reducing friction and delivering nutrients to chondrocytes, supporting cartilage maintenance [403].
Manual therapy also influences inflammation by modulating cytokine and growth factor expression [404]. Mechanical forces upregulate IL-10, an anti-inflammatory cytokine, suppressing pro-inflammatory pathways, while TGF-β enhances ECM synthesis and cellular differentiation, facilitating tissue remodeling and repair [405, 406].
Combining manual therapy with exercise therapy sustains mechanotransductive effects, improving joint strength and flexibility while reducing re-injury risk [407]. Exercise-induced mechanical loading further activates integrins, promoting ECM protein production and growth factor release [408]. Additionally, exercise upregulates genes involved in muscle hypertrophy via the mTOR pathway, enhancing muscle protein synthesis and functional recovery [409].
Study from Mellinger et al., [410] examines the role of manual therapy and mechanical loading interventions in treating knee injuries, particularly patellofemoral pain syndrome (PFPS) and ACL rehabilitation in runners. The research compares manual therapy techniques (joint mobilization, soft tissue therapy) combined with controlled loading exercises versus standard physiotherapy. The results showed that mechanical loading, when introduced in a structured manner, improved pain levels, enhanced running biomechanics, and reduced knee joint stress.
Last study from L NG et al., [411] study focuses on mechanobiology-based rehabilitation, emphasizing how manual therapy and controlled loading affect cellular healing and knee joint regeneration. The research explores how joint mobilization and external forces modulate chondrocyte mechanotransduction via YAP/TAZ and FAK signaling pathways, leading to enhanced cartilage repair. Findings indicate that gradual mechanical loading after knee injuries improves ligamentous remodeling (COL1A1 and COL3A1 expression) and enhances meniscus fibrocartilage integrity. The authors suggest that combining manual therapy (to modulate synovial fluid mechanics and joint congruency) with weight-bearing exercises (to promote collagen realignment) accelerates knee joint healing. This study provides strong evidence that physiotherapy protocols should integrate mechanobiology principles to maximize knee rehabilitation efficiency.
The integration of manual and exercise therapy optimizes rehabilitation by leveraging molecular pathways to enhance healing, reduce pain, and improve functional outcomes. This synergistic approach maximizes joint stability and tissue regeneration, providing a comprehensive strategy for long-term joint health.

4. Early Mechanical Loading
Early mechanical loading significantly impacts rehabilitation by stimulating molecular and cellular mechanisms that drive tissue repair [412]. For instance, a randomized controlled trial [412] found that partial weight-bearing exercises introduced within two weeks post-knee surgery led to more robust collagen alignment compared to delayed loading protocols, indicating that early intervention significantly enhances tissue quality. Another systematic review [413] reported that beginning light functional exercises within the first three weeks post-ACL reconstruction correlated with improved tendon structure and reduced postoperative stiffness, suggesting a narrow therapeutic window in which mechanotransduction can be most effectively harnessed.
Research suggests that controlled loading initiated within two weeks post-injury enhances collagen alignment and reduces postoperative stiffness, optimizing tendon and ligament structure [413].
Mechanotransduction plays a critical role by activating integrins, which link the ECM to the cytoskeleton [414]. Mechanical forces trigger integrin clustering, leading to FAK phosphorylation and activation of Src family kinases, MAPK (ERK, JNK, p38), and PI3K-Akt pathways [415, 416]. MAPK signaling regulates ECM protein synthesis (e.g., collagen, fibronectin), while PI3K-Akt promotes cell survival and mTOR-mediated protein synthesis, accelerating tissue regeneration [417]. Delayed loading beyond six weeks often results in suboptimal collagen organization and prolonged recovery [418].
Early controlled mechanical loading following orthobiologic procedures accelerates tissue repair and reduces pain by activating molecular pathways involved in mechanotransduction, including integrin-mediated focal adhesion kinase (FAK) signaling [48, 49]. Muscle contraction type also influences healing kinetics. Eccentric contractions generate greater tension and induce robust mechanosensitive responses compared to concentric contractions, while isometric contractions activate muscles without altering fiber length, maintaining joint stability and reducing pain [418-419].
Isometric exercises in early rehabilitation facilitate neural and structural adaptations, minimizing pain while improving muscle activation and proprioception [414]. Pain reduction through controlled loading enables increased joint range of motion, progressively transitioning to isotonic loading strategies that introduce further tensile and compressive stress, stimulating collagen synthesis and ECM reorganization. Load progression should integrate neural adaptation mechanisms involving proprioceptive feedback loops and motor unit recruitment patterns, ensuring effective tissue remodeling and functional recovery [415-416].
Early loading should be carefully monitored, progressing from isometric to isotonic exercises based on pain levels and healing progress [419, 420]. This gradual approach provides mechanical stimuli to activate cellular pathways without exacerbating injury, supporting a more functional repair process.
A major benefit of early mechanical loading is the reduction of excessive scar tissue formation, which can restrict mobility [421]. TGF-β signaling regulates fibroblast activity and ECM synthesis, promoting organized collagen deposition [422]. Clinical trials demonstrate that gentle weight-bearing within two weeks post-meniscal repair minimizes scar tissue and accelerates functional recovery [423].
This study from Mae et al., [424] examines the effects of early mechanical loading on graft tension following double-bundle ACL reconstruction. The research investigates how active knee extension exercises influence the biomechanical properties of the reconstructed ACL. The authors analyzed graft tension variations in response to quadriceps activation and knee extension angles, finding that certain loading conditions could improve graft integration while excessive stress could risk over-stretching the ligament. The molecular response to loading involved increased fibroblast proliferation, collagen synthesis (primarily COL1A1 and COL3A1), and extracellular matrix (ECM) remodeling, mediated by mechanotransduction pathways, such as TGF-β and integrin signaling. Understanding these loading parameters helps optimize post-operative rehabilitation strategies, allowing controlled early mechanical stimulation without compromising graft integrity.
Another study from Capin et al., [425] explores the long-term biomechanical effects of early weight-bearing mechanical loading following medial meniscectomy compared to meniscal repair in patients with ACL reconstruction. Using gait analysis, the study found that partial meniscectomy significantly altered knee joint kinematics and load distribution even two years post-surgery. These changes were associated with increased compressive forces on the medial tibiofemoral compartment, accelerated cartilage degradation, and a shift in subchondral bone remodeling. The researchers identified changes in cartilage proteoglycan content (aggrecan loss) and an upregulation of MMP-13 and ADAMTS5, enzymes involved in cartilage catabolism. In contrast, patients who underwent meniscal repair maintained more natural joint biomechanics, preserving type II collagen and chondrogenic markers such as SOX9. The study suggests that mechanical loading post-meniscectomy should be carefully regulated to minimize long-term degenerative changes.
Last study from Uzuner et al., [426] investigates how meniscectomy-induced mechanical changes affect ACL loading during weight-bearing activities. Researchers found that early mechanical loading post-meniscectomy led to a redistribution of forces across the knee joint, significantly increasing ACL strain and anterior tibial translation. Molecularly, the altered mechanical environment induced an upregulation of pro-inflammatory cytokines (IL-1β, TNF-α) and matrix-degrading enzymes (MMP-1, MMP-13), accelerating ACL microstructural damage. The study further showed that partial meniscectomy caused greater ACL loading asymmetry compared to total meniscectomy, suggesting that incomplete meniscus removal may create an uneven force distribution leading to focal stress on the ligament. The findings highlight the importance of adaptive neuromuscular training and early controlled weight-bearing exercises to mitigate excessive ACL loading while optimizing recovery.
In conclusion, incorporating early mechanical loading into rehabilitation leverages mechanotransduction to enhance recovery outcomes. By structuring controlled loading strategies—such as partial weight-bearing within two to three weeks post-injury—clinicians can optimize ECM remodeling, minimize scar tissue, and regulate inflammation, ultimately improving functional outcomes and reducing recovery time.

Rehabilitation Strategies Based on Musculoskeletal Healing Stages: Early Mechanical Loading

Rehabilitation strategies for musculoskeletal injuries must align with the distinct healing stages: inflammation, proliferation, and remodeling. Tailoring interventions to these stages optimizes tissue repair, restores function, and minimizes reinjury risk [427]. Molecular insights into cellular mechanisms enhance rehabilitation effectiveness [428].

During inflammation, the body responds with pro-inflammatory cytokines like IL-1 and TNF-α, which recruit immune cells to clear debris [429]. Rehabilitation at this stage aims to reduce inflammation while maintaining muscle activation through gentle range-of-motion exercises and isometric contractions, modulating inflammatory responses and preventing excessive tissue degradation [430].

The proliferation stage involves fibroblast activation and ECM synthesis, primarily collagen deposition, driven by growth factors such as TGF-β and VEGF [431]. Controlled mechanical loading is crucial for collagen fiber organization. Low-intensity resistance training and balance exercises stimulate IGF-1 expression, further enhancing tissue regeneration [432].

In remodeling, new tissue undergoes maturation through MMP-regulated ECM remodeling and collagen cross-linking [433]. Rehabilitation shifts toward progressive overload with increased exercise intensity. Plyometric exercises and sport-specific drills promote collagen alignment along mechanical stress lines, improving tissue strength and function [434, 435].

Stage-specific rehabilitation enhances healing, reduces reinjury risk, and accelerates functional recovery. Early mechanical loading, tailored to each stage, is critical. Molecular biology insights guide timing and intervention selection, ensuring rehabilitation strategies support natural healing at the cellular level (Table 7).

1. Inflammation Stage
The inflammation stage is the initial and crucial response to musculoskeletal injury, marked by vasodilation, platelet activation, and the recruitment of inflammatory cells, including neutrophils, monocytes, and macrophages [436]. These processes are regulated by complex chemical mediators such as histamine, bradykinin, and prostaglandin E2 (PGE2), each playing distinct roles in the inflammatory cascade. At the molecular level, interconnected signaling pathways orchestrate cellular responses to promote tissue repair and recovery [437].
Vasodilation increases blood flow to the injured site, ensuring the delivery of essential nutrients and immune cells. This response is mediated by histamine, bradykinin, and PGE2, which trigger the relaxation of vascular smooth muscle cells [438]. Histamine, released from mast cells, basophils, and platelets, binds to H1 receptors on endothelial cells, increasing vascular permeability and allowing immune cells to infiltrate the tissue. Additionally, histamine stimulates endothelial nitric oxide synthase (eNOS), producing nitric oxide (NO), a potent vasodilator that further enhances blood flow and nutrient exchange [439]. This activation involves secondary messengers such as cyclic AMP (cAMP) and intracellular calcium ions, amplifying the inflammatory response [440].
Bradykinin, generated from kininogen via kallikrein activity, binds to B2 receptors on endothelial cells, promoting NO and prostacyclin (PGI2) release, which dilate blood vessels and enhance permeability [441]. This process facilitates immune cell infiltration and supports tissue repair. Bradykinin also sensitizes nociceptors, increasing pain perception as a protective mechanism to limit movement and prevent further damage [442]. Intracellularly, bradykinin signaling activates phospholipase C (PLC), generating inositol triphosphate (IP3) and diacylglycerol (DAG), leading to calcium release and protein kinase C (PKC) activation, which propagate inflammatory responses [443].
PGE2, synthesized from arachidonic acid via the cyclooxygenase (COX) pathway, plays a pivotal role in inflammation. COX-2, upregulated in response to injury, drives PGE2 synthesis, which binds to EP2 and EP4 receptors on smooth muscle cells, increasing cAMP and causing vasodilation [444]. PGE2 also sensitizes sensory nerves, heightening pain perception. This biosynthetic cascade is tightly regulated by phospholipase A2 (PLA2), COX enzymes, and specific synthases, ensuring controlled inflammatory signaling [445].
Platelets initiate hemostasis and tissue repair by adhering to exposed subendothelial collagen and von Willebrand factor (vWF), which engage glycoprotein receptors GPVI and GPIb, triggering platelet activation [446]. Activated platelets release adenosine diphosphate (ADP) and thromboxane A2 (TXA2), amplifying aggregation and stabilizing the injury site [447]. They also secrete growth factors such as platelet-derived growth factor (PDGF) and transforming growth factor-beta (TGF-β), which recruit fibroblasts and smooth muscle cells to drive tissue regeneration [448]. These processes are mediated by PI3K/Akt signaling and small GTPases like Rap1, which regulate cytoskeletal reorganization and integrin activation, stabilizing thrombus formation [449].
Inflammatory cell recruitment is critical for clearing debris and orchestrating repair. Neutrophils, attracted by IL-8, complement C5a, and leukotriene B4 (LTB4), engulf pathogens via phagocytosis, release proteolytic enzymes (e.g., elastase, collagenase), and generate reactive oxygen species (ROS) to neutralize threats [450, 451]. Monocytes migrate to the injury site under the influence of monocyte chemoattractant protein-1 (MCP-1/CCL2) and differentiate into macrophages [452]. M1 macrophages produce IL-1, IL-6, and TNF-α, sustaining inflammation and promoting debris clearance, while M2 macrophages secrete IL-10 and TGF-β, resolving inflammation and supporting tissue repair. The transition from M1 to M2 macrophages is crucial for shifting from inflammation to healing, regulated by transcription factors such as NF-κB and STAT3 [453].
Key chemical mediators fine-tune these responses. Histamine facilitates vasodilation and immune cell influx, aiding debris clearance and early healing [454]. Bradykinin increases vascular permeability and nociceptor sensitivity, amplifying pain signaling and the inflammatory response. PGE2 enhances vasodilation and immune cell recruitment while modulating immune responses to transition from acute inflammation to tissue repair [455]. Leukotrienes, synthesized via the lipoxygenase pathway, act as potent neutrophil chemoattractants, while NO, produced by endothelial (eNOS) and inducible nitric oxide synthase (iNOS), promotes vasodilation and antimicrobial defense [456, 457].
At the molecular level, inflammation is tightly regulated by interconnected pathways. The NF-κB pathway is central to the transcriptional control of pro-inflammatory cytokines and adhesion molecules, governing leukocyte recruitment and activation. The MAPK and JAK-STAT pathways transduce cytokine and growth factor signals, activating genes involved in inflammation, cell proliferation, and survival [458, 459]. These pathways ensure the inflammatory response transitions efficiently into the proliferative phase, facilitating tissue repair. Understanding these molecular mechanisms provides insight into therapeutic targets for modulating inflammation and accelerating recovery.
In the early phase of healing, the immune system releases IL-1 and TNF-α to clear debris and prevent further injury [460]. Rehabilitation strategies aim to minimize excessive inflammation while preserving muscle activation and circulation. Gentle range-of-motion exercises and isometric contractions help stimulate blood flow, reduce stiffness, and prevent atrophy, balancing inflammatory responses and promoting efficient healing [461].
A randomized controlled trial [462] evaluated early-stage interventions for acute lateral ankle sprains. Patients who performed gentle dorsiflexion and plantarflexion exercises within 72 hours of injury demonstrated faster resolution of swelling, decreased pain, and improved proprioception compared to a control group receiving only immobilization. These findings support the idea that mild mechanical stimulation modulates local inflammation and helps prevent the deleterious effects of disuse, aligning with the molecular premise of curbing excessive cytokine-mediated tissue breakdown.
Mild loading in the inflammation stage can help regulate the expression of matrix metalloproteinases (MMPs), ensuring that collagen degradation does not outpace repair [463]. Early motion also encourages nutrient delivery to injured tissues, aiding the clearance of inflammatory byproducts.

2. Fibroblastic Stage
The fibroblastic stage follows the initial inflammatory response and involves the activation and proliferation of fibroblasts, which synthesize and organize extracellular matrix (ECM) components necessary for tissue repair [464]. Key growth factors, including Transforming Growth Factor-beta 1 (TGF-β1), Bone Morphogenetic Proteins (BMPs), and Connective Tissue Growth Factor (CTGF), play essential roles. TGF-β1 binds to TGF-β receptors (TGF-βRI and TGF-βRII) on fibroblasts, initiating the phosphorylation of Smad2/3 proteins [465]. These phosphorylated Smads form complexes with Smad4, translocating to the nucleus to regulate ECM gene transcription, promoting ECM synthesis and fibroblast proliferation [466].
TGF-β1 enhances type I and III collagen, fibronectin, and integrin production, stabilizing the ECM [468]. It suppresses matrix metalloproteinases (MMPs) to prevent premature ECM degradation, supporting robust tissue repair [469]. BMPs, binding to BMPR-I and BMPR-II, activate Smad1/5/8 proteins, which complex with Smad4 to promote fibroblast differentiation and ECM production [470]. BMPs drive fibroblast-to-myofibroblast differentiation, a key process in wound contraction and matrix organization [471]. BMP signaling via Smad1/5/8 regulates collagen synthesis, with myofibroblasts expressing alpha-smooth muscle actin (α-SMA) for mechanical tissue stabilization [473].
CTGF interacts with integrins and heparan sulfate proteoglycans, activating downstream MAPK/ERK signaling pathways to promote fibroblast proliferation, migration, and ECM synthesis [474]. This pathway upregulates genes controlling fibroblast adhesion, migration, and matrix remodeling [475]. CTGF enhances collagen, fibronectin, and proteoglycan synthesis, strengthening fibroblast-ECM interactions and optimizing structural recovery [476].
Fibroblast proliferation is essential for ECM production, regulated by the TGF-β/Smad, MAPK, and PI3K/Akt pathways [478]. Type I and III collagen synthesis forms a provisional matrix, initially arranged in a disorganized fashion, providing mechanical stability [479]. Over time, fibroblast-mediated mechanical forces attempt to align collagen fibers along tension lines, though this reorganization remains incomplete in scar tissue [480]. Lysyl oxidase (LOX) catalyzes collagen cross-linking, increasing ECM tensile strength and ensuring durability [482]. However, excessive fibroblast activity can lead to fibrosis, reducing tissue functionality and elasticity [481].
Other molecules, such as vascular endothelial growth factor (VEGF) and platelet-derived growth factor (PDGF), regulate fibroblast recruitment and angiogenesis, ensuring adequate oxygen and nutrient supply [485]. VEGF, via receptor tyrosine kinases, activates the PI3K/Akt and Ras/MAPK pathways, driving endothelial cell proliferation and vessel formation [487]. PDGF stimulates fibroblast migration and ECM synthesis, ensuring efficient tissue repair [486].
At the molecular level, the fibroblastic stage is governed by signaling pathways that coordinate fibroblast activity and ECM remodeling [488]. Regulation of these pathways ensures controlled collagen deposition and scar formation, improving clinical outcomes [489]. Understanding these molecular mechanisms enables targeted therapeutic strategies to enhance tissue repair while minimizing fibrosis.
Once inflammation subsides, fibroblasts and other repair cells proliferate and begin ECM synthesis, primarily collagen [490]. Growth factors such as TGF-β, VEGF, and insulin-like growth factor-1 (IGF-1) regulate angiogenesis, cell proliferation, and collagen deposition [491]. Controlled mechanical loading directs collagen fiber alignment, optimizing functional tissue recovery and reducing the risk of excessive scar formation.
In this study [492], patients recovering from partial Achilles tendon tears participated in a progressive loading protocol consisting of low-intensity resistance exercises and balance training. The intervention significantly increased local IGF-1 expression and improved collagen fiber alignment when compared to immobilization. Ultrasound imaging at 12 weeks showed better tissue echogenicity and organized fiber architecture in the intervention group, correlating with greater tensile strength.
Mechanical cues during the proliferation stage activate integrin-mediated signaling pathways (FAK, MAPK) in fibroblasts, stimulating collagen gene transcription and enhancing cross-link formation via enzymes such as lysyl oxidase [493]. These processes result in a more robust ECM scaffold capable of withstanding increasing loads.

3. Remodeling Stage
The remodeling stage enhances extracellular matrix (ECM) organization and mechanical properties through coordinated cellular, enzymatic, and signaling interactions [494]. Fibroblasts and myofibroblasts synthesize and remodel collagen, driving structural integrity. This stage is regulated by complex molecular pathways that control ECM turnover and tissue strengthening.
Collagen remodeling is central to this stage. Lysyl oxidase (LOX) catalyzes cross-linking between collagen molecules, enhancing ECM stability [495]. LOX modifies lysine residues, forming covalent bonds that reinforce ECM structure. Concurrently, matrix metalloproteinases (MMPs), particularly MMP-1 and MMP-9, degrade disorganized collagen, ensuring ECM homeostasis. Their activity is tightly regulated by tissue inhibitors of metalloproteinases (TIMPs) [496]. Myofibroblasts exert contractile forces that align collagen fibers along mechanical stress lines, improving tissue resilience [497]. This alignment is facilitated by integrin-mediated focal adhesion kinase (FAK) and RhoA/ROCK signaling, which drive cytoskeletal reorganization and ECM remodeling [498].
Fibroblasts and myofibroblasts continue ECM synthesis while generating contractile forces essential for tissue contraction and collagen fiber alignment [499]. Myofibroblast differentiation, regulated by Transforming Growth Factor-beta (TGF-β), activates SMAD and non-SMAD pathways, leading to α-SMA expression and enhanced matrix remodeling [500]. Integrin-mediated cell-ECM interactions activate intracellular signaling pathways, including MAPK/ERK and PI3K/Akt, promoting fibroblast survival, migration, and ECM production [501].
Key growth factors regulate ECM remodeling. TGF-β stimulates fibroblast proliferation, myofibroblast differentiation, and collagen synthesis, activating SMAD2/3 proteins that translocate to the nucleus and regulate ECM-related gene expression [502, 503]. TGF-β modulates MMP and TIMP expression, balancing ECM turnover. Connective Tissue Growth Factor (CTGF) enhances collagen synthesis and fibroblast adhesion, activating MAPK/ERK and PI3K/Akt pathways [504]. Platelet-Derived Growth Factor (PDGF) recruits fibroblasts and stimulates ECM production through receptor tyrosine kinases, activating Ras/MAPK and PI3K/Akt pathways [505].
ECM components such as fibronectin and elastin contribute to tissue stability. Fibronectin binds integrins, facilitating cell adhesion and migration, while elastin ensures resilience to mechanical stress. Elastin precursor tropoelastin undergoes LOX-mediated cross-linking to form stable elastic fibers [507]. Proteoglycans and glycosaminoglycans (GAGs) regulate ECM hydration, maintaining tissue viscoelasticity [508]. Their controlled production supports ECM integrity and functional recovery [509].
Excessive collagen synthesis can lead to fibrosis and tendon adhesions. Persistent fibroblast activation results in excessive ECM deposition, reducing elasticity and function [510]. Scar tissue, though mechanically supportive, lacks the biomechanical properties of native tissue, leading to impaired flexibility and function [511]. Continuous collagen deposition around tendons can cause adhesions, restricting mobility and necessitating therapeutic intervention [512].
Molecular pathways in adhesion formation involve inflammatory mediators. Chronic inflammation, driven by elevated IL-1 and TNF-α, activates NF-κB and JAK/STAT pathways, upregulating fibrotic genes and sustaining fibroblast and myofibroblast activity [514]. The fibroblast-to-myofibroblast transition, induced by TGF-β, enhances collagen production and ECM contraction, exacerbating adhesion formation [515]. Targeting TGF-β, MMP, and cytokine pathways may improve therapeutic interventions by reducing fibrosis and optimizing ECM remodeling [516].
Over time, tissue maturation involves ECM turnover, collagen realignment, and cross-linking to enhance mechanical properties [517]. MMPs regulate balanced ECM degradation, while progressive overload exercises align collagen fibers along stress lines, strengthening tissue and improving flexibility and function [518].
A prospective cohort study [519] followed athletes undergoing anterior cruciate ligament (ACL) reconstruction through a structured remodeling-phase program. Participants progressed from closed-chain exercises to plyometric drills and eventually to sport-specific agility training over four months. Biomechanical assessments and MRI evaluation revealed superior graft integrity, better neuromuscular control, and reduced re-injury rates in those who adhered to progressive loading principles compared to those in a less structured protocol.
Advanced loading protocols reinforce collagen cross-linking and ECM reorganization, partially mediated by signaling pathways such as SMAD (downstream of TGF-β) and NF-κB regulation of MMPs [520]. By carefully escalating mechanical demand, the tissue remodels efficiently without triggering excessive inflammatory or catabolic responses.

Blood Flow Restriction Training

Blood flow restriction (BFR) training offers a multidimensional molecular framework that can be harnessed in knee joint rehabilitation by driving controlled inflammation, hypoxia-mediated gene expression, anabolic hormone secretion, and fibrinolysis—all while preventing excessive mechanical stress on newly forming cartilage [521]. During low-load resistance exercises with partial venous occlusion, the localized hypoxia and metabolite accumulation trigger a cascade of intracellular signals that benefit not only skeletal muscle but also the cartilage matrix, tendons, and surrounding soft tissues [522].

Acute inflammation is induced by elevated shear stress upon reperfusion, which prompts the release of pro-inflammatory cytokines like interleukin-6 (IL-6) and tumor necrosis factor-alpha (TNF-α) [120]. IL-6 binds its IL-6R/gp130 receptor complex and activates Janus kinase (JAK1/JAK2/Tyk2), phosphorylating signal transducer and activator of transcription 3 (STAT3) [523]. Phosphorylated STAT3 translocates to the nucleus and induces genes that promote tissue repair, including processes relevant to chondroprogenitor cell recruitment. TNF-α, though harmful in chronic excess, can transiently assist cartilage healing by activating the IκB kinase (IKK) complex, freeing nuclear factor kappa B (NF-κB) to upregulate immune cell recruitment and debris clearance [524]. Critically, macrophage populations eventually shift toward an M2 anti-inflammatory state, supporting a more regenerative and less degradative environment conducive to cartilage matrix deposition and remodeling [525].

Simultaneously, reduced venous outflow stabilizes hypoxia-inducible factor-1 alpha (HIF-1α). Under sufficient hypoxia, prolyl hydroxylase domain (PHD) enzymes are inhibited, preventing HIF-1α degradation by the von Hippel–Lindau (VHL) pathway. Stabilized HIF-1α forms heterodimers with HIF-1β, binding hypoxia-responsive elements (HREs) to induce transcription of vascular endothelial growth factor (VEGF) and endothelial nitric oxide synthase (eNOS) [526]. VEGF promotes endothelial cell proliferation and vascular sprouting, potentially benefiting the subchondral bone region that supplies nutrients to the microfracture repair site. eNOS-derived nitric oxide (NO) augments local vasodilation, supporting blood flow within the vicinity of the forming cartilage. HIF-1α signaling is also central to chondrocyte viability in the low-oxygen niche of repaired cartilage, safeguarding ECM integrity while coordinating collagen and proteoglycan synthesis [527].

Metabolite buildup—particularly lactate and hydrogen ions—heightens sympathetic drive and fosters a systemic endocrine response that amplifies growth hormone (HGH) and insulin-like growth factor 1 (IGF-1) release. IGF-1, whether liver-derived or produced locally in skeletal muscle (e.g., mechano growth factor, MGF), binds the IGF-1 receptor to activate phosphoinositide 3-kinase (PI3K) and downstream AKT/mTORC1 signaling [528]. This pathway phosphorylates p70 S6 kinase 1 (S6K1) and eukaryotic initiation factor 4E-binding protein 1 (4E-BP1), unleashing cap-dependent translation of anabolic mRNAs [529]. For knee joint rehabilitation, the net effect is heightened protein synthesis in periarticular musculature as well as tendon and possibly joint tissues, reinforcing both skeletal support and the local environment in which new chondral tissue forms. IGF-1 further bolsters collagen gene transcription in tenocytes, which supports improved tendon stiffness—particularly valuable in joint stabilization during rehabilitation [530].

BFR preferentially recruits type II muscle fibers at loads well below those usually necessary for fast-twitch activation, an advantage for patients with arthrogenic muscle inhibition (AMI) or limited tolerance for high-intensity exercise [531] [532]. The hypoxic and metabolite-rich milieu triggers group IV afferent fibers, boosting central motor drive and promoting the contraction of higher-threshold motor units. For knee joint rehabilitation, preserving or increasing type II fiber mass helps stabilize the joint and stress-shielding from undue forces while allowing functional gains in strength [533].

Alongside these anabolic and immunomodulatory processes, partial occlusion stimulates tissue plasminogen activator (tPA) release from Weibel–Palade bodies in the vascular endothelium [534]. tPA catalyzes the conversion of plasminogen to plasmin, sustaining fibrinolysis and mitigating thrombotic concerns, while also degrading extraneous fibrin that could hinder nutrient diffusion or ECM organization in the microfracture site [535]. This safeguard, in concert with muscle contractions, maintains adequate fibrin turnover and limits the risk of deep vein thrombosis (DVT) [536].

Overall, BFR training weaves together transient inflammation, hypoxia-driven angiogenesis, robust anabolic signaling, and fibrinolytic activity to facilitate muscle and connective tissue repair while posing minimal mechanical stress on the knee joint [537]. This molecular synergy—manifested in cellular preservation, regulated macrophage activation, enhanced growth factor profiles, and stable clot remodeling—makes BFR an innovative option in designing comprehensive and effective rehabilitation programs for knee joint rehabilitation and performance [538] [539] [540].

Neuromuscular Fatigue and Recovery

The complexities of exercise-induced neuromuscular fatigue and recovery must be carefully considered, particularly when addressing different types of training such as explosive power exercises versus traditional strength training. While it's helpful to provide general recovery guidelines based on research, as Tim Gabbett's work [541] suggests, it is crucial to emphasize that recovery times are influenced by numerous factors, including the training modality, intensity, and volume. Generalized prescriptions can lead to misinterpretations and ineffective training plans.

A clear distinction should be made between different training parameters, such as velocity loss (VL), which significantly impacts both recovery time and supercompensation effects. Research indicates that a 10% velocity loss (VL10) threshold during resistance training results in similar total repetitions as a VL20 protocol, but VL10 induces faster recovery and potentially better supercompensation [542]. Trainers and therapists must be cautious, especially when applying eccentric training protocols, as these can induce earlier neuromuscular fatigue and more pronounced delayed onset muscle soreness (DOMS) compared to concentric training [543] [544] [545].

The central challenge is that protocols designed for concentric training often cannot be directly applied to eccentric exercises without resulting in excessive neuromuscular fatigue. Eccentric training generates greater central and peripheral fatigue, primarily through impaired excitation-contraction coupling, which requires adjusted parameters for effective results [546] [547]. For example, studies prescribing 12 repetitions per set in eccentric training may not account for this increased fatigue, leading to suboptimal recovery strategies.

In order to make significant progress in understanding neuromuscular fatigue, it is imperative that future studies define the specific training parameters under investigation. Currently, much of the literature tends to generalize recovery outcomes without sufficiently accounting for variations in training intensity, contraction types, and other critical factors [548] [549] [550].

For instance, studies on recovery rates between power and strength sessions highlight the importance of eccentric phases in inducing muscle damage and slower recovery times. Eccentric force and velocity, particularly in stretch-shortening cycle exercises, appear to contribute substantially to neuromuscular impairment [551]. While power-oriented sessions involving faster eccentric velocities and moderate loads can induce substantial mechanical stress, the recovery rates vary compared to heavier strength-oriented sessions. Both types of sessions can affect recovery, but the effects of eccentric phases during explosive exercises should be further explored [552] [553].

Trainers and therapists should take these variations into account, as generalized recovery times and protocols may not apply uniformly across different exercises, muscle groups, or training statuses. For instance, upper body muscles tend to sustain more damage and require longer recovery times than lower body muscles during eccentric exercise [552] [554], although recovery rates between traditional strength training for upper and lower body exercises appear similar [555] [556] [557].

Moreover, the balance between fatigue and potentiation or supercompensation is critical for optimizing training outcomes. While heavy loads and large exercise volumes can induce long-lasting neuromuscular fatigue, low-volume, high-intensity exercises may result in potentiation and enhanced performance, sometimes even after 24–48 hours [558]. These findings highlight the need for individualized approaches to both training and recovery, as supercompensation effects are often influenced by specific training volumes and intensities.

Lastly, the interpretation of subjective and objective recovery measures should be approached cautiously. While subjective recovery scores (e.g., PRS) can provide insight into an athlete’s perceived readiness, they may not always correlate with objective neuromuscular recovery markers [559] [560] [561] [562]. Trainers should use a combination of these measures to assess recovery status, ensuring a more comprehensive understanding of the athlete's neuromuscular readiness for the next session.

In summary, recovery from resistance training varies greatly depending on the specific training parameters, especially concerning eccentric versus concentric workloads [563]. To optimize both performance and recovery, trainers and therapists should consider adjusting protocols based on the specific demands of the training, rather than relying on generalized recovery guidelines [564].

In terms of future research and practical application, we must consider the cognitive load athletes experience during training. Fatigue and recovery are not just physical processes but involve significant cognitive dimensions [565]. Fatigue and recovery are not just about the physical demands placed on the body; they are multifaceted processes that require us to also consider the cognitive aspects of training. Research suggests that the anterior cingulate cortex (ACC) plays a key role in regulating attention, helping athletes maintain efficient activation of motor units even during fatigue [566]. This has a significant effect on both motor performance and likely also on recovery [567].

As we move forward, it is crucial to take into account the cognitive load placed on athletes during training tasks. The concept of motor-cognitive interference—how cognitive load impacts movement mechanics and efficiency, even more pronounced under circumstances of fatigue—must be integrated into our understanding of fatigue and recovery [568] [569]. Tasks involving more complex decision-making place higher cognitive demands, which can reduce movement efficiency and economy, ultimately increasing the load on the body and potentially extending recovery times [570].

The Influence of Loading History on Musculoskeletal Adaptations. Application to Structured Prevention Program

The mechanical environment that muscles, tendons, ligaments, and cartilage encounter profoundly influences their cellular and molecular responses [571] [572]. When these tissues are repeatedly challenged by physical exercise or rehabilitative protocols, they develop a protective adaptation often referred to as the repeated bout effect (RBE). This phenomenon is underpinned by intricate signaling cascades involving oxidative stress, inflammatory mediators, mechanotransduction pathways, and metabolic regulators [573] [574]. Moreover, the outcomes of these molecular processes are highly individualized, reflecting differences in loading history, genetic background, and the extent of prior tissue adaptation (Fig. 4 &5).

Fig. 4: The repeated bout effect describes how muscle damage is attenuated after multiple sessions of eccentric exercise. During an initial bout of unfamiliar eccentric contractions, muscle fibers experience microtears in the sarcomeres and disruption of the extracellular matrix. On the molecular level, the muscle damage triggers a pronounced inflammatory response, characterized by elevated pro-inflammatory cytokines and infiltration of neutrophils and macrophages. These immune cells help clear debris and release signals that activate satellite cells—muscle stem cells critical for fiber repair and growth. As the muscle adapts with repeated exposure, several protective and reparative processes are enhanced. First, the inflammatory response becomes more regulated, limiting excessive inflammation and tissue breakdown. Second, structural proteins such as titin, desmin, and nebulin are reinforced or reorganized, improving the muscle fiber’s cytoskeletal integrity and resilience to mechanical stress. Satellite cell activation and fusion also become more efficient, bolstering the myofiber’s capacity for repair and hypertrophy. Meanwhile, remodeling of the extracellular matrix—through changes in collagen deposition and modulation of matrix metalloproteinases—provides a sturdier scaffold for muscle tissue. Neural adaptations further contribute by optimizing motor unit recruitment and synchrony, and changes in muscle-tendon properties (e.g., improved stiffness and compliance) help distribute forces more evenly. Collectively, these molecular and structural modifications—spanning inflammation, cytoskeletal fortification, satellite cell activity, and neural coordination—culminate in reduced muscle damage, faster recovery, and less soreness following subsequent bouts of eccentric exercise. This phenomenon encapsulates the repeated bout effect, highlighting the body’s remarkable ability to adapt and protect itself against repeated mechanical stress.

Fig. 4

Fig. 4: The repeated bout effect describes how muscle damage is attenuated after multiple sessions of eccentric exercise. During an initial bout of unfamiliar eccentric contractions, muscle fibers experience microtears in the sarcomeres and disruption of the extracellular matrix. On the molecular level, the muscle damage triggers a pronounced inflammatory response, characterized by elevated pro-inflammatory cytokines and infiltration of neutrophils and macrophages. These immune cells help clear debris and release signals that activate satellite cells—muscle stem cells critical for fiber repair and growth. As the muscle adapts with repeated exposure, several protective and reparative processes are enhanced. First, the inflammatory response becomes more regulated, limiting excessive inflammation and tissue breakdown. Second, structural proteins such as titin, desmin, and nebulin are reinforced or reorganized, improving the muscle fiber’s cytoskeletal integrity and resilience to mechanical stress. Satellite cell activation and fusion also become more efficient, bolstering the myofiber’s capacity for repair and hypertrophy. Meanwhile, remodeling of the extracellular matrix—through changes in collagen deposition and modulation of matrix metalloproteinases—provides a sturdier scaffold for muscle tissue. Neural adaptations further contribute by optimizing motor unit recruitment and synchrony, and changes in muscle-tendon properties (e.g., improved stiffness and compliance) help distribute forces more evenly. Collectively, these molecular and structural modifications—spanning inflammation, cytoskeletal fortification, satellite cell activity, and neural coordination—culminate in reduced muscle damage, faster recovery, and less soreness following subsequent bouts of eccentric exercise. This phenomenon encapsulates the repeated bout effect, highlighting the body’s remarkable ability to adapt and protect itself against repeated mechanical stress.

Fig. 5: The figure illustrates a weekly microdosing plan around match days, where small, high-quality stimuli (e.g., ECC + plyo, high-force plyo, neuro-cognitive work, BFR strength, lumbopelvic control) are delivered frequently with long recovery gaps to cumulate adaptations while limiting fatigue. Mechanical stretching causes matrix deformation, which transmits force through integrins to the cytoskeleton, activates FAK/Src kinases, engages the MAPK cascade, and results in phosphorylated ERK. Phosphorylated ERK increases the expression of collagen genes such as COL1A1 and COL3A1 and upregulates post-translational regulators P4HA and LOX, enhancing collagen synthesis and maturation in the extracellular matrix. The programming principle for tendons and ligaments is to use short bouts of about 10 minutes at moderate force, separated by at least six hours of complete rest. Practically, one or two sessions per day—morning and evening—satisfy the spacing requirement, maximizing connective-tissue anabolism while minimizing overload risk. Abbreviations: MD, Match Day; MD±X, Day relative to the match (e.g., MD+3 = three days after MD; MD−2 = two days before); OFF, Rest day; AU, Arbitrary Units (training-load metric, often RPE × duration); ECC, Eccentric (eccentric muscle work); PLYO, Plyometrics; HFP, Horizontal Force Production (sprint emphasis); BFR, Blood Flow Restriction training; SS, Small-Sided (small-sided games/drills); Rondos, Keep-away/possession games in a small area; Prehab, Prehabilitation (injury-prevention work); CMJ, Countermovement Jump (power/monitoring test); TD, Total Distance (running volume); ACC/DEC, Acceleration/Deceleration exposures; PV, Peak Velocity (max sprint speed); UB, Upper Body; LPC, Lumbopelvic Control (trunk/hip stability); Neuro-cog, Neurocognitive (perception–decision–action tasks); Front-side mechanics, Sprint front-side mechanics (knee lift/thigh recovery); + / ++ / +++ and − / −−, Relative emphasis or magnitude (low/medium/high; reduced/very low).

Fig. 5

Fig. 5: The figure illustrates a weekly microdosing plan around match days, where small, high-quality stimuli (e.g., ECC + plyo, high-force plyo, neuro-cognitive work, BFR strength, lumbopelvic control) are delivered frequently with long recovery gaps to cumulate adaptations while limiting fatigue. Mechanical stretching causes matrix deformation, which transmits force through integrins to the cytoskeleton, activates FAK/Src kinases, engages the MAPK cascade, and results in phosphorylated ERK. Phosphorylated ERK increases the expression of collagen genes such as COL1A1 and COL3A1 and upregulates post-translational regulators P4HA and LOX, enhancing collagen synthesis and maturation in the extracellular matrix. The programming principle for tendons and ligaments is to use short bouts of about 10 minutes at moderate force, separated by at least six hours of complete rest. Practically, one or two sessions per day—morning and evening—satisfy the spacing requirement, maximizing connective-tissue anabolism while minimizing overload risk. Abbreviations: MD, Match Day; MD±X, Day relative to the match (e.g., MD+3 = three days after MD; MD−2 = two days before); OFF, Rest day; AU, Arbitrary Units (training-load metric, often RPE × duration); ECC, Eccentric (eccentric muscle work); PLYO, Plyometrics; HFP, Horizontal Force Production (sprint emphasis); BFR, Blood Flow Restriction training; SS, Small-Sided (small-sided games/drills); Rondos, Keep-away/possession games in a small area; Prehab, Prehabilitation (injury-prevention work); CMJ, Countermovement Jump (power/monitoring test); TD, Total Distance (running volume); ACC/DEC, Acceleration/Deceleration exposures; PV, Peak Velocity (max sprint speed); UB, Upper Body; LPC, Lumbopelvic Control (trunk/hip stability); Neuro-cog, Neurocognitive (perception–decision–action tasks); Front-side mechanics, Sprint front-side mechanics (knee lift/thigh recovery); + / ++ / +++ and − / −−, Relative emphasis or magnitude (low/medium/high; reduced/very low).

One of the key insights into muscle adaptation emerges from investigations into eccentric exercise. In a study exploring the RBE at the cellular level, it was found that initial eccentric contractions rapidly elevate reactive oxygen species (ROS) production, which serves as a signal to activate transcription factors involved in antioxidant defenses [575]. Central to this is the NRF2/KEAP1 axis: when intracellular ROS levels rise, NRF2 dissociates from KEAP1 and translocates to the nucleus, where it promotes the transcription of numerous antioxidant genes. The heightened oxidative defense established during this process diminishes muscle fiber damage in subsequent exercise bouts. Parallel proteomic analyses reveal the upregulation of FOXO3—an essential transcription factor for cellular stress responses—and heat shock proteins, which further enhance muscle resilience by stabilizing misfolded or damaged proteins. This suggests a form of molecular memory, where repeated exposure to mechanical and oxidative stress primes muscle fibers for future challenges, resulting in faster repair and improved functionality.

Table 7: The table outlines the healing stages of inflammation, fibroblastic, and remodeling, detailing cellular processes, biophysical characteristics, and therapeutic interventions. During inflammation, vasodilation and inflammatory cell invasion cause swelling and pain, treated with cryotherapy and NSAIDs; the fibroblastic stage involves growth factor-driven ECM synthesis, managed with manual therapy and therapeutic exercises; and the remodeling stage focuses on ECM organization and mechanical property enhancement, requiring tailored manual therapy and exercises to restore function and strength. Abbreviations: BMP, bone morphogenetic protein; CTGF, connective tissue growth factor; DOMS, delayed onset muscle soreness; ECM, extracellular matrix; ESWT, extracorporeal shock wave therapy; NSAIDs, non-steroidal anti-inflammatory drugs; PEMF, pulsed electromagnetic field therapy; BFR, blood flow restriciton; EPTE, percutaneous electrolysis therapy; PGE2, prostaglandin E2; ROM, range of motion; TGF-β1, transforming growth factor-β1

Table 7

Table 7: The table outlines the healing stages of inflammation, fibroblastic, and remodeling, detailing cellular processes, biophysical characteristics, and therapeutic interventions. During inflammation, vasodilation and inflammatory cell invasion cause swelling and pain, treated with cryotherapy and NSAIDs; the fibroblastic stage involves growth factor-driven ECM synthesis, managed with manual therapy and therapeutic exercises; and the remodeling stage focuses on ECM organization and mechanical property enhancement, requiring tailored manual therapy and exercises to restore function and strength. Abbreviations: BMP, bone morphogenetic protein; CTGF, connective tissue growth factor; DOMS, delayed onset muscle soreness; ECM, extracellular matrix; ESWT, extracorporeal shock wave therapy; NSAIDs, non-steroidal anti-inflammatory drugs; PEMF, pulsed electromagnetic field therapy; BFR, blood flow restriciton; EPTE, percutaneous electrolysis therapy; PGE2, prostaglandin E2; ROM, range of motion; TGF-β1, transforming growth factor-β1

Complementing these antioxidant adaptations are changes in muscle regenerative capacity. Exercise-induced muscle damage (EIMD) activates satellite cells that reside between the basal lamina and sarcolemma of muscle fibers [576]. These progenitor cells are governed by the Pax7/Myf5/MyoD pathway, a hierarchical network of myogenic regulatory factors crucial for muscle repair. Upon mechanical injury, Pax7+ satellite cells proliferate, then differentiate under the guidance of Myf5 and MyoD, ultimately fusing to damaged fibers or forming new fibers. Over time, repeated loading reduces the surge of pro-inflammatory cytokines such as IL-6 and TNF-α, while simultaneously stabilizing mitochondrial function through the PGC-1α signaling axis. PGC-1α co-activates genes involved in oxidative phosphorylation and mitochondrial biogenesis, fostering an environment where muscle cells recover more quickly and sustain higher workloads with fewer signs of damage.

Adaptations to repeated bouts of mechanical loading are not confined to muscle alone. In the knee joint, for instance, chondrocytes and fibroblasts respond to repeated compression or tensile forces with marked changes in gene expression and extracellular matrix (ECM) remodeling [577]. Integrin-FAK signaling is pivotal here: integrins on the cell surface detect mechanical distortion of the ECM, transmitting signals via focal adhesion kinase (FAK) to initiate cascades that increase the synthesis of collagen type II, lubricin, and other ECM components. Repeated activation of this pathway, however, leads to a refined homeostatic balance in subsequent loading sessions—initially, NF-κB drives a transient inflammatory response, but repeated exposure triggers upregulation of anti-inflammatory mediators like IL-10 and the inhibition of NF-κB. This modulatory capacity underscores the tissue’s ability to “learn” from prior mechanical stress, bolstering cartilage lubrication and reducing wear over time.

These joint-level adaptations are part of a broader systemic network of mechanotransduction. Mechanosensitive ion channels such as PIEZO1 and TRPV4 open in response to fluid shear or tensile strain, permitting calcium influx that activates downstream pathways relevant to both muscle hypertrophy and tendon/ligament remodeling [578]. In tandem, metabolic regulators such as AMPK, SIRT1, and PGC-1α promote adjustments in energy substrate utilization, mitochondrial density, and overall cellular endurance. This synergy of mechanical and metabolic signaling optimizes tissue remodeling, fosters adaptation to higher training loads, and minimizes catabolic or pro-inflammatory cascades that might otherwise impair recovery.

Intriguingly, the evolution of these molecular adaptations varies significantly among individuals. Personal training history, prior injuries, and even epigenetic modifications can influence how robustly these pathways are activated. Individuals with a long-standing background of consistent loading—be it through endurance sports, resistance training, or repetitive occupational tasks—often exhibit rapid upregulation of antioxidant and anti-inflammatory defenses, swiftly recalibrating the muscle’s or joint’s response to stress. Conversely, novices or those with limited conditioning may encounter more pronounced inflammatory and oxidative responses, necessitating a more cautious progression in exercise or rehabilitation to avoid injury.

Taken together, these molecular insights underscore the importance of systematically structured loading protocols that consider not only the intensity, volume, and frequency of exercise but also each individual’s unique loading history. When appropriately calibrated, repeated mechanical stresses harness the beneficial effects of the RBE, augmenting antioxidant capacity, enhancing tissue repair mechanisms, and refining inflammatory responses. Through these interconnected pathways, muscles, tendons, ligaments, and cartilage become progressively more resilient, a transformation that holds significant promise for both athletic performance and long-term joint health. Ultimately, by acknowledging and leveraging the body’s capacity for molecular adaptation, clinicians and strength and conditioning professionals can develop periodized training and rehabilitation programs that maximize recovery and minimize injury risk, tailored to the nuanced biochemical and biomechanical profiles of each individual.

Conclusion

This review highlights the impact of mechanical loading on the knee joint at the molecular and cellular levels, emphasizing the pathways and factors involved in cartilage maintenance, synovial fluid regulation, and structural integrity. By analyzing these mechanisms, the study provides a scientific foundation for developing precise rehabilitation programs that adapt loading conditions to individual patient needs. Beyond advancing knee joint biomechanics, the findings support the translation of mechanobiological insights into clinical practice, aiming to accelerate recovery, prevent overuse injuries, and improve therapeutic outcomes.

The analysis of mechanotransduction mechanisms in cartilage, synovium, ligaments, and tendons underscores the critical role of different loading modalities—compression, tension, shear, and hydrostatic pressure—in shaping tissue responses. Understanding how key cells, such as chondrocytes, synoviocytes, and fibroblasts, process mechanical stimuli through integrins, ion channels, and signaling pathways like MAPK, NF-κB, and Wnt, is essential for optimizing rehabilitation strategies.

A mechanobiology-driven approach to rehabilitation enables the personalization of therapeutic interventions, including controlled loading, exercise regimens, manual therapy, and biophysical stimulation. By integrating biomechanics with cellular biology, these strategies enhance tissue repair, restore joint function, and prevent further degeneration. Ultimately, this review establishes a comprehensive framework for improving knee joint health and optimizing rehabilitation outcomes, contributing to more effective and patient-centered musculoskeletal therapies.

Limitations

Despite the promising insights into how mechanical loading can be harnessed to optimize knee joint health, several limitations and considerations warrant attention. First, while in vitro and animal studies have shed light on molecular pathways, translating these findings to human clinical practice can be challenging due to inter-individual variability in genetics, comorbidities, and lifestyles. Mechanotransduction pathways are highly interconnected, and their responses can be influenced by factors such as inflammation, hormonal changes, and biomechanical compensation patterns. Consequently, a one-size-fits-all loading paradigm may overlook the nuanced ways in which individuals respond to different mechanical stimuli.

Second, the complexity of knee pathologies—ranging from degenerative osteoarthritis to acute ligament injuries—demands tailored approaches that consider not only mechanical but also biochemical and inflammatory contexts. Inconsistencies in the literature regarding optimal loading protocols underscore the need for robust, controlled clinical trials that can validate specific dosing regimens of exercise or physical therapy. Furthermore, while emerging technologies (e.g., wearable sensors, motion-capture systems) hold promise for monitoring joint mechanics in real-time, their cost, accessibility, and integration into standard clinical workflows remain practical hurdles.

Third, ensuring adherence to personalized rehabilitation programs can be difficult, especially given varying patient motivations, socioeconomic barriers, and differences in healthcare access. Long-term patient follow-up and engagement are critical for maintaining therapeutic gains, yet these aspects are often underreported or inconsistently addressed in current research. Lastly, mechanobiological interventions cannot be viewed in isolation; complementary strategies—such as nutritional support, pharmacological management of pain and inflammation, and psychosocial interventions—must be integrated for truly holistic musculoskeletal rehabilitation.

By recognizing these challenges and systematically addressing them in future research, clinicians and scientists can refine mechanobiology-driven protocols to achieve more reliable and generalized benefits for knee joint health. This critical perspective ensures that the field continues to evolve toward evidence-based, individualized rehabilitation practices that maximize therapeutic impact while minimizing risks.

References