In Vivo Sarcomere Lengths Become More Non-uniform upon Activation in Intact Whole Muscle

Effect of pre-activation force on active force generation in skeletal muscle

Cross bridges play a central role in skeletal muscle force generation. The level of force per cross bridge and the number of attached cross bridges are thought to determine muscle performance. Recent studies propose the so-called myosin-activation hypothesis, which suggests that stress exerted on myosin filaments increases the number of attached cross bridges and, hence, active force. This study was aimed at investigating the influence of passive stress magnitude exerted at the onset of activation on active force in a whole muscle preparation. The tibialis anterior (TA) muscle-tendon unit (MTU) of mice (N = 8) was stretched uniaxially in situ to long lengths where substantial viscoelastic passive force relaxation occurs. Muscle stress upon activation was varied by activating the TA either immediately at the end of the passive stretch (high passive force), or following nearly complete passive force relaxation (low passive force). Total forces with and without activation were measured from every MTU. Active forces were calculated by subtracting the passive force relaxation curve from the total force measured over a 1.13-s activation. We found that active force generated by the TA at low passive stress was 5-13 % higher than that at high passive stress. While the results seem contradictory to the myosin-activation hypothesis, we speculate that the results arose either from length adjustments between muscle and tendon during passive force relaxation, from excessive lattice spacing compression, or from unfavourable alterations of myosin conformation by high passive stress. Further research is required to improve our understanding of active force generation under the influence of viscoelasticity of muscle and tendon.

Regional differences in amplitude and spatial homogeneity of muscle activity in the biceps femoris long head

PURPOSE

Hamstring injuries, particularly in the proximal Biceps femoris long head (BFlh), remain frequent in sports involving sprints and accelerations despite extensive research. Non-uniform muscle activity may contribute to these injuries by causing uneven load distribution. This study examines spatial homogeneity of muscle activity and amplitude in the proximal and distal BFlh at different knee flexion torque levels and muscle-tendon unit (MTU) lengths under controlled isometric conditions.

METHODS

Fifteen male recreational athletes performed unilateral isometric knee flexion contractions at three MTU lengths (0°, 45°, 90° hip flexion) and torque levels (30% MVC90, 60% MVC90, 90%MVC90) with high-density surface electromyography (HDsEMG) assessing proximal and distal activity.

RESULTS

The proximal BFlh exhibited lower spatial homogeneity and amplitude compared to the distal region across all conditions, with the largest homogeneity differences at lower torques and longer MTU lengths. Proximal homogeneity increased with torque and decreased with MTU length, while the distal region remained consistent. Amplitudes were lower proximally and decreased with MTU length in both regions.

CONCLUSION

The proximal-distal differences in spatial homogeneity and amplitude within the BFlh reflect non-uniform activation patterns along the BFlh and the proximal regions lower spatial homogeneity and amplitude of activation reflect non-uniform patterns, possibly contributing to injury risk.

Phenomenological Muscle Constitutive Model With Actin-Titin Binding for Simulating Active Stretching

The force produced by a muscle depends on its contractile history, yet human movement simulations typically employ muscle models that define the force-length relationship from measurements of fiber force during isometric contractions. In these muscle models, the total force-length curve can have a negative slope at fiber lengths greater than the fiber length at which peak isometric force is produced. This region of negative stiffness can cause numerical instability in simulations. Experiments have found that the steady-state force in a muscle fiber following active stretching is greater than the force produced during a purely isometric contraction. This behavior is called residual force enhancement. We present a constitutive model that exhibits force enhancement, implemented as a hyperelastic material in the febio finite element software. There is no consensus on the mechanisms responsible for force enhancement; we adopt the assumption that the passive fiber force depends on the sarcomere length at the instant that the muscle is activated above a threshold. We demonstrate the numerical stability of our model using an eigenvalue analysis and by simulating a muscle whose fibers are of different lengths. We then use a three-dimensional muscle geometry to verify the effect of force enhancement on the development of stress and the distribution of fiber lengths. Our proposed muscle material model is one of the few models available that exhibits force enhancement and is suitable for simulations of active lengthening. We provide our implementation in febio so that others can reproduce and extend our results.

Paraspinal muscle fibre structural and contractile characteristics demonstrate distinct irregularities in patients with spinal degeneration and deformity

BACKGROUND

Paraspinal and spinopelvic muscular dysfunction are hypothesized to be a causative factor for spinal degeneration and deformity; however, our fundamental understanding of paraspinal muscle (dys)function remains limited.

METHODS

Twelve surgical patients with spinal degeneration were recruited and categorized into group DEG (four patients) with no sagittal imbalance and no usage of compensatory mechanisms; group DEG-COMP (four patients) with no sagittal imbalance through use of compensatory mechanisms; and group DEG-COMP-UNBAL (four patients) with sagittal imbalance despite use of compensatory mechanisms. From each patient, four biopsies were collected from right and left multifidus (MULT) and longissimus (LONG) for single fibre contractile and structural measurements.

RESULTS

Eight of 48 (17%) biopsies did not exhibit any contractile properties. Specific force was not different between groups for the MULT (p = 0.47) but was greater in group DEG compared to group DEG-COMP-UNBAL for the LONG (p = 0.02). Force sarcomere-length properties were unusually variable both within and amongst patients in all groups. Thin filament (actin) lengths were in general shorter and more variable than published norms for human muscle.

CONCLUSION

This study is the first to show a heightened intrinsic contractile muscle disorder (i.e. impaired specific force generation) in patients with spinal degeneration who are sagittally imbalanced (compared to patients without deformity). Additionally, there are clear indications that patients with spinal degeneration (all groups) have intrinsic force sarcomere-length properties that are dysregulated. This provides important insight into the pathophysiology of muscle weakness in this patient group.

Finite Element Model of the Shoulder with Active Rotator Cuff Muscles: Application to Wheelchair Propulsion

The rotator cuff is prone to injury, remarkably so for manual wheelchair users. To understand its pathomechanisms, finite element models incorporating three-dimensional activated muscles are needed to predict soft tissue strains during given tasks. This study aimed to develop such a model to understand pathomechanisms associated with wheelchair propulsion. We developed an active muscle model associating a passive fiber-reinforced isotropic matrix with an activation law linking calcium ion concentration to tissue tension. This model was first evaluated against known physiological muscle behavior; then used to activate the rotator cuff during a wheelchair propulsion cycle. Here, experimental kinematics and electromyography data was used to drive a shoulder finite element model. Finally, we evaluated the importance of muscle activation by comparing the results of activated and non-activated rotator cuff muscles during both propulsion and isometric contractions. Qualitatively, the muscle constitutive law reasonably reproduced the classical Hill model force-length curve and the behavior of a transversally loaded muscle. During wheelchair propulsion, the deformation and fiber stretch of the supraspinatus muscle-tendon unit pointed towards the possibility for this tendon to develop tendinosis due to the multiaxial loading imposed by the kinematics of propulsion. Finally, differences in local stretch and positions of the lines of action between activated and non-activated models were only observed at activation levels higher than 30%. Our novel finite element model with active muscles is a promising tool for understanding the pathomechanisms of the rotator cuff for various dynamic tasks, especially those with high muscle activation levels.

The effects of muscle starting length on work loop power output of isolated mouse soleus and extensor digitorum longus muscle

Force-length relationships derived from isometric activations may not directly apply to muscle force production during dynamic contractions. As such, different muscle starting lengths between isometric and dynamic conditions could be required to achieve maximal force and power. Therefore, this study examined the effects of starting length [±5-10% of length corresponding to maximal twitch force (L0)] on work loop (WL) power output (PO), across a range of cycle frequencies, of the soleus (SOL) and extensor digitorum longus muscle (EDL; N=8-10) isolated from ∼8 week old C57 mice. Furthermore, passive work was examined at a fixed cycle frequency to determine the association of passive work and active net work. Starting length affected maximal WL PO of the SOL and EDL across evaluated cycle frequencies (P<0.030, ηp2>0.494). For the SOL, PO produced at -5% L0 was greater than that at most starting lengths (P<0.015, Cohen's d>0.6), except -10% L0 (P=0.135, d<0.4). However, PO produced at -10% L0 versus L0 did not differ (P=0.138, d=0.35-0.49), indicating -5% L0 is optimal for maximal SOL WL PO. For the EDL, WL PO produced at -10% L0 was lower than that at most starting lengths (P<0.032, d>1.08), except versus -5% L0 (P=0.124, d<0.97). PO produced at other starting lengths did not differ (P>0.163, d<1.04). For the SOL, higher passive work was associated with reduced PO (Spearman's r=0.709, P<0.001), but no relationship was observed between passive work and PO of the EDL (Pearson's r=0.191, r2=0.04, P=0.184). This study suggests that starting length should be optimised for both static and dynamic contractions and confirms that the force-length curve during dynamic contractions is muscle specific.

Gaining new understanding of sarcomere length non-uniformities in skeletal muscles

Sarcomere lengths are non-uniform on all structural levels of mammalian skeletal muscle. These non-uniformities have been associated with a variety of mechanical properties, including residual force enhancement and depression, creep, increased force capacity, and extension of the plateau of the force-length relationship. However, the nature of sarcomere length non-uniformities has not been explored systematically. The purpose of this study was to determine the properties of sarcomere length non-uniformities in active and passive muscle. Single myofibrils of rabbit psoas (n = 20; with 412 individual sarcomeres) were subjected to three activation/deactivation cycles and individual sarcomere lengths were measured at 4 passive and 3 active points during the activation/deactivation cycles. The myofibrils were divided into three groups based on their initial average sarcomere lengths: short, intermediate, and long average sarcomere lengths of 2.7, 3.2, and 3.6 µm. The primary results were that sarcomere length non-uniformities did not occur randomly but were governed by some structural and/or contractile properties of the sarcomeres and that sarcomere length non-uniformities increased when myofibrils went from the passive to the active state. We propose that the mechanisms that govern the systematic sarcomere lengths non-uniformities observed in active and passive myofibrils may be associated with the variable number of contractile proteins and the variable number and the adjustable stiffness of titin filaments in individual sarcomeres.

A Conceptual Exploration of Hamstring Muscle-Tendon Functioning during the Late-Swing Phase of Sprinting: The Importance of Evidence-Based Hamstring Training Frameworks

An eccentrically lengthening, energy-absorbing, brake-driven model of hamstring function during the late-swing phase of sprinting has been widely touted within the existing literature. In contrast, an isometrically contracting, spring-driven model of hamstring function has recently been proposed. This theory has gained substantial traction within the applied sporting world, influencing understandings of hamstring function while sprinting, as well as the development and adoption of certain types of hamstring-specific exercises. Across the animal kingdom, both spring- and motor-driven muscle-tendon unit (MTU) functioning are frequently observed, with both models of locomotive functioning commonly utilising some degree of active muscle lengthening to draw upon force enhancement mechanisms. However, a method to accurately assess hamstring muscle-tendon functioning when sprinting does not exist. Accordingly, the aims of this review article are three-fold: (1) to comprehensively explore current terminology, theories and models surrounding muscle-tendon functioning during locomotion, (2) to relate these models to potential hamstring function when sprinting by examining a variety of hamstring-specific research and (3) to highlight the importance of developing and utilising evidence-based frameworks to guide hamstring training in athletes required to sprint. Due to the intensity of movement, large musculotendinous stretches and high mechanical loads experienced in the hamstrings when sprinting, it is anticipated that the hamstring MTUs adopt a model of functioning that has some reliance upon active muscle lengthening and muscle actuators during this particular task. However, each individual hamstring MTU is expected to adopt various combinations of spring-, brake- and motor-driven functioning when sprinting, in accordance with their architectural arrangement and activation patterns. Muscle function is intricate and dependent upon complex interactions between musculoskeletal kinematics and kinetics, muscle activation patterns and the neuromechanical regulation of tensions and stiffness, and loads applied by the environment, among other important variables. Accordingly, hamstring function when sprinting is anticipated to be unique to this particular activity. It is therefore proposed that the adoption of hamstring-specific exercises should not be founded on unvalidated claims of replicating hamstring function when sprinting, as has been suggested in the literature. Adaptive benefits may potentially be derived from a range of hamstring-specific exercises that vary in the stimuli they provide. Therefore, a more rigorous approach is to select hamstring-specific exercises based on thoroughly constructed evidence-based frameworks surrounding the specific stimulus provided by the exercise, the accompanying adaptations elicited by the exercise, and the effects of these adaptations on hamstring functioning and injury risk mitigation when sprinting.

Impact of stretch on sarcomere length variability in isolated fully relaxed rat cardiac myocytes

The contractility of cardiac muscle is greatly affected by preload via the Frank-Starling mechanism (FSM). It is based on preload-dependent activation of sarcomeres-the elementary contractile units in muscle cells. Recent findings show a natural variability in sarcomere length (SL) in resting cardiomyocytes that, moreover, is altered in an actively contracting myocyte. SL variability may contribute to the FSM, but it remains unresolved whether the change in the SL variability is regulated by activation process per se or simply by changes in cell stretch, i.e., average SL. To separate the roles of activation and SL, we characterized SL variability in isolated, fully relaxed rat ventricular cardiomyocytes (n = 12) subjected to a longitudinal stretch with the carbon fiber (CF) technique. Each cell was tested in three states: without CF attachment (control, no preload), with CF attachment without stretch, and with CF attachment and ~ 10% stretch of initial SL. The cells were imaged by transmitted light microscopy to retrieve and analyze individual SL and SL variability off-line by multiple quantitative measures such as coefficient of variation or median absolute deviation. We found that CF attachment without stretch did not affect the extent of SL variability nor average SL. In stretched myocytes, the averaged SL significantly increased, while the SL variability remained unchanged. This result clearly indicates that the non-uniformity of individual SL is not sensitive to the average SL itself in fully relaxed myocytes. We conclude that SL variability per se does not contribute to the FSM in the heart.

Impact of stretch on sarcomere length variability in isolated fully relaxed rat cardiac myocytes

The contractility of cardiac muscle is greatly affected by preload via the Frank-Starling Mechanism (FSM). It is based on the preload-dependent activation of sarcomeres - the elementary contractile units in muscle cells. Recent findings show a natural variability in sarcomere length (SL) in resting cardiomyocytes that, moreover, is altered in an actively contracting myocyte. SL variability may contribute to the FSM but it remains unresolved whether the change in the SL variability is regulated by activation process per se or simply by changes in cell stretch, i.e. average SL. To separate the roles of activation and SL, we characterized SL variability in isolated fully relaxed rat ventricular cardiomyocytes ( n  = 12) subjected to a longitudinal stretch with the carbon fiber (CF) technique. Each cell was tested in three states: without CF attachment (control, no preload), with CF attachment without stretch, and with CF attachment and ~ 10% stretch of initial SL. The cells were imaged by transmitted light microscopy to retrieve and analyze individual SL and SL variability off-line by multiple quantitative measures like coefficient of variation or median absolute deviation. We found that CF attachment without stretch did not affect the extent of SL variability and averaged SL. In stretched myocytes, the averaged SL significantly increased while the SL variability remained unchanged. This result clearly indicates that the non-uniformity of individual SL is not sensitive to the average SL itself in fully relaxed myocytes. We conclude that SL variability per se does not contribute to the FSM in the heart.

Characterizing residual and passive force enhancements in cardiac myofibrils

Residual force enhancement (RFE), an increase in isometric force after active stretching of a muscle compared with the purely isometric force at the corresponding length, has been consistently observed throughout the structural hierarchy of skeletal muscle. Similar to RFE, passive force enhancement (PFE) is also observable in skeletal muscle and is defined as an increase in passive force when a muscle is deactivated after it has been actively stretched compared with the passive force following deactivation of a purely isometric contraction. These history-dependent properties have been investigated abundantly in skeletal muscle, but their presence in cardiac muscle remains unresolved and controversial. The purpose of this study was to investigate whether RFE and PFE exist in cardiac myofibrils and whether the magnitudes of RFE and PFE increase with increasing stretch magnitudes. Cardiac myofibrils were prepared from the left ventricles of New Zealand White rabbits, and the history-dependent properties were tested at three different final average sarcomere lengths (n = 8 for each), 1.8, 2, and 2.2 μm, while the stretch magnitude was kept at 0.2 μm/sarcomere. The same experiment was repeated with a final average sarcomere length of 2.2 μm and a stretching magnitude of 0.4 μm/sarcomere (n = 8). All 32 cardiac myofibrils exhibited increased forces after active stretching compared with the corresponding purely isometric reference conditions (p < 0.05). Furthermore, the magnitude of RFE was greater when myofibrils were stretched by 0.4 compared with 0.2 μm/sarcomere (p < 0.05). We conclude that, like in skeletal muscle, RFE and PFE are properties of cardiac myofibrils and are dependent on stretch magnitude.

Cardiomyocyte sarcomere length variability: Membrane fluorescence versus second harmonic generation myosin imaging

Sarcomere length (SL) and its variation along the myofibril strongly regulate integrated coordinated myocyte contraction. It is therefore important to obtain individual SL properties. Optical imaging by confocal fluorescence (for example, using ANEPPS) or transmitted light microscopy is often used for this purpose. However, this allows for the visualization of structures related to Z-disks only. In contrast, second-harmonic generation (SHG) microscopy visualizes A-band sarcomeric structures directly. Here, we compared averaged SL and its variability in isolated relaxed rat cardiomyocytes by imaging with ANEPPS and SHG. We found that SL variability, evaluated by several absolute and relative measures, is two times smaller using SHG vs. ANEPPS, while both optical methods give the same average (median) SL. We conclude that optical methods with similar optical spatial resolution provide valid estimations of average SL, but the use of SHG microscopy for visualization of sarcomeric A-bands may be the "gold standard" for evaluation of SL variability due to the absence of optical interference between the sarcomere center and non-sarcomeric structures. This contrasts with sarcomere edges where t-tubules may not consistently colocalize to Z-disks. The use of SHG microscopy instead of fluorescent imaging can be a prospective tool to map sarcomere variability both in vitro and in vivo conditions and to reveal its role in the functional behavior of living myocardium.

Linking muscle architecture and function in vivo: conceptual or methodological limitations?

Background

Despite the clear theoretical link between sarcomere arrangement and force production, the relationship between muscle architecture and function remain ambiguous in vivo.

Methods

We used two frequently used ultrasound-based approaches to assess the relationships between vastus lateralis architecture parameters obtained in three common conditions of muscle lengths and contractile states, and the mechanical output of the muscle in twenty-one healthy subjects. The relationship between outcomes obtained in different conditions were also examined. Muscle architecture was analysed in panoramic ultrasound scans at rest with the knee fully extended and in regular scans at an angle close to maximum force (60°), at rest and under maximum contraction. Isokinetic and isometric strength tests were used to estimate muscle force production at various fascicle velocities.

Results

Measurements of fascicle length, pennation angle and thickness obtained under different experimental conditions correlated moderately with each other (r = 0.40-.74). Fascicle length measured at 60° at rest correlated with force during high-velocity knee extension (r = 0.46 at 400° s^-1) and joint work during isokinetic knee extension (r = 0.44 at 200° s^-1 and r = 0.57 at 100° s^-1). Muscle thickness was related to maximum force for all measurement methods (r = 0.44-0.73). However, we found no significant correlations between fascicle length or pennation angle and any measures of muscle force or work. Most correlations between architecture and force were stronger when architecture was measured at rest close to optimal length.

Conclusion

These findings reflect methodological limitations of current approaches to measure fascicle length and pennation angle in vivo. They also highlight the limited value of static architecture measurements when reported in isolation or without direct experimental context.

Titin force in muscle cells alters lattice order, thick and thin filament protein formation

Skeletal muscle force production is increased at longer compared to shorter muscle lengths because of length-dependent priming of thick filament proteins in the contractile unit before contraction. Using small-angle X-ray diffraction in combination with a mouse model that specifically cleaves the stretch-sensitive titin protein, we found that titin cleavage diminished the length-dependent priming of the thick filament. Strikingly, a titin-sensitive, length-dependent priming was also present in thin filaments, which seems only possible via bridge proteins between thick and thin filaments in resting muscle, potentially myosin-binding protein C. We further show that these bridges can be forcibly ruptured via high-speed stretches. Our results advance a paradigm shift to the fundamental regulation of length-dependent priming, with titin as the key driver.

Contractile State Dependent Sarcomere Length Variability in Isolated Guinea-Pig Cardiomyocytes

Cardiomyocytes contract keeping their sarcomere length (SL) close to optimal values for force generation. Transmural heterogeneity in SL across the ventricular wall coordinates the contractility of the whole-ventricle. SL heterogeneity (variability) exists not only at the tissue (macroscale) level, but also presents at the level of a single cardiomyocyte (microscale level). However, transmural differences in intracellular SL variability and its possible dependence on the state of contraction (e.g. end-diastole or end-systole) have not been previously reported. In the present study, we studied three aspects of sarcomere-to-sarcomere variability in intact cardiomyocytes isolated from the left ventricle of healthy guinea-pig: 1) transmural differences in SL distribution between subepi- (EPI) and subendocardial (ENDO) cardiomyocytes; 2) the dependence of intracellular variability in SL upon the state of contraction; 3) local differences in SL variability, comparing SL distributions between central and peripheral regions within the cardiomyocyte. To characterize the intracellular variability of SL, we used different normality tests for the assessment of SL distributions, as well as nonparametric coefficients to quantify the variability. We found that individual SL values in the end-systolic state of contraction followed a normal distribution to a lesser extent as compared to the end-diastolic state of contraction (∼1.3-fold and ∼1.6-fold in ENDO and EPI, respectively). The relative and absolute coefficients of sarcomere-to-sarcomere variability in end-systolic SL were significantly greater (∼1.3-fold) as compared to end-diastolic SL. This was independent of both the transmural region across the left ventricle and the intracellular region within the cardiomyocyte. We conclude that the intracellular variability in SL, which exists in normal intact guinea-pig cardiomyocytes, is affected by the contractile state of the myocyte. This phenomenon may play a role in inter-sarcomere communication in the beating heart.

Ultrastructural and Molecular Development of the Myotendinous Junction Triggered by Stretching Prior to Resistance Exercise

The myotendinous junction (MTJ) is a highly specialized region of the locomotor apparatus. Here, we investigated the ultrastructural and molecular effects in the MTJ region after static stretching prior to the ladder-based resistance training. Thirty-two male, 60-day old Wistar rats were divided into four groups: Sedentary, Resistance Training, Stretching, and Stretching-Resistance Training. The gastrocnemius muscle was processed for transmission electron microscopy techniques and Western blot assay. We observed that the static stretching prior to the ladder-based resistance training increased the MTJ components, the fibroblast growth factor (FGF)-2 and FGF-6 protein expression. Also, we demonstrated the lower transforming growth factor expression and no difference in the lysyl oxidase expression after combined training. The MTJ alterations in response to combined training demonstrate adaptive mechanisms which can be used for the prescription or development of methods to reduce or prevent injuries in humans and promote the myotendinous interface benefit.

Variability of in vivo Sarcomere Length Measures in the Upper Limb Obtained With Second Harmonic Generation Microendoscopy

The lengths of a muscle’s sarcomeres are a primary determinant of its ability to contract and produce force. In addition, sarcomere length is a critical parameter that is required to make meaningful comparisons of both the force-generating and excursion capacities of different muscles. Until recently, in vivo sarcomere length data have been limited to invasive or intraoperative measurement techniques. With the advent of second harmonic generation microendoscopy, minimally invasive measures of sarcomere length can be made for the first time. This imaging technique expands our ability to study muscle adaptation due to changes in stimulus, use, or disease. However, due to past inability to measure sarcomeres outside of surgery or biopsy, little is known about the natural, anatomical variability in sarcomere length in living human subjects. To develop robust experimental protocols that ensure data provide accurate representations of a muscle’s sarcomere lengths, we sought to quantify experimental uncertainty associated with in vivo measures of sarcomere lengths. Specifically, we assessed the variability in sarcomere length measured (1) within a single image, along a muscle fiber, (2) across images captured within a single trial, across trials, and across days, as well as (3) across locations in the muscle using second harmonic generation in two upper limb muscles with different muscle architectures, functions, and sizes. Across all of our measures of variability we estimate that the magnitude of the uncertainty for in vivo sarcomere length is on the order of ∼0.25 μm. In the two upper limb muscles studied we found larger variability in sarcomere lengths within a single insertion than across locations. We also developed custom code to make measures of sarcomere length variability across a single fiber and determined that this codes’ accuracy is an order of magnitude smaller than our measurement uncertainty due to sarcomere variability. Together, our findings provide guidance for the development of robust experimental design and analysis of in vivo sarcomere lengths in the upper limb.

Synchrony of sarcomeric movement regulates left ventricular pump function in the in vivo beating mouse heart

Sarcomeric contraction in cardiomyocytes serves as the basis for the heart's pump functions. It has generally been considered that in cardiac muscle as well as in skeletal muscle, sarcomeres equally contribute to myofibrillar dynamics in myocytes at varying loads by producing similar levels of active and passive force. In the present study, we expressed α-actinin-AcGFP in Z-disks to analyze dynamic behaviors of sequentially connected individual sarcomeres along a myofibril in a left ventricular (LV) myocyte of the in vivo beating mouse heart. To quantify the magnitude of the contribution of individual sarcomeres to myofibrillar dynamics, we introduced the novel parameter "contribution index" (CI) to measure the synchrony in movements between a sarcomere and a myofibril (from -1 [complete asynchrony] to 1 [complete synchrony]). First, CI varied markedly between sarcomeres, with an average value of ∼0.3 during normal systole. Second, when the movements between adjacent sarcomeres were asynchronous (CI < 0), a sarcomere and the ones next to the adjacent sarcomeres and farther away moved in synchrony (CI > 0) along a myofibril. Third, when difference in LV pressure in diastole and systole (ΔLVP) was lowered to <10 mm Hg, diastolic sarcomere length increased. Under depressed conditions, the movements between adjacent sarcomeres were in marked asynchrony (CI, -0.3 to -0.4), and, as a result, average CI was linearly decreased in association with a decrease in ΔLVP. These findings suggest that in the left ventricle of the in vivo beating mouse heart, (1) sarcomeres heterogeneously contribute to myofibrillar dynamics due to an imbalance of active and passive force between neighboring sarcomeres, (2) the force imbalance is pronounced under depressed conditions coupled with a marked increase in passive force and the ensuing tug-of-war between sarcomeres, and (3) sarcomere synchrony via the distal intersarcomere interaction regulates the heart's pump function in coordination with myofibrillar contractility.

Muscle in Variable Gravity: "I Do Not Know Where I Am, But I Know What to Do"

Purpose: Fascicle and sarcomere lengths are important predictors of muscle mechanical performance. However, their regulation during stretch-shortening cycle (SSC) activities in usual and challenging conditions is poorly understood. In this study, we aimed to investigate muscle fascicle and sarcomere behavior during drop jumps (a common SSC activity) in conditions of variable gravity.

Methods: Fifteen volunteers performed repeated drop jumps in 1 g, hypo-gravity (0 to 1 g), and hyper-gravity (1 to 2 g) during a parabolic flight. Gastrocnemius medialis (GM) electromyographic activity and fascicle length (Lf) were measured at drop-off, ground contact (GC), minimum ankle joint angle (MAJ), and push-off. GM sarcomere number was estimated by dividing Lf, measured by ultrasound at rest, by published data on GM sarcomere length, and measured in vivo at the same joint angle. Changes in sarcomere length were estimated by dividing GM Lf in each jump phase by sarcomere number calculated individually. The sarcomere force-generating capacity in each jump phase was estimated from the sarcomere length-tension relationship previously reported in the literature.

Results: The results showed that, regardless of the gravity level, GM sarcomeres operated in the ascending portion of their length-tension relationship in all the jump phases. Interestingly, although in hypo-gravity and hyper-gravity during the braking phase (GC-MAJ) GM fascicles and sarcomeres experienced a stretch (as opposed to the quasi-isometric behavior in 1 g), at MAJ they reached similar lengths as in 1 g, allowing sarcomeres to develop about the 70% of their maximum force.

Conclusion: The observed fascicle behavior during drop jumping seems useful for anchoring the tendon, enabling storage of elastic energy and its release in the subsequent push-off phase for effectively re-bouncing in all gravity levels, suggesting that an innate neuromuscular wisdom enables to perform SSC movements also in challenging conditions.

Muscle and tendon adaptations to moderate load eccentric vs. concentric resistance exercise in young and older males

Resistance exercise training (RET) is well-known to counteract negative age-related changes in both muscle and tendon tissue. Traditional RET consists of both concentric (CON) and eccentric (ECC) contractions; nevertheless, isolated ECC contractions are metabolically less demanding and, thus, may be more suitable for older populations. However, whether submaximal (60% 1RM) CON or ECC contractions differ in their effectiveness is relatively unknown. Further, whether the time course of muscle and tendon adaptations differs to the above is also unknown. Therefore, this study aimed to establish the time course of muscle and tendon adaptations to submaximal CON and ECC RET. Twenty healthy young (24.5 ± 5.1 years) and 17 older males (68.1 ± 2.4 years) were randomly allocated to either isolated CON or ECC RET which took place 3/week for 8 weeks. Tendon biomechanical properties, muscle architecture and maximal voluntary contraction were assessed every 2 weeks and quadriceps muscle volume every 4 weeks. Positive changes in tendon Young's modulus were observed after 4 weeks in all groups after which adaptations in young males plateaued but continued to increase in older males, suggesting a dampened rate of adaptation with age. However, both CON and ECC resulted in similar overall changes in tendon Young's modulus, in all groups. Muscle hypertrophy and strength increases were similar between CON and ECC in all groups. However, pennation angle increases were greater in CON, and fascicle length changes were greater in ECC. Notably, muscle and tendon adaptations appeared to occur in synergy, presumably to maintain the efficacy of the muscle-tendon unit.

The Energy of Muscle Contraction. III. Kinetic Energy During Cyclic Contractions

During muscle contraction, chemical energy is converted to mechanical energy when ATP is hydrolysed during cross-bridge cycling. This mechanical energy is then distributed and stored in the tissue as the muscle deforms or is used to perform external work. We previously showed how energy is distributed through contracting muscle during fixed-end contractions; however, it is not clear how the distribution of tissue energy is altered by the kinetic energy of muscle mass during dynamic contractions. In this study we conducted simulations of a 3D continuum muscle model that accounts for tissue mass, as well as force-velocity effects, in which the muscle underwent sinusoidal work-loop contractions coupled with bursts of excitation. We found that increasing muscle size, and therefore mass, increased the kinetic energy per unit volume of the muscle. In addition to greater relative kinetic energy per cycle, relatively more energy was also stored in the aponeurosis, and less was stored in the base material, which represented the intra and extracellular tissue components apart from the myofibrils. These energy changes in larger muscles due to greater mass were associated lower mass-specific mechanical work output per cycle, and this reduction in mass-specific work was greatest for smaller initial pennation angles. When we compared the effects of mass on the model tissue behaviour to that of in situ muscle with added mass during comparable work-loop trials, we found that greater mass led to lower maximum and higher minimum acceleration in the longitudinal (x) direction near the middle of the muscle compared to at the non-fixed end, which indicates that greater mass contributes to tissue non-uniformity in whole muscle. These comparable results for the simulated and in situ muscle also show that this modelling framework behaves in ways that are consistent with experimental muscle. Overall, the results of this study highlight that muscle mass is an important determinant of whole muscle behaviour.

Sarcomere length of the vastus intermedius with the knee joint angle change

The force-length relation of the skeletal muscles is an important factor influencing the joint torque at a given joint angle. We aimed to clarify the relationship between the resting sarcomere length and knee joint angle in the vastus intermedius (VI) and to compare it with that of the vastus lateralis (VL). The left and right legs were fixed at knee joint angles of 0° and 90°, respectively, in seven cadavers (age at the time of death: 70-91 years). Muscle tissues were dissected by necropsy of the VL and the VI, and electron microscopy images were obtained to calculate the sarcomere length. At knee joint angles of 0° and 90°, the VL sarcomere length was 2.28 ± 0.49 μm and 2.30 ± 0.48 μm, respectively, and the VI sarcomere length was 2.19 ± 0.35 μm and 2.46 ± 0.53 μm, respectively, with a significant difference between the two (p = 0.028). The magnitude of sarcomere length changes with knee joint angle changes was significantly greater for the VI (0.27 ± 0.20 μm) than for the VL (0.02 ± 0.09 μm) (p = 0.009). Thus, knee joint angle changes may affect the passive and active tension produced by the VI more than those produced by the VL.

Sex-Related Differences After a Single Bout of Maximal Eccentric Exercise in Response to Acute Effects: A Systematic Review and Meta-analysis

Morawetz, D, Blank, C, Koller, A, Arvandi, M, Siebert, U, and Schobersberger, W. Sex-related differences after a single bout of maximal eccentric exercise in response to acute effects: a systematic review and meta-analysis. J Strength Cond Res 34(9): 2697-2707, 2020-The most prominent effects after unaccustomed eccentric exercise are muscle damage, muscle soreness, strength loss, and higher concentrations of muscle proteins in the plasma. The aim of this systematic review is to evaluate sex-related differences in these acute effects. A systematic literature search in MEDLINE following the PRISMA guidelines was performed. Inclusion criteria were the difference in absolute outcomes between sexes in eccentric muscle strength, strength loss after eccentric exercise, blood concentrations of creatine kinase (CK), and delayed onset muscle soreness (DOMS). Results for maximal eccentric torque and CK data were pooled using a random-effect meta-analysis. A meta-regression was conducted to explain heterogeneity. Based on the 23 included trials, men showed significantly higher absolute eccentric strength. No sex-related differences were detected when normalizing strength for body mass, cross-sectional area of the muscle, or fat-free mass. Women displayed a tendency toward greater relative strength loss immediately after exercise. The absolute CK concentrations of men were significantly higher after exercise-induced muscle damage. No significant difference was found between sexes in DOMS. Untrained men and women display similar responses in all measures of relative muscle strength and DOMS. Apart from the enzymatic activity after exercise and the levels of absolute eccentric torque, there is no evidence for sex-related differences immediately after eccentric exercise. Therefore, eccentric training might have the same impact on men and women. One potential sex difference with practical relevance would be the possible difference in fatigue pattern immediately after eccentric exercise.

The Time-Course of Changes in Muscle Mass, Architecture and Power During 6 Weeks of Plyometric Training

Purpose

To investigate the time-course of changes in knee-extensors muscle mass, architecture and function in response to plyometric training (PLT) performed on a novel training device, the Tramp-Trainer. This machine consists in a trampoline connected to an inclined sledge which allows the performance of repeated jumps while the subject is sitting on a chair.

Methods

Eight healthy males (173.6 ± 4.7 cm, 69.7 ± 13.5 kg, 25.3 ± 4.6 years) underwent 6 weeks of bilateral PLT on the tramp-trainer machine. Training was performed three times per week (between 120 and 150 bounces per session). Knee-extensor maximum voluntary torque (MVT) and power, quadriceps femoris (QF) volume (VOL), cross-sectional area from the 20% to the 60% of femur length and CSA_mean, together with vastus lateralis (VL) architecture (fascicle length, Lf, and pennation angle, PA) were assessed after 2, 4, and 6 weeks of PLT.

Results

All results are presented as changes versus baseline values. MVT increased by 17.8% (week 2, p < 0.001) and 22.2% (week 4, p < 0.01), respectively, and declined to 13.3% (p < 0.05) at week 6 of PLT. Power increased by 18.2% (week 4, p < 0.05) and 19.7% (week 6, p < 0.05). QF VOL increased by 4.7% (week 4, p < 0.05) and 5.8% (week 6, p < 0.01); VL VOL increased by 5.2%, (p < 0.05), 8.2%, (p < 0.01), and 9.6% (p < 0.05) at weeks 2, 4, and 6, respectively. An increase in Lf was detected already at wk 2 (2.2%, p < 0.05), with further increase at 4 and 6 weeks of PLT (4 and 4.4%, respectively, p < 0.01). PA increased by 5.8% (p < 0.05) at week 6. Significant positive correlations were found between CSA_mean and Power (R² = 0.46, p < 0.001) and between QF VOL and Power (R² = 0.44, p < 0.024).

Conclusions

PLT induced rapid increases in muscle volume, fascicle length, pennation angle, torque and power in healthy younger adults. Notably, changes in VL VOL and Lf were detectable already after 2 weeks, followed by increases in knee extensors VOL and power from week 4 of PLT. Since the increase in CSA_mean and QF VOL cannot fully explain the increment in muscle power, it is likely that other factors (such as adaptations in neural drive or tendon mechanical properties) may have contributed to such fucntional changes.

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.

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.

Current Understanding of Residual Force Enhancement: Cross-Bridge Component and Non-Cross-Bridge Component

Muscle contraction is initiated by the interaction between actin and myosin filaments. The sliding of actin filaments relative to myosin filaments is produced by cross-bridge cycling, which is governed by the theoretical framework of the cross-bridge theory. The cross-bridge theory explains well a number of mechanical responses, such as isometric and concentric contractions. However, some experimental observations cannot be explained with the cross-bridge theory; for example, the increased isometric force after eccentric contractions. The steady-state, isometric force after an eccentric contraction is greater than that attained in a purely isometric contraction at the same muscle length and same activation level. This well-acknowledged and universally observed property is referred to as residual force enhancement (rFE). Since rFE cannot be explained by the cross-bridge theory, alternative mechanisms for explaining this force response have been proposed. In this review, we introduce the basic concepts of sarcomere length non-uniformity and titin elasticity, which are the primary candidates that have been used for explaining rFE, and discuss unresolved problems regarding these mechanisms, and how to proceed with future experiments in this exciting area of research.

On the Shape of the Force-Velocity Relationship in Skeletal Muscles: The Linear, the Hyperbolic, and the Double-Hyperbolic

The shape of the force-velocity (F-V) relationship has important implications for different aspects of muscle physiology, such as muscle efficiency and fatigue, the understanding of the pathophysiology of several myopathies or the mechanisms of muscle contraction per se, and may be of relevance for other fields, such as the development of robotics and prosthetic applications featuring natural muscle-like properties. However, different opinions regarding the shape of the F-V relationship and the underlying mechanisms exist in the literature. In this review, we summarize relevant evidence on the shape of the F-V relationship obtained over the last century. Studies performed at multiple scales ranging from the sarcomere to the organism level have described the concentric F-V relationship as linear, hyperbolic or double-hyperbolic. While the F-V relationship has most frequently been described as a rectangular hyperbola, a large number of studies have found deviations from the hyperbolic function at both ends of the F-V relation. Indeed, current evidence suggests that the F-V relation in skeletal muscles follows a double-hyperbolic pattern, with a breakpoint located at very high forces/low velocities, which may be a direct consequence of the kinetic properties of myofilament cross-bridge formation. Deviations at low forces/high velocities, by contrast, may be related to a recently discovered, calcium-independent regulatory mechanism of muscle contraction, which may also explain the low metabolic cost of very fast muscle shortening contractions. Controversial results have also been reported regarding the eccentric F-V relationship, with studies in prepared muscle specimens suggesting that maximum eccentric force is substantially greater than isometric force, whereas in vivo studies in humans show only a modest increase, no change, or even a decrease in force in lengthening contractions. This review discusses possible reasons reported in the literature for these discrepant findings, including the testing procedures (familiarization, pre-load condition, and temperature) and a potential neural inhibition at higher lengthening velocities. Finally, some unresolved questions and recommendations for F-V testing in humans are reported at the end of this document.

Bouncing Back! Counteracting Muscle Aging With Plyometric Muscle Loading

The preservation of muscle power is crucial in aging for maintaining mobility and performing daily tasks. Resistance training involving high movement velocities represents a valid strategy to slow down the rate of sarcopenia, counteracting the loss of muscle mass and muscle power. Plyometric exercise may represent an effective training modality for increasing muscle power; however, its application in older populations has been sparingly investigated, as the high impact actions involved may reduce its feasibility for older individuals. By adopting a safer modality of plyometric training, we investigated if a 6-week plyometric training intervention could increase knee extensor muscle size, architecture, force and power in 14 young (YM, age = 25.4 ± 3.5 y; means ± SD) and nine older males (OM, age = 69.7 ± 3.4 y). Volunteers trained 3 times/week using a device similar to a leg press machine where the user was required to bounce against his body mass on a trampoline. Pre-to-post training changes in isometric maximum voluntary torque (MVT), leg extension power and vastus lateralis (VL) architecture were assessed. Muscle power increased in both groups (+27% OM -P < 0.001, 20% YM -P < 0.001), although the total external work performed during the training period was significantly lower for OM (i.e., ~-47%). Both groups showed significant increases in muscle thickness (MT) (+5.8 OM -P < 0.01 vs. +3.8% YM -P < 0.01), fascicle length (Lf) (+8% OM -P < 0.001 vs. +6% YM -P < 0.001), and pennation angle (PA) (+7.5% OM -P < 0.001 vs. +4.1% YM -P < 0.001). The current study shows that trampoline-based plyometric training is an effective intervention producing a rapid increase in muscle mass and power in both young and older individuals. The training modality used in this study seems to particularly benefit the older population, targeting the morphological and functional effects of sarcopenia in human muscle.

Different Segments within Vertebrate Muscles Can Operate on Different Regions of Their Force-Length Relationships

To relate in vivo behavior of fascicle segments within a muscle to their in vitro force-length relationships, we examined the strain behavior of paired segments within each of three vertebrate muscles. After determining in vivo muscle activity patterns and length changes of in-series segments within the semimembranosus muscle (SM) in the American Toad (Bufo americanus) during hopping and within the sternohyoid (SH) muscle in the rat (Rattus rattus) during swallowing, and of spatially separated fascicles within the medial gastrocnemius (MG) muscle in the rat during trotting, we measured their corresponding in vitro (toad) or in situ (rat) force-length relationships (FLRs). For all three muscles, in vivo strain heterogeneity lasted for about 36-57% of the behavior cycle, during which one segment or fascicle shortened while the other segment or fascicle simultaneously lengthened. In the toad SM, the proximal segment shortened from the descending limb across the plateau of its FLR from 1.12 to 0.91 of its optimal length (Lo), while the distal segment lengthened (by 0.04 ± 0.04 Lo) before shortening down the ascending limb from 0.94 to 0.83 Lo. In the rat SH muscle, the proximal segment tended to shorten on its ascending limb from 0.90 to 0.85 Lo while the distal segment tended to lengthen across Lo (0.96-1.12 Lo). In the rat MG muscle, in vivo strains of proximal fascicles ranged from 0.72 to 1.02 Lo, while the distal fascicles ranged from 0.88 to 1.11 Lo. Even though the timing of muscle activation patterns were similar between segments, the heterogeneous strain patterns of fascicle segments measured in vivo coincided with different operating ranges across their FLRs simultaneously, implying differences in force-velocity behavior as well. The three vertebrate skeletal muscles represent a diversity of fiber architectures and functions and suggest that patterns of in vivo contractile strain and the operating range over the FLR in one muscle region does not necessarily represent other regions within the same muscle.

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.

Time Course and Association of Functional and Biochemical Markers in Severe Semitendinosus Damage Following Intensive Eccentric Leg Curls: Differences between and within Subjects

Purpose: To investigate the extent and evolution of hamstring muscle damage caused by an intensive bout of eccentric leg curls (ELCs) by (1) assessing the time course and association of different indirect markers of muscle damage such as changes in the force-generating capacity (FGC), functional magnetic resonance (fMRI), and serum muscle enzyme levels and (2) analyzing differences in the degree of hamstring muscle damage between and within subjects (limb-to-limb comparison).

Methods: Thirteen male participants performed six sets of 10 repetitions of an ELC with each leg. Before and at regular intervals over 7 days after the exercise, FGC was measured with maximal isometric voluntary contraction (MVC). Serum enzyme levels, fMRI transverse relaxation time (T2) and perceived muscle soreness were also assessed and compared against the FGC.

Results: Two groups of subjects were identified according to the extent of hamstring muscle damage based on decreased FGC and increased serum enzyme levels: high responders (n = 10, severe muscle damage) and moderate responders (n = 3, moderate muscle damage). In the high responders, fMRI T2 analysis revealed that the semitendinosus (ST) muscle suffered severe damage in the three regions measured (proximal, middle, and distal). The biceps femoris short head (BFsh) muscle was also damaged and there were significant differences in the FGC within subjects in the high responders.

Conclusion: FGC and serum enzyme levels measured in 10 of the subjects from the sample were consistent with severe muscle damage. However, the results showed a wide range of peak MVC reductions, reflecting different degrees of damage between subjects (high and moderate responders). fMRI analysis confirmed that the ST was the hamstring muscle most damaged by ELCs, with uniform T2 changes across all the measured sections of this muscle. During intensive ELCs, the ST muscle could suffer an anomalous recruitment pattern due to fatigue and damage, placing an excessive load on the BFsh and causing it to perform a synergistic compensation that leads to structural damage. Finally, T2 and MVC values did not correlate for the leg with the smaller FGC decrease in the hamstring muscles, suggesting that long-lasting increases in T2 signals after FGC markers have returned to baseline values might indicate an adaptive process rather than damage.

Optical prediction of single muscle fiber force production using a combined biomechatronics and second harmonic generation imaging approach

Skeletal muscle is an archetypal organ whose structure is tuned to match function. The magnitude of order in muscle fibers and myofibrils containing motor protein polymers determines the directed force output of the summed force vectors and, therefore, the muscle's power performance on the structural level. Structure and function can change dramatically during disease states involving chronic remodeling. Cellular remodeling of the cytoarchitecture has been pursued using noninvasive and label-free multiphoton second harmonic generation (SHG) microscopy. Hereby, structure parameters can be extracted as a measure of myofibrillar order and thus are suggestive of the force output that a remodeled structure can still achieve. However, to date, the parameters have only been an indirect measure, and a precise calibration of optical SHG assessment for an exerted force has been elusive as no technology in existence correlates these factors.  We engineered a novel, automated, high-precision biomechatronics system into a multiphoton microscope allows simultaneous isometric Ca²⁺-graded force or passive viscoelasticity measurements and SHG recordings. Using this MechaMorph system, we studied force and SHG in single EDL muscle fibers from wt and mdx mice; the latter serves as a model for compromised force and abnormal myofibrillar structure. We present Ca²⁺-graded isometric force, pCa-force curves, passive viscoelastic parameters and 3D structure in the same fiber for the first time. Furthermore, we provide a direct calibration of isometric force to morphology, which allows noninvasive prediction of the force output of single fibers from only multiphoton images, suggesting a potential application in the diagnosis of myopathies.