Corresponding author: Kirill Schaaf Tel: +4922149825390 Fax: +4922149828370 E-mail INTRODUCTION
Human skeletal muscle is of vital importance for the maintenance of health and performance over the life span because it ensures metabolic and cognitive health, increases strength and reduces mortality [1,2]. One major factor for the age-associated loss in muscle mass is the general reduction of motor units. Particularly fast motor units are lost consequently reducing the number of fast type II fibers [3] which are important to maintain strength and reduce the risk of falling [4]. One mechanistic cause for the motoneuron loss is believed to be exerted by a reduced mitochondrial functionality which maintains the functional cellular environment of the motoric endplate [5]. A decline in mitochondrial quality control via mitophagy is one driver for this phenomenon [6]. However, endurance exercise (EE) but also a single bout of resistance exercise (RE) can promote the induction of mitophagy [7] emphasizing the potential of exercise for maintaining metabolic health. Indeed, the maintenance of motor units can be supported by lifelong physical activity e.g. running [8].
However, unlike RE, and although EE sustains motoneuron activity for extended time courses it does not effectively stimulate myofibrillar muscle protein synthesis (MPS) [9] to increase skeletal muscle mass and strength. Hence, mechanical tension induced by RE exceeds and exerts a yet undefined stimulation pattern on sarcomeres to substantially stimulate an adaptation pattern which cannot be induced by EE. The divergent adaptations elicited by resistance and endurance exercise originate from exercise-specific activation patterns of transcription factors and signaling pathways. These differences are encoded by distinct magnitudes and temporal profiles of mechanical, metabolic, and growth factor–dependent signals within the sarcomeric environment of actively recruited skeletal muscle fibers [10].
Due to its unique ability to act on muscle mass and strength, RE has emerged as a unique training tool which is becoming more and more popular [11].
As a consequence of the mass of scientific data, the incorporation of RE next to EE is advocated since 2020 in the WHO guidelines and now-adays considered as a lifelong training strategy for nearly all age groups [12]. Although being highly beneficial for increasing strength and muscle mass, skeletal muscle is subjected to a wide range of stressors during RE. Thereby, when compared with other exercise modalities, skeletal muscle is predominantly subjected to immense mechanical stress which constantly threatens its structural integrity [13]. In line with extensive prior work, our research group has shown in recent years, that one inevitable consequence of high-intensity RE is the induction of myofibrillar damage in skeletal muscle sarcomeres (Fig. 1A) [14–16]. This requires the repair and disposal of defective proteins as well as strategies to strengthen sarcomeric substructures [12,17]. In contrast to a vastly accumulated amount of data concerning the regulation of muscle protein synthesis after RE, much less is known about acute responses to muscle damage, mechanisms of skeletal muscle repair, the role of MPS in this process, and how these mechanisms build the invisible foundation for muscle growth and adaptation. In this narrative review we briefly summarize key signaling elements for the induction of muscle growth after RE but particularly highlight chaperone-mediated acute responses to muscle damage, mechanisms that adapt the skeletal muscle sarcomeric environment. We finally propose a working model for a less explored mechanism in the context of RE, linking RE-stimulated chaperone responses with muscle anabolism.
Fig. 1.
Fig. 1.(A) Mechanical tension during RE can cause damage within the myofibrillar apparatus. (B) This can manifest as Z-disk streaming and focal disruption of Z-disk–associated structures as early microstructural lesions. (C) Damaged regions show an acute lesion-centered repair response with accumulation of chaperones and proteostatic factors; BAG3 locally suppresses mTORC1 to prioritize turnover. Via chaperone-assisted selective autophagy (CASA), damaged clients (e.g., FLNC) are targeted for autophagic degradation, supporting structural repair while surrounding regions remain permissive for anabolic signaling. (D) Mechanical tension activates mTORC1-dependent anabolic signaling to drive muscle protein synthesis: mechanotransduction inputs (e.g., PLD/PA) converge on mTORC1–raptor, promoting phosphorylation of p70S6K and 4E-BP1 to enhance translation initiation. In addition, costameric signaling via FAK can activate p70S6K and thereby further reinforces mTORC1 signaling (including feedback phosphorylation at Ser2448). Concurrently, mTORC1-mediated ULK1 phosphorylation suppresses autophagy, biasing the response toward net anabolism. (E) Amino acid availability—particularly leucine uptake through LAT1—further stimulates mTORC1 activity and supports protein synthesis in human skeletal muscle. (F) Conversely, removal of mechanical loading (e.g., bedrest) rapidly promotes atrophy, in part through proteasomal degradation pathways, with increased MuRF1 and Atrogin-1 expression within a few days of unloading. (G) In the long term, initial bouts of RE induce damage, shifting the post-exercise response toward repair rather than net growth. With repeated loading, damage is reduced (repeated-bout effect), lowering repair demand and allowing more of the adaptive response to support myofibrillar accretion and hypertrophy. (H) These adaptations arise across mechanically coupled subsystems of the force-transmission network, including the myofibrillar Z-disk and intermediate filament system (and associated costameric/ECM interfaces).
1. Regulation of muscle anabolism after resistance exercise
The major molecular signal induced by RE to increase muscle mass is believed to be the mechanical stimulation of the sarcomeres within recruited skeletal muscle fibers [18]. Mechanical stimulation of skeletal muscle via gravity alone is so vital for its maintenance that once it is completely unloaded, e.g. via bedrest, skeletal muscle mass is rapidly reduced by proteasomal degradation of sarcomeric proteins. MURF-1 (Muscle RING-finger protein-1) as well as Atrogin 1 (Muscle atrophy F-box protein or FBXO32), two major components of the proteasomal degradation process of sarcomeric proteins and myogenic factors rapidly increase within 5 days of unloading (Fig. 1F) [19]. In contrast, increased skeletal muscle activity, particularly via means of RE, activates molecular signaling pathways that transiently elevate skeletal muscle protein synthesis [20] which in the long term culminates in muscle growth.
Protein synthesis in skeletal muscle is controlled by the mammalian target of rapamycin protein complex (mTOR). The fundamental role of mTOR activation for skeletal muscle growth has so far been extensively investigated and scientifically proven from animal to human skeletal muscle. In a landmark experiment it has been shown that the transient inhibition of mTORC-1 via rapamycin administration abolishes RE-induced protein synthesis in human skeletal muscle [21]. mTOR integrates mechanical stimulation, nutrient availability and energy status to adjust protein synthesis rates to varying cellular conditions within the cell [22]. Its activation is driven by different factors which we will briefly introduce here but particularly highlight the role of specific mechanically-induced mechanisms almost uniquely exerted by RE.
2. Growth factor and nutrient-dependent activation of mTOR signaling
The mTOR complex consists of mTORC-1 and mTORC-2 with distinct functions. mTORC-1 is sensitive to the energy status, nutrients, growth factors and mechanical stimulation in skeletal muscle and is the major regulator of protein synthesis. mTORC-1 associates with raptor (Regulatory-associated protein of mTOR) and other proteins to regulate the initiation of protein synthesis by phosphorylation of downstream proteins p70s6k (Ribosomal protein S6 kinase beta-1) and 4E-BP1 (Eukaryotic translation initiation factor 4E-binding protein 1). mTORC-1 also phosphorylates ULK1 (Unc-51 Like Kinase 1) to shut down autophagy, a major degradation pathway in cells (Fig. 1D) [23]. Thus, mTOR is also considered as a master switch between energy consuming cell growth and longevity-associated degradation processes [24]. Early findings assumed IGF-1 to be a major driver of muscle protein synthesis after resistance-exercise [25]. Upon IGF-1 and Insulin binding to the insulin receptor, PI3-kinase-dependent signaling phosphorylates and activates AKT (Protein kinase B) at Thr 308 which phosphorylates and inactivates TSC2 (tuberous sclerosis complex) relieving the inhibitory effects of TSC2 proteins on mTORC-1. Under such conditions mTOR becomes autophosphorylated at Ser 2481 and activates its own kinase activity. However, the growth factor-dependent phosphorylation and activation of AKT at Thr 308 is believed to be not a direct phosphorylation and activation event of mTORC-1 phosphorylation [26]. Upon activation, mTORC1 phosphorylates the downstream kinase p70s6k at Thr 389. P70s6kinase then enhances as positive feedback loop mTORC-1 phosphorylation at Ser2448 (Fig. 1D) [27].
mTORC-2 is associated with Rictor (Rapamycin-insensitive companion of mTOR) controls glucose uptake, is involved in the cytoskeleton regulation but also moderates the phosphorylation of AKT at Ser473 as positive feedback to fully activate AKT [28]. At least in mice, the constitutive activation of AKT has been shown to significantly increase muscle mass and mTORC-1 activation [29]. However, growth-factor dependent AKT signaling does not alone provide the molecular basis to explain RE-induced mTORC-1 activation. Moreover, animals lacking a functional IGF-1 receptor also showed overload-induced hypertrophy [30].
The uptake of amino acids, particularly the branched-chain amino acid leucine, via the large amino acid transporter 1 (LAT1) further stimulates protein synthesis in human skeletal muscle via mTORC-1 (Fig. 1E). mTOR activation is coordinated at the membranes of lysosomes. These organelles represent major protein degradation sites where the final steps of autophagy take place and amino acids are released. While under resting conditions mTORC-1 is localized in the sarcoplasm, increased leucine availability induces the recruitment of mTORC-1 to the lysosomal membrane where it is activated. This mechanism is facilitated by a complex interplay between Sestrin2 (SESN2) which senses leucine, RAG (RAS-related GTP-binding protein) GTPases which recruit mTORC1 to the membrane and RheB GTPases (Ras-homologue enriched in brain) which finally activates mTORC-1. Together with GA-TOR 1 and 2 (GTPase-Activating Protein Activity Toward Rags) as well as Raptor these GTPase proteins orchestrate the activation of mTOR [31]. Importantly, the administration of leucine alone increases LAT-1 expression in human skeletal muscle [32]. However, mechanical stimulation via RE alone and without leucine administration, significantly enhances the abundance and membrane-associated localization of amino acid transporters LAT1 (large amino acid transporter 1) in human skeletal muscle [33]. So far, several studies have determined that protein administration with or without additional leucine in combination with RE induces superior effects on the magnitude of muscle protein synthesis after RE than compared to leucine administration alone [34,35]. This illustrates the importance of contraction-induced mechanical signals enhancing the molecular events leading to muscle growth.
3. Mechanically stimulated activation of cellular events regulating protein synthesis
A single bout of RE rapidly activates mTOR-related and mechanosensitive signaling within human skeletal muscle fibers [36]. How exactly RE-induced mechanical stimulation triggers the activation of the mTOR pathway is yet under investigation and not entirely clear [18]. It has been proposed that mechanical stress directly phosphorylates p70s6k via focal adhesion kinases (FAK) and independently of mTOR (Fig. 1D) [37]. Mechanosensitive networks also localized to the force bearing Z-bands and at costameres are believed to sense and transduce mechanical tension and translate it into cellular signaling [38]. Mechanical stimulation of the sarcomere further stimulates phospholipase D (PLD) which facilitates the release of phosphatidic acid (PA). PA binds to the FRB domain of mTORC-1 contributing to mechanically-induced mTOR signaling (Fig. 1D) [39].
To further differentiate the molecular mechanisms differing between RE and EE-induced stimulation, a very recent research paper published in Nature Metabolism [40] revealed that RE communicates its unique signal also via selective phosphorylation of MAPKinaseKinases (MKKs) possessing a high sensitivity to mechanical stimulation. After resistance training specifically, MKK3b phosphorylation at Ser 218 (Mitogen-Activated Protein Kinase Kinase 3, isoform b) and MKK6 at Ser 207 (Mitogen-Activated protein kinase kinase 6) correlated with the magnitude of muscle protein synthesis. This signaling event was not observed after EE. These kinases have widespread effects on the activation of MYC (Myelocytomatosis oncogene) which regulates ribosome biogenesis but further p38 MAPK kinase (p38) and MK2 (MAPKAPK2; MAPK-activated protein kinase 2) both acting independently of mTOR. Interestingly, upon MKK3b phosphorylation and the subsequent activation of p38 and MK2, CRYAB (alpha-B crystallin) is predicted to be a substrate of MK2 [40]. Once phosphorylated at Ser 59 (CRYABSer59), CRYAB plays a crucial role in the acute cellular stress response and is tightly linked to damage prevention and protection across multiple cell types, including skeletal muscle (see following chapters). In line with this, we observed that CRYAB is highly sensitive towards RE-induced mechanical loading in human skeletal muscle as being rapidly phosphorylated at Ser 59 within 30 min post exercise [17,41].
Although the precise mechanism by which mechanical strain of skeletal muscle fibers is translated into increased protein synthesis is not clear, we will introduce in chapter 4 one important but overseen mechanism that regulates spatially coordinated damage resolution and mTORC-1 activation.
FROM RE-INDUCED MECHANICAL STRESS TO STRUCTURAL DISRUPTION AND SKELETAL MUSCLE ADAPTATION
High mechanical strain induced by RE constantly challenges sarcomeric structures to maintain proteostasis. Indeed, RE induces myofibrillar damage as an inevitable consequence of intense muscle contractions [42]. This phenomenon was observed as early as in 1971 [43] and the proteostatic mechanisms involved emerged meanwhile as its own area of research [11]. Although involving intricate and yet not fully understood mechanisms, the molecular damage control has obviously a considerable impact for practical training approaches aiming for skeletal muscle hypertrophy. Damas et al. [44] determined in a human study that hypertrophy only occurs when muscle damage is reduced. This indicates that damage is not per se a direct activator of muscle growth but instead has to be reduced. Therefore, on the way to muscle growth, molecular mechanisms seemingly have to pave the sarcomeric road by regulating recurrent muscle proteostasis and prepare the structural and molecular sarcomeric environment to and add muscle proteins on top of already stable structures. To arrange the protection, degradation, restoration and synthesis of sarcomeric proteins specific mechanisms have to be coordinated with high synchronicity, simultaneously and with high resolution in its spatial localization.
Thus, mechanical stress signaling and associated mechanisms may bridge the gap between damage and growth. In the following chapters we first describe which structural components of human skeletal muscle are most vulnerable to mechanically induced damage and how this damage can be defined. Next, we summarize the acute mechanoprotective responses that prevent damage and pave the way for longer-term cytoskeletal adaptation.
1. Microdamage and mechanotransduction
RE exerts a significant mechanical and metabolic stress on skeletal muscle fibers, inducing microstructural damage that requires highly coordinated molecular repair processes [11]. Some damaged regions within skeletal muscle fibers, show often highly localized regions with specific accumulation of sarcomeric proteins and proteostatic factors (Fig. 1C) also termed lesions [45]. Such microlesions are readily visualized as remarkable accumulations of a variety of proteins but most prominently of xin actin-binding repeat-containing proteins (XIRP), reflecting impaired mechanical integrity and disturbed lateral force transmission [45,46]. Such alterations initiate a cascade of proteostatic and cytoprotective responses that collectively determine whether the stimulus will ultimately promote adaptation.
Mechanically induced muscle damage can be viewed as a spectrum ranging from reversible, focal microlesions to obvious fiber injury. Within the myofibril, the Z-disk is a mechanically stressed area where tension and lateral force are integrated [47]. Consequently, protocols involving large eccentric strain often result in Z-disk streaming and focal disruption of Z-disk-associated structures (Fig. 1B) [48].
In parallel, the costameric axis (integrin-FAK-vinculin complexes) connects sarcomeres to the sarcolemma and the extracellular matrix [49]. Moreover, intermediate filaments, especially desmin, which links neighboring myofibrils, are essential to the mechanical integrity of the contractile apparatus [50]. Disorganization or perturbations to these connections can therefore impair lateral force transmission, even when there is no significant membrane rupture [51].
A third, partially independent component comprises sarcolemma microdisruptions [51]. These can be associated with transient defects in muscle contraction and may contribute to early strength loss following unfamiliar exercise [52].
One important practical implication is that “ damage” varies greatly across individuals, muscles, contraction modes, and training history. This variability complicates the interpretation of structural disruption based on indicators such as soreness alone and emphasizes the need for more specific markers, such as lesion-associated protein accumulations, to quantify microdamage in human biopsies.
Lesion markers, such as XIRP accumulation, provide a useful functional definition and quantification of microdamage (Fig. 1C) [45,46]. They highlight the specific locations where stress induced by mechanical strain has caused local structural disruptions which at those sites require targeted proteostasis [13]. Importantly, these subcellular locations are also where mechanotransduction occurs [53]. Therefore, microlesions should not be interpreted as merely “ breakage”, but rather as mechanically specific microenvironments that simultaneously recruit repair machinery and modulate signaling.
2. The Acute mechanoprotective responses to damage
In order to maintain contractile function during repeated bouts of mechanical strain induced by RE, skeletal muscle rapidly activates mechanoprotective programs that limit lesion expansion and initiate tissue repair. One of the earliest molecular responses is the post-translational modification and translocation of heat shock proteins (HSPs), including HSP70 and especially small heat shock proteins such as HSP27 (HSPB1), HSP22 (HSPB8) and αB-crystallin (CRYAB, HSPB5) (Fig. 1C) [41,54–56].
During high-intensity or unfamiliar RE, skeletal muscle fibers experience mechanical and metabolic stress, which leads to perturbations of the cytoskeleton and contractile apparatus [57]. In human skeletal muscle, HSPB1 and CRYAB translocated from the soluble to the cytoskeletal fraction within 1 hour post-exercise, indicating immediate stress on cytoskeletal structures even when overt myofibrillar disruption was minimal [41,54]. Mechanistically, both HSPB1 [58] and CRYAB [59] have been shown to form oligomers, which once phosphorylated disassemble into smaller species increasing their chaperone activity and enabling binding to mechanically strained clients.
As previously stated, a central pathway that coordinates selective protein turnover of these clients particularly Filamin C (FLNC) is the chaperone-assisted selective autophagy mechanism (CASA) [14]. It targets damaged FLNC towards autophagic degradation via a complex of BAG3, HSP70 and small HSPs like HSPB1 and HSPB8 (Fig. 1C) [60]. FLNC is a heterodimeric protein which anchors actin filaments in the z-disk [61], interacts with numerous other Z-disk proteins (e.g., alpha actinin [62] or myotilin [63]) and contributes to force transfer to the extracellular matrix [64]. CASA links FLNC damage or unfolding to its coordinated disposal via autophagy but also controls FLNC expression [65]. In human skeletal muscle we have shown that sarcomeric damage at the Z-disk is associated with the unfolding of FLNC and its rapid degradation via CASA in damaged skeletal muscle fibers [14]. We recently demonstrated that mechanical forces trigger the dephosphorylation of cochaperone BAG3, which promotes CASA complex assembly and recruits membrane-trafficking regulators that are required for efficient CASA execution [66]. Following activation, BAG3 recognizes the exposed WW-binding domain of unfolded FLNC at affected Z-disks and orchestrates the recruitment of further CASA proteins like HSPB8, p62 and HSC-70 [16] to degrade FLNC via autophagosomal incorporation and lysosomal degradation. We determined that acute and high intense eccentric but not low-intense RE induces significant Z-disk streaming and damage leading to FLNC degradation via CASA within 24 hours in human skeletal muscle [14]. CASA acts as essential mechanism to ensure the degradation of sarcomeric FLNC and to keep muscle functional [65]. BAG3 is of considerable importance, as mutations in BAG3 lead to early muscle dysfunction and mortality in patients [67].
Building on this lesion-centred CASA framework, a key question is how the fiber stabilizes mechanically stressed structures while this selective turnover is initiated. Small heat shock proteins provide precisely this rapid, phosphorylation-regulated layer of mechanoprotection by mobilizing to areas of cytoskeletal strain, binding affected proteins, and organizing the local chaperone environment in which CASA operates. In this context, CRYAB has emerged as a prominent early responder to RE [17,41,54].
Following RE, CRYAB is phosphorylated at Ser 59, with the extent of this phosphorylation being dependent on both the exercise intensity and the number of contractions performed [41]. While being largely soluble in resting muscle [68], following RE CRYAB translocates to the cytoskeletal structures surrounding, and especially into the damaged areas (Fig. 1) [41].
At the Z-disks, phosphorylated CRYAB binds to structural proteins, such as desmin [69], a muscle-specific intermediate filament, that connects neighboring myofibrils at the Z-disks, integrates organelles such as mitochondria and nuclei into the cytoskeletal system and establishes connections to the contractile apparatus [50]. This interaction has been demonstrated to stabilize the cytoskeleton during periods of stress, thereby preventing filament aggregation [70]. Its functional significance is underscored by human desmin- related myopathies caused by mutations that impair CRYAB-desmin binding [71].
Alongside CRYAB activation, our group recently demonstrated that desmin itself exhibits acute, load-dependent post-translational modulation [15]. Mechanical loading has been demonstrated to trigger dephosphorylation of desmin at specific residues. In untrained human muscle, a single bout of RE has been shown to induce a significant dephosphorylation of desmin at Ser 31, a site known to modulate filament stability [15]. This dephosphorylation appears to increase the structural resilience of desmin filaments by reducing their vulnerability to calpain-mediated cleavage [72]. Notably, this phosphor-switch is more evident in trained muscle, indicating an adaptive increase in the sensitivity of the cytoskeleton to mechanical loading [15].
Collectively, these acute responses indicate that mechanoprotection is achieved through a coordinated program that couples stabilization, selective turnover, and locally restricted signaling. Firstly, the stabilization of vulnerable filament systems is achieved through a phosphorylation-dependent cytoskeletal remodeling process, accompanied by the recruitment of small heat shock proteins (sHSPs) to regions of mechanical strain. The resulting chaperone layer is integrated with selective autophagy via CASA to remove the unfolded, damaged structures and enable the replacement of these areas with newly synthesized proteins. The ability of these processes to coexist is contingent upon the precise lesion-centered regulation of signaling. Local inhibition of mTORC1 at lesion sites enables autophagy-mediated degradation [16,73], while the surrounding cytoplasm remains competent for protein synthesis. This, in turn, sustains robust whole-muscle anabolic signaling alongside local proteostasis (Fig. 1D).
CYTOSKELETAL REMODELING AND STRUCTURAL ADAPTATIONS
Beyond acute mechanoprotective mechanisms, resistance training (RT) drives longer-term structural remodeling that increases the mechanical resilience of skeletal muscle. These adaptations occur across multiple mechanically coupled subsystems within the force-transmission network, ranging from the myofibrillar Z-disk and the intermediate filament network [74] to costameres [75] and the extracellular matrix [76] (Fig. 1H). Collectively, they reinforce force transmission and reduce the likelihood and impact of recurrent microlesions (Fig. 1G). In parallel, cellular remodeling such as satellite cell activation and myonuclear accretion expand transcriptional capacity and support sustained repair and growth during hypertrophic adaptation.
Longitudinal human RT studies have demonstrated remodeling of the Z-disk region and its associated scaffolding proteins. This remodeling can be interpreted as an architectural adaptation that improves mechanical stress distribution and reduces the susceptibility to focal Z-disk disruption during RE [77]. For instance, Z-disk-associated connecting elements such as FLNC [14], myotilin [78] and other Z-disk proteins [79], have been observed to increase in expression in response to repeated RE (Fig. 1H).
In the same paper where we have determined the acute degradation of FLNC after intense RE we also determined the increased expression and incorporation of FLNC in human skeletal muscle after 6 weeks of repeated RE [14]. It has to be noted, that this occurred particularly after progressive RE but not after constant RE where subjects trained with constant weight loading [14]. After already 2 weeks of RE carried our thrice per week, skeletal muscle damage was significantly reduced (Fig. 1G) [14].
This highlights on the one hand the importance of FLNC for the mechanical stability of the sarcomere but also the requirement for mechanically challenging contraction patterns for human skeletal muscle to initiate CASA and increased FLNC expression. In addition, remodeling responses have been demonstrated to show a proportional relationship with exercise intensity [41]. Furthermore, titin, which is anchored at the Z-disk and spans the sarcomere, acts as an adjustable molecular spring and mechanosensor [80]. Its kinase-based signaling axis links mechanical strain to pathways that regulate muscle gene expression and protein turnover, thereby contributing to sarcomeric remodeling [81]. Upon mechanical stimulation, titin kinase (TK) interacts with the zinc-finger protein nbr1, which in turn recruits p62/SQSTM1 to the sarcomere, where p62 links to the muscle-specific E3 ligase MuRF2 to regulate the serum response transcription factor (SRF) [82], a pathway implicated in hypertrophic remodeling [81]. During mechanical inactivity, MuRF2 translocates to the nucleus, reduces nuclear SRF, and thereby represses transcription [82].
While Z-disk scaffolds redistribute mechanical stress within the sarcomere, intermediate filaments provide a complementary stabilization layer by linking myofibrils and supporting lateral force transmission [50]. As outlined above, desmin is subject to acute, load-dependent post-translational regulation that modulates filament stability, as well as longer-term quantitative remodeling over the period of training. For instance, our group determined significant increases in desmin content following multiple RT bouts over 5 weeks [17]. This reinforcement has been demonstrated to enhance lateral force transmission and stabilize sarcomeres, reducing vulnerability to RE-induced damage. In support of this mechanoprotective model, our group observed coordinated regulation between desmin remodeling and acute chaperone-based protection reflected by CRYAB Ser59 phosphorylation dynamics across training, detraining, and re-exposure protocols [17]. Intermediate filament reinforcement has been observed to coincide with a progressive decrease in CRYAB phosphorylation, which is consistent with reduced damage during RE [17].
In contrast to desmin, the intermediate filament protein vimentin expression is typically low in mature myofibers but becomes prominent in remodeling-associated contexts. Vimentin levels are increased during regeneration [83] and mechanical overload [84], and recent proteomic analysis of human skeletal muscle demonstrated significantly increased sarcolemmal and extracellular-matrix-associated vimentin after 10 weeks of RT [84], supporting the idea that vimentin may participate in the remodeling environment accompanying hypertrophy. During the process of muscle regeneration, vimentin is observed to appear in satellite cells and nascent myotubes prior to desmin incorporation, thereby underscoring its role as an early structural scaffold in the process of cytoskeletal renewal [85]. Although there is evidence of a correlation between vimentin and mTORC1 signaling, suggesting the potential of vimentin to modulate anabolic signaling pathways associated with hypertrophy [86], the direct causal evidence in adult human myofibers during hypertrophy remains limited and should be interpreted with caution.
Structural reinforcement extends beyond the myofibril to the sarcolemma, where costameres connect sarcomeres to the ECM and act as key structural elements for both force transmission and mechanosensitive signaling [87]. Human loading studies have indicated that costameric anchors (integrin–talin–vinculin–FAK complexes) remodel in a load-responsive manner [75]. In young men, alterations in β1-integrin, meta-vinculin, and FAK were associated with loading status, thereby supporting the dynamic turnover of membrane anchoring complexes that couple myofibrils to the sarcolemma [75]. Complementary integrin-focused reviews further propose that α7β1-integrin signaling integrates mechanical load with anabolic control pathways, including mTORC1-related signaling, during hypertrophy [88]. Importantly, membrane-associated remodeling of integrin complexes occurs in parallel with reinforcement of the intermediate filament network, and together these coupled systems can enhance lateral force transmission and improve mechanical stability under high loads [89]. Collectively, these structural reinforcements support a model in which repeated loading strengthens the cytoskeleton– costamere–ECM continuum, consistent with the repeated bout effect in which functional deficits and stress-marker responses are attenuated after repeated exposure to a given stimulus.
FROM DAMAGE CONTROL TO HYPERTROPHIC REMODELING
Although structural reinforcements of the sarcomeric cytoskeleton contribute to enhanced stability of the sarcomere, myofibrillar repair proteostasis is always needed. It is highly likely that structural damage of skeletal muscle sarcomeres will occur permanently during training and daily use. However, exact data on that assumption are not available. However, we have determined (own observations, unpublished) that although the percentage of fibers containing lesions after RE is reduced about 30-40% when subjects performed continued RE for six weeks, many fibers still show lesions.
Under such conditions it becomes clear that muscle anabolism and degradation must occur at the same time and in dependency of the localization of damage.
We have demonstrated that BAG3 inhibits mTOR signaling in the damaged areas, creating a lesion-centered response that prioritizes turnover and repair of mechanically strained cytoskeletal proteins (Fig. 1C) [16]. Specifically, at sites of damage, TSC1 (Tuberous sclerosis complex) proteins recognize the WW binding site of BAG 3 which initiates binding of TSC1 to BAG3. This also initiates the TSC1-induced inhibition of mTOR at those sites while in the remaining sarcoplasm protein synthesis can be pursued [16]. Via such mechanisms, skeletal muscle can coordinate damage, repair and at the same time RE-induced muscle protein synthesis under recurring mechanical stress and by differentiating damaged and not damaged areas.
Force-induced unfolding of FLNC and the BAG3-dependent mTOR inhibition links also mechanical stress towards an emphasized protein synthesis response in skeletal muscle contributing to the multifactorial orchestration of muscle anabolism.
The structural reinforcement of sarcomeres through repeated mechanical loading has been demonstrated to be associated with functional benefits. Following an initial bout, subsequent bouts typically result in reduced muscle damage (Fig. 1G). This phenomenon is referred to as the repeated bout effect [90]. In parallel, as the architecture of the cytoskeleton is reinforced, markers of mechanical stress (e.g., p CRYAB Ser59, pJNK) are progressively blunted in adapted muscle despite comparable loading [74]. As a consequence, downstream anabolic signaling (e.g., mTORC1, p70S6K) also becomes less responsive over time [74]. A more straightforward interpretation is that the repeated bout effect minimizes structural disruption and cellular stress for a given workload, thereby reducing the “ repair tax” after each session and shifting the post-exercise response from predominantly repair-directed turnover to a greater net accretion of myofibrillar proteins as training progresses.
A central framework for understanding this transition has been articulated by Damas et al. [44], who emphasize that early RT is characterized by high disruption and broad remodeling demands, whereas measurable hypertrophy emerges more consistently only once indices of damage are attenuated. The central argument advanced here is not that damage is “ good” or “ required,” but rather, that the early post-exercise anabolic response is partly allocated to the repair and remodeling of stressed structures [44]. This allocation can mask or delay true hypertrophy during the initial weeks of training. Early increases in cross-sectional area may be confounded by edema-related swelling [91], while a substantial fraction of the post-exercise synthetic response is likely directed toward repair and remodeling rather than net accretion of contractile proteins [44].
As training progresses and indices of muscle damage attenuate (repeated bout effect), the acute magnitude of MPS and myofibrillar protein synthesis (MyoPS) and mTORC1-related phosphorylation often become smaller [92]. However, the relationship between synthesis and hypertrophy appears to become more strongly associated with longer resistance training [93]. This finding aligns with the hypothesis of a reduced “ repair tax” and an increased coupling of post-exercise synthesis to myofibrillar accretion.
Consistent with this interpretation, Damas et al. present evidence that training results in a more “ focused” synthesis response toward phenotype-relevant fractions (e.g., predominantly myofibrillar after RT), whereas untrained muscle exhibits a broader remodeling response that may include non-myofibrillar processes [92,94]. In alignment with the principles outlined in the specified allocation framework, tracer studies conducted by Wilkinson et al. [9] have indicated that untrained muscle exhibits a more extensive post-exercise synthesis response across protein fractions. However, following a period of RT, the response undergoes a shift toward myofibrillar synthesis, thereby substantiating a more pronounced coupling of post-exercise anabolism to contractile remodeling in trained muscle [9,92].
A major practical conclusion is that muscle damage is not a necessary mediator of hypertrophy. Hypertrophic gains are more consistently observed once damage markers decline, and protocols that cause less overt damage can still produce comparable hypertrophy over time [44].
Conceptually, this supports a two-track model in which damage control (stabilization, turnover, restoration of force transmission) and hypertrophic remodeling run in parallel [95] but shift in relative weighting across training. In the initial sessions, the primary focus is on repair, while progressive structural reinforcement enables a greater proportion of the anabolic response to support net growth (Fig. 1G).
1. Chaperones at the intersection between damage control and hypertrophy
In addition to its role as a cytosolic chaperone, recent evidence suggests that CRYAB may also fulfill an additional nuclear function. In a recent study [96] our group demonstrated that CRYAB localizes to the nuclei of human skeletal muscle fibers at rest, where it colocalizes with the transcriptional mediator Mothers against decapentaplegic homolog 4 (SMAD4), a key effector of Transforming growth factor β (TGF-β)/ myostatin signaling (Fig. 2) [97].
Fig. 2.
Fig. 2.Proposed nuclear role of CRYAB in modulating SMAD4-dependent myostatin/TGF-β signaling in human skeletal muscle: (A) At rest, within myonuclei, αB-crystallin (CRYAB) colocalizes with SMAD4, the central transcriptional mediator of basal myostatin/TGF-β signaling. This suggests a potential chaperone function in stabilizing or regulating SMAD4-containing transcriptional complexes, thereby contributing to the restraint of excessive anabolic activity. (B) Following acute resistance exercise, nuclear CRYAB content decreases while CRYAB phosphorylation at Ser59 increases, accompanied by reduced nuclear and cytosolic SMAD4. These changes suggest a transient disruption of the CRYAB–SMAD4 interaction, potentially promoting TIF1γ-mediated proteasomal degradation of SMAD4, thereby attenuating SMAD-dependent inhibitory signaling and facilitating anabolic programs. Concurrently, enhanced mTOR–rpS6 signaling further promotes the anabolic response.
While at rest nuclear SMAD4 acts as a central transcriptional mediator of basal TGF-β/myostatin signaling, contributing to the maintenance of muscle homeostasis by restraining excessive anabolic activity [98]. Under these conditions, the nuclear colocalization of CRYAB with SMAD4 suggests that αB-crystallin, by acting as a molecular chaperone within the nucleus, may participate in the stabilization or regulation of SMAD4-containing transcriptional complexes (Fig. 2A) [96,99].
Following acute RE, we observed reduced nuclear CRYAB content alongside increased phosphorylation at Ser59, accompanied by a decline in nuclear and cytosolic SMAD4 and a parallel activation of the mTOR and rpS6 pathways [96]. The observations reported here appear to be consistent with a transient disruption of the CRYAB–SMAD4 interaction following acute RE. This disruption enables the subsequent ubiquitination of SMAD4 via TIF1γ, which ultimately leads to its proteosomal degradation [96]. Such a mechanism could reduce SMAD-dependent inhibitory signaling and thereby permissively support anabolic programs, linking mechanical stress responses to transcriptional control of growth and atrophy (Fig. 2B). Given that SMAD4 serves as a shared cofactor for multiple TGF-β pathways, including myostatin associated atrophy signaling and BMP signaling, which is often linked to anabolic remodeling [98,100]. A stress- and phosphorylation-dependent redistribution of CRYAB could modulate the availability or trafficking of SMAD4, thereby biasing the transcriptional balance during the early post-exercise period.
Our working model is that acute mechanical stress triggers (I) the phosphorylation of cytosolic CRYAB and its recruitment to stabilize cytoskeletal clients and (II) a parallel reduction in the interaction between nuclear CRYAB and SMAD4, which transiently dampens SMAD-dependent inhibitory signaling. This creates a permissive environment for mTOR-driven translation and pro-growth transcriptional programs [96] while the structural repair machinery resolves microlesions.
CONCLUSION AND PRACTICAL IMPLICATIONS FOR RESISTANCE TRAINING DESIGN
Taken together, these findings establish CRYAB as a molecular switch that functions at the intersection of damage repair and growth regulation. Through its various subcellular functions and phosphorylation-dependent interactions, CRYAB integrates mechanical stress signals, proteostatic control, and anabolic signaling to orchestrate the fine balance between protection and adaptation. Further exploration of this interplay, including its relation to desmin remodeling, CASA machinery, and SMAD-mediated transcription, may ultimately refine our understanding of how resistance training shapes muscle plasticity at the molecular level.
Microlesions, which are frequently induced by RE, are the result of high mechanical loading. They are best conceptualized as local areas where mechanotransduction and proteostasis occur. Acute mechanoprotective programs contain these disruptions and preserve contractile function through rapid sHSP, recruitment, CASA-mediated turnover, and lesion-specific signaling (e.g., local mTORC1 suppression to permit autophagic processing), while maintaining anabolic signaling in less affected regions of the fiber.
With RT, the sarcomere-costamere-cytoskeleton continuum is structurally reinforced, reducing lesion burden and enabling a larger fraction of the post-exercise anabolic response to support hypertrophic remodeling. Defining how chaperones such as CRYAB coordinate cytoskeletal quality control with nuclear growth signaling will improve mechanistic models of how resistance training produces long-lasting muscle mass gains.
From an applied perspective, the mechanoprotective framework developed here challenges the idea that soreness or early strength loss are desirable indicators of an “ effective” hypertrophic stimulus. Rather, the repeated bout effect and progressive reinforcement of the force-transmission network imply that hypertrophy occurs when training promotes adaptation while minimizing excessive disruption that requires substantial repair. This supports the use of progressive overload strategies that allow tissue resilience to develop, such as gradual increases in volume or eccentric load. These strategies are particularly important for untrained individuals because early sessions are likely to bias post-exercise remodeling toward repair-directed turnover. In trained muscle, where the disruption burden is typically lower, maintaining sufficient mechanical tension and proximity to failure is more important for sustaining myofibrillar remodeling than maximizing damage intentionally. Finally, the lesion-centered model implies that, rather than relying on soreness alone, monitoring training progression should incorporate performance-based and structural-readiness indicators (e.g., recovery of force output and tolerance to repeated loading), as the same external workload can differ substantially in its underlying disruption burden across individuals and training status.
KEY MESSAGES
1. Hypertrophy does not require damage
Excessive muscle damage reallocates post-exercise protein synthesis toward repair and turnover rather than net accretion and subsequent growth.
2. Early resistance training is repair-dominated
In untrained muscle, mechanical strain strongly engages stress responses and proteostasis (chaperone activation and selective autophagy) alongside cytoskeletal remodeling, which can dominate the early post-exercise response.
3. Progressive loading reduces the “repair tax”
With repeated exposure, structural reinforcement of the cytoskeleton (including Z-disk and filament remodeling/expression) is accompanied by attenuated damage and a shift toward more efficient hypertrophic remodeling.
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