Preservation of shortening velocity and power output in single muscle fibres from patients with idiopathic inflammatory myopathies

Idiopathic inflammatory myopathies (IIMs) are autoimmune disorders of skeletal muscle causing weakness and disability. Utilizing single fibre contractility studies, we have previously shown that contractility is affected in muscle fibres from individuals with IIMs. For the current study, we hypothesized that a compensatory increase in shortening velocity occurs in muscle fibres from individuals with IIMs in an effort to maintain power output. We performed in vitro single fibre contractility studies to assess force-velocity relationships and maximum shortening velocity (V_max) of muscle fibres from individuals with IIMs (25 type I and 58 type IIA) and healthy controls (66 type I and 27 type IIA) and calculated maximum power output (W_max) for each fibre. We found significantly higher V_max (mean ± SEM) of fibres from individuals with IIMs, for both type I (1.40 ± 0.31 fibre lengths/s, n = vs. 0.63 ± 0.13 fibre lengths/s; p = 0.0019) and type IIA fibres (2.00 ± 0.17 fibre lengths/s vs 0.77 ± 0.10 fibre lengths/s; p < 0.0001). Furthermore, W_max (mean ± SEM) was maintained compared to fibres from healthy controls, again for both type I and type IIA fibres (4.10 ± 1.00 kN/m²·fibre lengths/s vs. 2.00 ± 0.16 kN/m²·fibre lengths/s; p = ns and 9.00 ± 0.64 kN/m²·fibre lengths/s vs. 6.00 ± 0.67 kN/m²·fibre lengths/s; p = ns respectively). In addition, type I muscle fibres from individuals with IIMs was able to develop maximum power output at lower relative force. The findings of this study suggest that compensatory responses to maintain power output, including increased maximum shortening velocity and improved efficiency, may occur in muscle of individuals with IIMs. The mechanism underlying this response is unclear, and different hypotheses are discussed.

Transcriptomic profiles of muscular dystrophy with myositis (mdm) in extensor digitorum longus, psoas, and soleus muscles from mice

Background

Titinopathies are inherited muscular diseases triggered by genetic mutations in the titin gene. Muscular dystrophy with myositis (mdm) is one such disease caused by a LINE repeat insertion, leading to exon skipping and an 83-amino acid residue deletion in the N2A-PEVK region of mouse titin. This region has been implicated in a number of titin—titin ligand interactions, hence are important for myocyte signaling and health. Mice with this mdm mutation develop a severe and progressive muscle degeneration. The range of phenotypic differences observed in mdm mice shows that the deletion of this region induces a cascade of transcriptional changes extending to numerous signaling pathways affected by the titin filament. Previous research has focused on correlating phenotypic differences with muscle function in mdm mice. These studies have provided understanding of the downstream physiological effects resulting from the mdm mutation but only provide insights on processes that can be physiologically observed and measured. We used differential gene expression (DGE) to compare the transcriptomes of extensor digitorum longus (EDL), psoas and soleus muscles from wild-type and mdm mice to develop a deeper understand of these tissue-specific responses.

Results

The overall expression pattern observed shows a well-differentiated transcriptional signature in mdm muscles compared to wild type. Muscle-specific clusters observed within the mdm transcriptome highlight the level of variability of each muscle to the deletion. Differential gene expression and weighted gene co-expression network analysis showed a strong directional response in oxidative respiration-associated mitochondrial genes, which aligns with the poor shivering and non-shivering thermogenesis previously observed. Sln, which is a marker associated with shivering and non-shivering thermogenesis, showed the strongest expression change in fast-fibered muscles. No drastic changes in MYH expression levels were reported, which indicated an absence of major fiber-type switching events. Overall expression shifts in MYH isoforms, MARPs, and extracellular matrix associated genes demonstrated the transcriptional complexity associated with mdm mutation. The expression alterations in mitochondrial respiration and metabolism related genes in the mdm muscle dominated over other transcriptomic changes, and likely account for the late stage cellular responses in the mdm muscles.

Conclusions

We were able to demonstrate that the complex nature of mdm mutation extends beyond a simple rearrangement in titin gene. EDL, psoas and soleus exemplify unique response modes observed in skeletal muscles with mdm mutation. Our data also raises the possibility that failure to maintain proper energy homeostasis in mdm muscles may contribute to the pathogenesis of the degenerative phenotype in mdm mice. Understanding the full disease-causing molecular cascade is difficult using bulk RNA sequencing techniques due to intricate nature of the disease. The development of the mdm phenotype is temporally and spatially regulated, hence future studies should focus on single fiber level investigations.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08873-2.

Contributions of Titin and Collagen to Passive Stress in Muscles from mdm Mice with a Small Deletion in Titin’s Molecular Spring

Muscular dystrophy with myositis (mdm) is a naturally occurring mutation in the mouse Ttn gene that results in higher passive stress in muscle fibers and intact muscles compared to wild-type (WT). The goal of this study was to test whether alternative splicing of titin exons occurs in mdm muscles, which contain a small deletion in the N2A-PEVK regions of titin, and to test whether splicing changes are associated with an increase in titin-based passive tension. Although higher levels of collagen have been reported previously in mdm muscles, here we demonstrate alternative splicing of titin in mdm skeletal muscle fibers. We identified Z-band, PEVK, and C-terminus Mex5 exons as splicing hotspots in mdm titin using RNA sequencing data and further reported upregulation in ECM-associated genes. We also treated skinned mdm soleus fiber bundles with trypsin, trypsin + KCl, and trypsin + KCL + KI to degrade titin. The results showed that passive stress dropped significantly more after trypsin treatment in mdm fibers (11 ± 1.6 mN/mm²) than in WT fibers (4.8 ± 1 mN/mm²; p = 0.0004). The finding that treatment with trypsin reduces titin-based passive tension more in mdm than in WT fibers supports the hypothesis that exon splicing leads to the expression of a stiffer and shorter titin isoform in mdm fibers. After titin extraction by trypsin + KCl + KI, mdm fibers (6.7 ± 1.27 mN/mm²) had significantly higher collagen-based passive stress remaining than WT fibers (2.6 ± 1.3 mN/mm²; p = 0.0014). We conclude that both titin and collagen contribute to higher passive tension of mdm muscles.

Understanding skeletal muscle in cerebral palsy: a path to personalized medicine?

Until recently, there has been little interest in understanding the intrinsic features associated with the pathomorphology of skeletal muscle in cerebral palsy (CP). Coupled with emerging evidence that challenges the role of spasticity as a determinant of gross motor function and in the development of fixed muscle contractures, it has become increasingly important to further elucidate the underlying mechanisms responsible for muscle alterations in CP. This knowledge can help clinicians to understand and apply treatment modalities that take these aspects into account. Thus, the inherent heterogeneity of the CP phenotype allows for the potential of personalized medicine through the understanding of muscle pathomorphology on an individual basis and tailoring treatment approaches accordingly. This review aims to summarize recent developments in the understanding of CP muscle and their relationship to musculoskeletal manifestations, in addition to proposing a treatment paradigm that incorporates this new knowledge.

Functional and structural differences between skinned and intact muscle preparations

Myofilaments and their associated proteins, which together constitute the sarcomeres, provide the molecular-level basis for contractile function in all muscle types. In intact muscle, sarcomere-level contraction is strongly coupled to other cellular subsystems, in particular the sarcolemmal membrane. Skinned muscle preparations (where the sarcolemma has been removed or permeabilized) are an experimental system designed to probe contractile mechanisms independently of the sarcolemma. Over the last few decades, experiments performed using permeabilized preparations have been invaluable for clarifying the understanding of contractile mechanisms in both skeletal and cardiac muscle. Today, the technique is increasingly harnessed for preclinical and/or pharmacological studies that seek to understand how interventions will impact intact muscle contraction. In this context, intrinsic functional and structural differences between skinned and intact muscle pose a major interpretational challenge. This review first surveys measurements that highlight these differences in terms of the sarcomere structure, passive and active tension generation, and calcium dependence. We then highlight the main practical challenges and caveats faced by experimentalists seeking to emulate the physiological conditions of intact muscle. Gaining an awareness of these complexities is essential for putting experiments in due perspective.

Residual and passive force enhancement in skinned cardiac fibre bundles

In skeletal muscle, steady-state force is consistently greater following active stretch than during a purely isometric contraction at the same length (residual force enhancement; RFE). Similarly, when deactivated, the force remains higher following active stretch than following an isometric condition (passive force enhancement; PFE). RFE and PFE have been associated with the sarcomere protein titin, but skeletal and cardiac titin have different structures, and results regarding RFE in cardiac muscle have been inconsistent and contradictory. Therefore, the purpose of this study was to determine if cardiac muscle exhibits RFE and PFE. Skinned fibre bundles (n = 10) were activated isometrically at a sarcomere length of 2.2 μm and actively stretched by 15% of their length. The resultant active and passive forces were compared to the corresponding forces obtained for purely isometric contractions at the long length. RFE was observed in all fibre bundles, averaging 5.5 ± 2.5% (ranging from 2.3 to 9.4%). PFE was observed in nine of the ten bundles, averaging 11.1 ± 6.5% (ranging from -2.1 to 18.7%). Stiffness was not different between the active isometric and the force enhanced conditions, but was higher following deactivation from the force-enhanced compared to the isometric reference state. We conclude that there is RFE and PFE in cardiac muscle. We speculate that cardiac muscle has the same RFE capability as skeletal muscle, and that the most likely mechanism for the RFE and PFE is the engagement of a passive structural element during active stretching.

Sarcomere Lengths Become More Uniform Over Time in Intact Muscle-Tendon Unit During Isometric Contractions

The seemingly uniform striation pattern of skeletal muscles, quantified in terms of sarcomere lengths (SLs), is inherently non-uniform across all hierarchical levels. The SL non-uniformity theory has been used to explain the force creep in isometric contractions, force depression following shortening of activated muscle, and residual force enhancement following lengthening of activated muscle. Our understanding of sarcomere contraction dynamics has been derived primarily from in vitro experiments using regular bright-field light microscopy or laser diffraction techniques to measure striation/diffraction patterns in isolated muscle fibers or myofibrils. However, the collagenous extracellular matrices present around the muscle fibers, as well as the complex architecture in the whole muscles may lead to different contraction dynamics of sarcomeres than seen in the in vitro studies. Here, we used multi-photon excitation microscopy to visualize in situ individual sarcomeres in intact muscle tendon units (MTUs) of mouse tibialis anterior (TA), and quantified the temporal changes of SL distribution as a function of SLs in relaxed and maximally activated muscles for quasi-steady state, fixed-end isometric conditions. The corresponding muscle forces were simultaneously measured using a force transducer. We found that SL non-uniformity, quantified by the coefficient of variation (CV) of SLs, decreased at a rate of 1.9–3.1%/s in the activated muscles, but remained constant in the relaxed muscles. The force loss during the quasi-steady state likely did not play a role in the decrease of SL non-uniformity, as similar force losses were found in the activated and relaxed muscles, but the CV of SLs in the relaxed muscles underwent negligible change over time. We conclude that sarcomeres in the mid-belly of maximally contracting whole muscles constantly re-organize their lengths into a more uniform pattern over time. The molecular mechanisms accounting for SL non-uniformity appear to differ in active and passive muscles, and need further elucidation, as do the functional implications of the SL non-uniformity.

N2A Titin: Signaling Hub and Mechanical Switch in Skeletal Muscle

Since its belated discovery, our understanding of the giant protein titin has grown exponentially from its humble beginning as a sarcomeric scaffold to recent recognition of its critical mechanical and signaling functions in active muscle. One uniquely useful model to unravel titin’s functions, muscular dystrophy with myositis (mdm), arose spontaneously in mice as a transposon-like LINE repeat insertion that results in a small deletion in the N2A region of titin. This small deletion profoundly affects hypertrophic signaling and muscle mechanics, thereby providing insights into the function of this specific region and the consequences of its dysfunction. The impact of this mutation is profound, affecting diverse aspects of the phenotype including muscle mechanics, developmental hypertrophy, and thermoregulation. In this review, we explore accumulating evidence that points to the N2A region of titin as a dynamic “switch” that is critical for both mechanical and signaling functions in skeletal muscle. Calcium-dependent binding of N2A titin to actin filaments triggers a cascade of changes in titin that affect mechanical properties such as elastic energy storage and return, as well as hypertrophic signaling. The mdm phenotype also points to the existence of as yet unidentified signaling pathways for muscle hypertrophy and thermoregulation, likely involving titin’s PEVK region as well as the N2A signalosome.

The sarcomere force-length relationship in an intact muscle-tendon unit

The periodic striation pattern in skeletal muscle reflects the length of the basic contractile unit: the sarcomere. More than half a century ago, Gordon, Huxley and Julian provided strong support for the 'sliding filament' theory through experiments with single muscle fibres. The sarcomere force-length (FL) relationship has since been extrapolated to whole muscles in an attempt to unravel in vivo muscle function. However, these extrapolations were frequently associated with non-trivial assumptions, such as muscle length changes corresponding linearly to SL changes. Here, we determined the in situ sarcomere FL relationship in a whole muscle preparation by simultaneously measuring muscle force and individual SLs in an intact muscle-tendon unit (MTU) using state-of-the-art multi-photon excitation microscopy. We found that despite great SL non-uniformity, the mean value of SLs measured from a minute volume of the mid-belly, equivalent to about 5×10-6% of the total muscle volume, agrees well with the theoretically predicted FL relationship, but only if the precise contractile filament lengths are known, and if passive forces from parallel elastic components and activation-associated sarcomere shortening are considered properly. As SLs are not uniformly distributed across the whole muscle and changes in SL with muscle length are location dependent, our results may not be valid for the proximal or distal parts of the muscle. The approach described here, and our findings, may encourage future studies to determine the role of SL non-uniformity in influencing sarcomere FL properties in different muscles and for different locations within single muscles.

Effects of a titin mutation on force enhancement and force depression in mouse soleus muscles

The active isometric force produced by muscles varies with muscle length in accordance with the force-length relationship. Compared with isometric contractions at the same final length, force increases after active lengthening (force enhancement) and decreases after active shortening (force depression). In addition to cross-bridges, titin has been suggested to contribute to force enhancement and depression. Although titin is too compliant in passive muscles to contribute to active tension at short sarcomere lengths on the ascending limb and plateau of the force-length relationship, recent evidence suggests that activation increases titin stiffness. To test the hypothesis that titin plays a role in force enhancement and depression, we investigated isovelocity stretching and shortening in active and passive wild-type and mdm (muscular dystrophy with myositis) soleus muscles. Skeletal muscles from mdm mice have a small deletion in the N2A region of titin and show no increase in titin stiffness during active stretch. We found that: (1) force enhancement and depression were reduced in mdm soleus compared with wild-type muscles relative to passive force after stretch or shortening to the same final length; (2) force enhancement and force depression increased with amplitude of stretch across all activation levels in wild-type muscles; and (3) maximum shortening velocity of wild-type and mdm muscles estimated from isovelocity experiments was similar, although active stress was reduced in mdm compared with wild-type muscles. The results of this study suggest a role for titin in force enhancement and depression, which contribute importantly to muscle force during natural movements.

Hurry Up and Get Out of the Way! Exploring the Limits of Muscle-Based Latch Systems for Power Amplification

Animals can amplify the mechanical power output of their muscles as they jump to escape predators or strike to capture prey. One mechanism for amplification involves muscle-tendon unit (MT) systems in which a spring element (series elastic element [SEE]) is pre-stretched while held in place by a "latch" that prevents immediate transmission of muscle (or contractile element, CE) power to the load. In principle, this storage phase is followed by a triggered release of the latch, and elastic energy released from the SEE enables power amplification (PRATIO=PLOAD/PCE,max >1.0), whereby the peak power delivered from MT to the load exceeds the maximum power limit of the CE in isolation. Latches enable power amplification by increasing the muscle work generated during storage and reducing the duration over which that stored energy is released to power a movement. Previously described biological "latches" include: skeletal levers, anatomical triggers, accessory appendages, and even antagonist muscles. In fact, many species that rely on high-powered movements also have a large number of muscles arranged in antagonist pairs. Here, we examine whether a decaying antagonist force (e.g., from a muscle) could be useful as an active latch to achieve controlled energy transmission and modulate peak output power. We developed a computer model of a frog hindlimb driven by a compliant MT. We simulated MT power generated against an inertial load in the presence of an antagonist force "latch" (AFL) with relaxation time varying from very fast (10 ms) to very slow (1000 ms) to mirror physiological ranges of antagonist muscle. The fastest AFL produced power amplification (PRATIO=5.0) while the slowest AFL produced power attenuation (PRATIO=0.43). Notably, AFLs with relaxation times shorter than ∼300 ms also yielded greater power amplification (PRATIO>1.20) than the system driving the same inertial load using only an agonist MT without any AFL. Thus, animals that utilize a sufficiently fast relaxing AFL ought to be capable of achieving greater power output than systems confined to a single agonist MT tuned for maximum PRATIO against the same load.

Myasthenia gravis: Historical achievements and the "golden age" of clinical trials

Since the death of Chief Opechankanough >350 years ago, the myasthenia gravis (MG) community has gained extensive knowledge about MG and how to treat it. This review highlights key milestones in the history of treatment and discusses the current "golden age" of clinical trials. Although originally thought by many clinicians to be a disorder of hysteria and fluctuating weakness without observable cause, MG is one the most understood autoimmune neurologic disorders. However, studying it in clinical trials has been challenging due to the fluctuating nature of the medical condition which impacts MG clinical outcomes. Clinical trials must also account for the possibility of a placebo effect. Because MG is a rare incurable autoimmune disorder, it limits the number of potential patients available to participate in clinical trials. In the last 15 years, however, significant progress has been made with MG randomized clinical trials, resulting in a new drug (eculizumab) for physicians' treatment repertoire and an old technique (thymectomy) confirmed effective for MG. Some of the therapies (eg, thymectomy, corticosteroids, plasma exchange, and intravenous immunoglobulin [IVIg]) have survived the test of time. Others (eg, eculizumab and neonatal Fc receptor inhibitor) are novel and hold promise.

Optimal length, calcium sensitivity and twitch characteristics of skeletal muscles from mdm mice with a deletion in N2A titin

During isometric contractions, the optimal length of skeletal muscles increases with decreasing activation. The underlying mechanism for this phenomenon is thought to be linked to length dependence of Ca²⁺ sensitivity. Muscular dystrophy with myositis (mdm), a recessive titin mutation in mice, was used as a tool to study the role of titin in activation dependence of optimal length and length dependence of Ca²⁺ sensitivity. We measured the shift in optimal length between tetanic and twitch stimulation in mdm and wild-type muscles, and the length dependence of Ca²⁺ sensitivity at short and long sarcomere lengths in mdm and wild-type fiber bundles. The results indicate that the mdm mutation leads to a loss of activation dependence of optimal length without the expected change in length dependence of Ca²⁺ sensitivity, demonstrating that these properties are not linked, as previously suggested. Furthermore, mdm muscles produced maximum tetanic stress during sub-optimal filament overlap at lengths similar to twitch contractions in both genotypes, but the difference explains less than half of the observed reduction in active force of mdm muscles. Mdm muscles also exhibited increased electromechanical delay, contraction and relaxation times, and decreased rate of force development in twitch contractions. We conclude that the small deletion in titin associated with mdm in skeletal muscles alters force production, suggesting an important regulatory role for titin in active force production. The molecular mechanisms for titin's role in regulating muscle force production remain to be elucidated.

Myasthenia gravis

Myasthenia gravis (MG) is an autoimmune disease caused by antibodies against the acetylcholine receptor (AChR), muscle-specific kinase (MuSK) or other AChR-related proteins in the postsynaptic muscle membrane. Localized or general muscle weakness is the predominant symptom and is induced by the antibodies. Patients are grouped according to the presence of antibodies, symptoms, age at onset and thymus pathology. Diagnosis is straightforward in most patients with typical symptoms and a positive antibody test, although a detailed clinical and neurophysiological examination is important in antibody-negative patients. MG therapy should be ambitious and aim for clinical remission or only mild symptoms with near-normal function and quality of life. Treatment should be based on MG subgroup and includes symptomatic treatment using acetylcholinesterase inhibitors, thymectomy and immunotherapy. Intravenous immunoglobulin and plasma exchange are fast-acting treatments used for disease exacerbations, and intensive care is necessary during exacerbations with respiratory failure. Comorbidity is frequent, particularly in elderly patients. Active physical training should be encouraged.

Body stiffness and damping depend sensitively on the timing of muscle activation in lampreys

Unlike most manmade machines, animals move through their world using flexible bodies and appendages, which bend due to internal muscle and body forces, and also due to forces from the environment. Fishes in particular must cope with fluid dynamic forces that not only resist their overall swimming movements but also may have unsteady flow patterns, vortices, and turbulence, many of which occur more rapidly than what the nervous system can process. Has natural selection led to mechanical properties of fish bodies and their component tissues that can respond very quickly to environmental perturbations? Here, we focus on the mechanical properties of isolated muscle tissue and of the entire intact body in the silver lamprey, Ichthyomyzon unicuspis. We developed two modified work loop protocols to determine the effect of small perturbations on the whole body and on isolated segments of muscle as a function of muscle activation and phase within the swimming cycle. First, we examined how the mechanical properties of the whole lamprey body change depending on the timing of muscle activity. Relative to passive muscle, muscle activation can modulate the effective stiffness by about two-fold and modulate the effective damping by >10-fold depending on the activation phase. Next, we performed a standard work loop test on small sections of axial musculature while adding low-amplitude sinusoidal perturbations at specific frequencies. We modeled the data using a new system identification technique based on time-periodic system analysis and harmonic transfer functions (HTFs) and used the resulting models to predict muscle function under novel conditions. We found that the effective stiffness and damping of muscle varies during the swimming cycle, and that the timing of activation can alter both the magnitude and timing of peak stiffness and damping. Moreover, the response of the isolated muscle was highly nonlinear and length dependent, but the body's response was much more linear. We applied the resulting HTFs from our experiments to explore the effect of pairs of antagonistic muscles. The results suggest that when muscles work against each other as antagonists, the combined system has weaker nonlinearities than either muscle segment alone. Together, these results begin to provide an integrative understanding of how activation timing can tune the mechanical response properties of muscles, enabling fish to swim effectively in their complex and unpredictable environment.

On sarcomere length stability during isometric and post-active-stretch isometric contractions

Sarcomere length (SL) instability and SL non-uniformity have been used to explain fundamental properties of skeletal muscles, such as creep, force depression following active muscle shortening, and residual force enhancement following active stretching of muscles. Regarding residual force enhancement, it has been argued that active muscle stretching causes SL instability, thereby increasing SL non-uniformity. However, we recently showed that SL non-uniformity is not increased by active muscle stretching, but it remains unclear if SL stability is affected by active stretching. Here, we used single myofibrils of rabbit psoas and measured SL non-uniformity and SL instability during isometric contractions and for isometric contractions following active stretching at average SLs corresponding to the descending limb of the force-length relationship. We defined isometric contractions as contractions during which mean SL remained constant. SL instability was quantified by the rate of change of individual SLs over the course of steady state, isometric force; and SL non-uniformity was defined as deviations of SLs from the mean SL at an instant of time. We found that while the mean SL remained constant during isometric contraction, by definition, individual SLs did not. SLs were more stable in the force-enhanced, isometric state following active stretching compared to the isometric reference state. We also found that SL instability was not correlated with the rate of change of SL non-uniformity. Also, SL non-uniformity was not different in the isometric and the post-stretch isometric contractions. We conclude that since SL is more stable but similarly non-uniform in the force-enhanced compared to the corresponding isometric reference contraction, it appears unlikely that either SL instability or SL non-uniformity contribute to the residual force enhancement property of skeletal muscle.

Single sarcomere contraction dynamics in a whole muscle

The instantaneous sarcomere length (SL) is regarded as an important indicator of the functional properties of striated muscle. Previously, we found greater sarcomere elongations at the distal end compared to the mid-portion in the mouse tibialis anterior (TA) when the muscle was stretched passively. Here, we wanted to see if SL dispersions increase with activation, as has been observed in single myofibrils, and if SL dispersions differ for different locations in a muscle. Sarcomere lengths were measured at a mid- and a distal location of the TA in live mice using second harmonic generation imaging. Muscle force was measured using a tendon force transducer. We found that SL dispersions increased substantially from the passive to the active state, and were the same for the mid- and distal portions of TA. Sarcomere length non-uniformities within a segment of ~30 serial sarcomeres were up to 1.0 µm. We conclude from these findings that passive, mean SLs obtained from a single location are not necessarily representative of the distribution of SL in active muscle, and thus may be misinterpreted when deriving muscle mechanical properties, such as the force-length relationship. In view of these findings, it seems crucial to determine how SL distributions within a muscle relate to the most fundamental properties of muscle, such as the maximal isometric force.

Effects of a titin mutation on negative work during stretch-shortening cycles in skeletal muscles

Negative work occurs in muscles during braking movements such as downhill walking or landing after a jump. When performing negative work during stretch-shortening cycles, viscoelastic structures within muscles store energy during stretch, return a fraction of this energy during shortening, and dissipate the remaining energy as heat. Because tendons and extracellular matrix are relatively elastic rather than viscoelastic, energy is mainly dissipated by cross bridges and titin. Recent studies demonstrate that titin stiffness increases in active skeletal muscles, suggesting that titin contributions to negative work may have been underestimated in previous studies. The muscular dystrophy with myositis (mdm) mutation in mice results in a deletion in titin that leads to reduced titin stiffness in active muscle, providing an opportunity to investigate the contribution of titin to negative work in stretch-shortening cycles. Using the work loop technique, extensor digitorum longus and soleus muscles from mdm and wild type mice were stimulated during the stretch phase of stretch-shortening cycles to investigate negative work. The results demonstrate that, compared to wild type muscles, negative work is reduced in muscles from mdm mice. We suggest that changes in the viscoelastic properties of mdm titin reduce energy storage by muscles during stretch and energy dissipation during shortening. Maximum isometric stress is also reduced in muscles from mdm mice, possibly due to impaired transmission of cross bridge force, impaired cross bridge function, or both. Functionally, the reduction in negative work could lead to increased muscle damage during eccentric contractions that occur during braking movements.