Geometric models to explore mechanisms of dynamic shape change in skeletal muscle

The first and second phases of the muscle compound action potential in the thumb are differently affected by electrical stimulation trains

Sarcolemmal membrane excitability is often evaluated by considering the peak-to-peak amplitude of the compound muscle action potential (M wave). However, the first and second M-wave phases represent distinct properties of the muscle action potential, which are differentially affected by sarcolemma properties and other factors such as muscle architecture. Contrasting with previous studies in which voluntary contractions have been used to induce muscle fatigue, we used repeated electrically induced tetanic contractions of the adductor pollicis muscle and assessed the kinetics of M-wave properties during the course of the contractions. Eighteen participants (24 ± 6 yr; means ± SD) underwent 30 electrically evoked tetanic contractions delivered at 30 Hz, each lasting 3 s with 1 s intervals. We recorded the amplitudes of the first and second M-wave phases for each stimulation. During the initial stimulation train, the first and second M-wave phases exhibited distinct kinetics. The first phase amplitude showed a rapid decrease to reach ∼59% of its initial value (P < 0.001), whereas the second phase amplitude displayed an initial transient increase of ∼19% (P = 0.007). Within subsequent trains, both the first and second phase amplitudes consistently decreased as fatigue developed with a reduction during the last train reaching ∼47% of its initial value (P < 0.001). Analyzing the first M wave of each stimulation train unveiled different kinetics for the first and second phases during the initial trains, but these distinctions disappeared as fatigue progressed. These findings underscore the interplay of factors affecting the M wave and emphasize the significance of separately scrutinizing its first and second phases when assessing membrane excitability adjustments during muscle contractions.NEW & NOTEWORTHY Our understanding of how the first and second phases of the compound muscle action potential (M wave) behave during fatigue remains incomplete. Using electrically evoked repeated tetanic contractions of the adductor pollicis, we showed that the first and second phases of the M wave followed distinct kinetics only during the early stages of fatigue development. This suggests that the factors affecting the M-wave first and second phases may change as fatigue develops.

M-wave changes caused by brief voluntary and stimulated isometric contractions

INTRODUCTION

Under isometric conditions, the increase in muscle force is accompanied by a reduction in the fibers' length. The effects of muscle shortening on the compound muscle action potential (M wave) have so far been investigated only by computer simulation. This study was undertaken to assess experimentally the M-wave changes caused by brief voluntary and stimulated isometric contractions.

METHODS

Two different methods of inducing muscle shortening under isometric condition were adopted: (1) applying a brief (1 s) tetanic contraction and (2) performing brief voluntary contractions of different intensities. In both methods, supramaximal stimulation was applied to the brachial plexus and femoral nerves to evoke M waves. In the first method, electrical stimulation (20 Hz) was delivered with the muscle at rest, whereas in the second, stimulation was applied while participants performed 5-s stepwise isometric contractions at 10, 20, 30, 40, 50, 60, 70, and 100% MVC. The amplitude and duration of the first and second M-wave phases were computed.

RESULTS

The main findings were: (1) on application of tetanic stimulation, the amplitude of the M-wave first phase decreased (~ 10%, P < 0.05), that of the second phase increased (~ 50%, P < 0.05), and the M-wave duration decreased (~ 20%, P < 0.05) across the first five M waves of the tetanic train and then plateaued for the subsequent responses; (2) when superimposing a single electrical stimulus on muscle contractions of increasing forces, the amplitude of the M-wave first phase decreased (~ 20%, P < 0.05), that of the second phase increased (~ 30%, P < 0.05), and M-wave duration decreased (~ 30%, P < 0.05) as force was raised from 0 to 60-70% MVC force.

CONCLUSIONS

The present results will help to identify the adjustments in the M-wave profile caused by muscle shortening and also contribute to differentiate these adjustments from those caused by muscle fatigue and/or changes in Na+-K+ pump activity.

A review of the efforts to develop muscle and musculoskeletal models for biomechanics in the last 50 years

Both the Hill and the Huxley muscle models had already been described by the time the International Society of Biomechanics was founded 50 years ago, but had seen little use before the 1970s due to the lack of computing. As computers and computational methods became available in the 1970s, the field of musculoskeletal modeling developed and Hill type muscle models were adopted by biomechanists due to their relative computational simplicity as compared to Huxley type muscle models. Muscle forces computed by Hill type muscle models provide good agreement in conditions similar to the initial studies, i.e. for small muscles contracting under steady and controlled conditions. However, more recent validation studies have identified that Hill type muscle models are least accurate for natural in vivo locomotor behaviours at submaximal activations, fast speeds and for larger muscles, and thus need to be improved for their use in understanding human movements. Developments in muscle modelling have tackled these shortcomings. However, over the last 50 years musculoskeletal simulations have been largely based on traditional Hill type muscle models or even simplifications of this model that neglected the interaction of the muscle with a compliant tendon. The introduction of direct collocation in musculoskeletal simulations about 15 years ago along with further improvements in computational power and numerical methods enabled the use of more complex muscle models in simulations of whole-body movement. Whereas Hill type models are still the norm, we may finally be ready to adopt more complex muscle models into musculoskeletal simulations of human movement.

Toward Soft Wearable Strain Sensors for Muscle Activity Monitoring

The force-generating capacity of skeletal muscle is an important metric in the evaluation and diagnosis of musculoskeletal health. Measuring changes in muscle force exertion is essential for tracking the progress of athletes during training, for evaluating patients’ recovery after muscle injury, and also for assisting the diagnosis of conditions such as muscular dystrophy, multiple sclerosis, or Parkinson’s disease. Traditional hardware for strength evaluation requires technical training for operation, generates discrete time points for muscle assessment, and is implemented in controlled settings. The ability to continuously monitor muscle force without restricting the range of motion or adapting the exercise protocol to suit specific hardware would allow for a richer dataset that can help unlock critical features of muscle health and strength evaluation. In this paper, we employ wearable, ultra-sensitive soft strain sensors for tracking changes in muscle deformation during contractions. We demonstrate the sensors’ sensitivity to isometric contractions, as well as the sensors’ capacity to track changes in peak torque over the course of an isokinetic fatiguing protocol for the knee extensors. The wearable soft system was able to efficiently estimate peak joint torque reduction caused by muscle fatigue (mean NRMSE = 0.15±0.03).

The harder the climb the better the view: The impact of substrate stiffness on cardiomyocyte fate

The quest for novel methods to mature human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) for cardiac regeneration, modelling and drug testing has emphasized a need to create microenvironments with physiological features. Many studies have reported on how cardiomyocytes sense substrate stiffness and adapt their morphological and functional properties. However, these observations have raised new biological questions and a shared vision to translate it into a tissue or organ context is still elusive. In this review, we will focus on the relevance of substrates mimicking cardiac extracellular matrix (cECM) rigidity for the understanding of the biomechanical crosstalk between the extracellular and intracellular environment. The ability to opportunely modulate these pathways could be a key to regulate in vitro hiPSC-CM maturation. Therefore, both hiPSC-CM models and substrate stiffness appear as intriguing tools for the investigation of cECM-cell interactions. More understanding of these mechanisms may provide novel insights on how cECM affects cardiac cell function in the context of genetic cardiomyopathies.

Quantity versus quality: Age-related differences in muscle volume, intramuscular fat, and mechanical properties in the triceps surae

With aging comes reductions in the quality and size of skeletal muscle. These changes influence the force-generating capacity of skeletal muscle and contribute to movement deficits that accompany aging. Although declines in strength remain a significant barrier to mobility in older adults, the association between age-related changes in muscle structure and function remain unresolved. In this study, we compared age-related differences in (i) muscle volume and architecture, (ii) the quantity and distribution of intramuscular fat, and (iii) muscle shear modulus (an index of stiffness) in the triceps surae in 21 younger (24.6 ± 4.3 years) and 15 older (70.4 ± 2.4 years) healthy adults. Additionally, we explored the relationship between muscle volume, architecture, intramuscular fat and ankle plantar flexion strength in young and older adults. Magnetic resonance imaging was used to determine muscle volume and intramuscular fat content. B-mode ultrasound was used to quantify muscle architecture, shear-wave elastography was used to measure shear modulus, and ankle strength was measured during maximal isometric plantar flexion contractions. We found that older adults displayed higher levels of intramuscular fat yet similar muscle volumes in the medial (MG) and lateral gastrocnemius (LG) and soleus, compared to younger adults. These age-related higher levels of intramuscular fat were associated with lower muscle shear modulus in the LG and MG. We also found that muscle physiological cross-sectional area (PCSA) that accounted for age-associated differences in intramuscular fat showed a modest increase in its association with ankle strength compared to PCSA that did not account for fat content. This highlights that skeletal muscle fat infiltration plays a role in age-related strength deficits, but does not fully explain the age-related loss in muscle strength, suggesting that other factors play a more significant role.

Qualitative ultrasonography scale of the intensity of local twitch response during dry needling and its association with modified joint range of motion: a cross-sectional study

Background

The relevance of local twitch response (LTR) during dry needling technique (DNT) is controversial, and it is questioned whether LTR is necessary for successful outcomes. Furthermore, because the LTR during the deep DNT may be evoked with different intensities, it is unknown whether the magnitude of LTR intensity is associated with optimal clinical results, especially concerning to the effects of joint maximal range of motion (ROM). This study aimed to (i) determine whether visual inspections can quantify the LTR intensity during the DNT through a qualitative ultrasonography scale of LTR intensity (US-LTR scale), and (ii) assess the differences of US-LTR scale associated with changes in the maximal joint ROM.

Methods

Using a cross-sectional design, seven asymptomatic individuals were treated with DNT in the latent myofascial trigger point in both medial gastrocnemius muscles. During DNT, three consecutive LTRs were collected. The US-LTR scale was used to classify the LTRs into strong, medium, and weak intensities. The categories of US-LTR were differentiated by the velocity of LTRs using the optical flow algorithm. ROM changes in ankle dorsiflexion and knee extension were assessed before and immediately after DNT.

Results

The US-LTR scale showed the third LTR was significantly smaller than the first one (p < 0.05). A significant difference in velocity was observed between US-LTR categories (p < 0.001). A significant difference in the ROM was observed between the strong and weak-medium intensity (p < 0.05).

Conclusions

The present findings suggest that the LTR intensity can be assessed using a qualitative US-LTR scale, and the effects of DNT on joint maximal ROM is maximized with higher LTR intensity. This study reports a novel qualitative method for LTR analysis with potential applications in research and clinical settings. However, further research is needed to achieve a broader application.

Muscle Belly Gearing Positively Affects the Force-Velocity and Power-Velocity Relationships During Explosive Dynamic Contractions

Changes in muscle shape could play an important role during contraction allowing to circumvent some limits imposed by the fascicle force–velocity (F–V) and power–velocity (P–V) relationships. Indeed, during low-force high-velocity contractions, muscle belly shortening velocity could exceed muscle fascicles shortening velocity, allowing the muscles to operate at higher F–V and P–V potentials (i.e., at a higher fraction of maximal force/power in accordance to the F–V and P–V relationships). By using an ultrafast ultrasound, we investigated the role of muscle shape changes (vastus lateralis) in determining belly gearing (muscle belly velocity/fascicle velocity) and the explosive torque during explosive dynamic contractions (EDC) at angular accelerations ranging from 1000 to 4000°.s^–2. By means of ultrasound and dynamometric data, the F–V and P–V relationships both for fascicles and for the muscle belly were assessed. During EDC, fascicle velocity, belly velocity, belly gearing, and knee extensors torque data were analysed from 0 to 150 ms after torque onset; the fascicles and belly F–V and P–V potentials were thus calculated for each EDC. Absolute torque decreased as a function of angular acceleration (from 80 to 71 Nm, for EDC at 1000 and 4000°.s^–1, respectively), whereas fascicle velocity and belly velocity increased with angular acceleration (P < 0.001). Belly gearing increased from 1.11 to 1.23 (or EDC at 1000 and 4000°.s^–1, respectively) and was positively corelated with the changes in muscle thickness and pennation angle (the changes in latter two equally contributing to belly gearing changes). For the same amount of muscle’s mechanical output (force or power), the fascicles operated at higher F–V and P–V potential than the muscle belly (e.g., P–V potential from 0.70 to 0.56 for fascicles and from 0.65 to 0.41 for the muscle belly, respectively). The present results experimentally demonstrate that belly gearing could play an important role during explosive contractions, accommodating the largest part of changes in contraction velocity and allowing the fascicle to operate at higher F–V and P–V potentials.

The Influence of Age and Obesity-Altered Muscle Tissue Composition on Muscular Dimensional Changes: Impact on Strength and Function

The purpose of this study was to determine if muscular dimensional changes with increases in torque production are influenced by age- and obesity-related increases in intramuscular fat, and its relationship to percent body fat (%BF), echo intensity (EI), strength, and maximum walking speed. Sixty-six healthy men were categorized into 3 groups based on age and body mass index status (young normal weight [YNW], older normal weight [ONW], and older obese [OB]). Participants underwent %BF assessments, resting ultrasonography to determine muscle size (cross-sectional area [CSA]) and EI of the superficial quadriceps, and a 10-m maximum walking speed assessment. Maximal and submaximal (rest–100% MVC in 10% increments) isometric leg extension strength was assessed while changes in rectus femoris (RF) CSA, width, and depth were obtained with ultrasonography. Echo intensity and %BF were different among all groups (p ≤ .007), with the YNW and OB groups exhibiting the lowest and highest %BF and EI values, respectively. The RF increased in depth and decreased in width with increases in torque intensity for all groups. The ONW group demonstrated no change (−0.08%) in RF CSA across torque intensities, whereas the YNW group (−11.5%) showed the greatest decrease in CSA, and the OB group showed a more subtle decrease (−4.6%). Among older men, a greater change in RF CSA was related to poorer EI (r = −0.355) and higher %BF (r = −0.346), while a greater decrease in RF width was associated with faster walking speeds (r = −0.431). Examining muscular dimensional changes during contraction is a unique model to investigate the influence of muscle composition on functional performance.

The Structure and Role of Intramuscular Connective Tissue in Muscle Function

Extracellular matrix (ECM) structures within skeletal muscle play an important, but under-appreciated, role in muscle development, function and adaptation. Each individual muscle is surrounded by epimysial connective tissue and within the muscle there are two distinct extracellular matrix (ECM) structures, the perimysium and endomysium. Together, these three ECM structures make up the intramuscular connective tissue (IMCT). There are large variations in the amount and composition of IMCT between functionally different muscles. Although IMCT acts as a scaffold for muscle fiber development and growth and acts as a carrier for blood vessels and nerves to the muscle cells, the variability in IMCT between different muscles points to a role in the variations in active and passive mechanical properties of muscles. Some traditional measures of the contribution of endomysial IMCT to passive muscle elasticity relied upon tensile measurements on single fiber preparations. These types of measurements may now be thought to be missing the important point that endomysial IMCT networks within a muscle fascicle coordinate forces and displacements between adjacent muscle cells by shear and that active contractile forces can be transmitted by this route (myofascial force transmission). The amount and geometry of the perimysial ECM network separating muscle fascicles varies more between different muscle than does the amount of endomysium. While there is some evidence for myofascial force transmission between fascicles via the perimysium, the variations in this ECM network appears to be linked to the amount of shear displacements between fascicles that must necessarily occur when the whole muscle contracts and changes shape. Fast growth of muscle by fiber hypertrophy is not always associated with a high turnover of ECM components, but slower rates of growth and muscle wasting may be associated with IMCT remodeling. A hypothesis arising from this observation is that the level of cell signaling via shear between integrin and dystroglycan linkages on the surface of the muscle cells and the overlying endomysium may be the controlling factor for IMCT turnover, although this idea is yet to be tested.

Ultrasound imaging to assess skeletal muscle architecture during movements: a systematic review of methods, reliability, and challenges

B-mode ultrasound is often used to quantify muscle architecture during movements. Our objectives were to 1) systematically review the reliability of fascicle length (FL) and pennation angles (PA) measured using ultrasound during movements involving voluntary contractions; 2) systematically review the methods used in studies reporting reliability, discuss associated challenges, and provide recommendations to improve the reliability and validity of dynamic ultrasound measurements; and 3) provide an overview of computational approaches for quantifying fascicle architecture, their validity, agreement with manual quantification of fascicle architecture, and advantages and drawbacks. Three databases were searched until June 2019. Studies among healthy human individuals aged 17-85 yr that investigated the reliability of FL or PA in lower-extremity muscles during isoinertial movements and that were written in English were included. Thirty studies (n = 340 participants) were included for reliability analyses. Between-session reliability as measured by coefficient of multiple correlations (CMC), and coefficient of variation (CV) was FL CMC: 0.89-0.96; CV: 8.3% and PA CMC: 0.87-0.90; CV: 4.5-9.6%. Within-session reliability was FL CMC: 0.82-0.99; CV: 0.0-6.7% and PA CMC: 0.91; CV: 0.0-15.0%. Manual analysis reliability was FL CMC: 0.89-0.96; CV: 0.0-15.9%; PA CMC: 0.84-0.90; and CV: 2.0-9.8%. Computational analysis FL CMC was 0.82-0.99, and PA CV was 14.0-15.0%. Eighteen computational approaches were identified, and these generally showed high agreement with manual analysis and high validity compared with phantoms or synthetic images. B-mode ultrasound is a reliable method to quantify fascicle architecture during movement. Additionally, computational approaches can provide a reliable and valid estimation of fascicle architecture.

Sprint versus isolated eccentric training: Comparative effects on hamstring architecture and performance in soccer players

AIMS

The purpose of this study was to compare the effects of hamstring eccentric (NHE) strength training versus sprint training programmed as complements to regular soccer practice, on sprint performance and its mechanical underpinnings, as well as biceps femoris long head (BFlh) architecture.

METHODS

In this prospective interventional control study, sprint performance, sprint mechanics and BFlh architecture variables were compared before versus after six weeks of training during the first six preseason weeks, and between three different random match-pair groups of soccer players: "Soccer group" (n = 10), "Nordic group" (n = 12) and "Sprint group" (n = 10).

RESULTS

For sprint performance and mechanics, small to large pre-post improvements were reported in "Sprint group" (except maximal running velocity), whereas only trivial to small negative changes were reported in "Soccer group" and "Nordic group". For BFlh architecture variables, "Sprint" group showed moderate increase in fascicle length compared to smaller augment for the "Nordic" group with trivial changes for "Soccer group". Only "Nordic" group presented small increases at pennation angle.

CONCLUSIONS

The results suggest that sprint training was superior to NHE in order to increase BFlh fascicle length although only the sprint training was able to both provide a preventive stimulus (increase fascicle length) and at the same time improve both sprint performance and mechanics. Further studies with advanced imaging techniques are needed to confirm the validity of the findings.

Post-buckling behaviour of a growing elastic rod

We consider mechanically-induced pattern formation within the framework of a growing, planar, elastic rod attached to an elastic foundation. Through a combination of weakly nonlinear analysis and numerical methods, we identify how the shape and type of buckling (super- or subcritical) depend on material parameters, and a complex phase-space of transition from super- to subcritical is uncovered. We then examine the effect of heterogeneity on buckling and post-buckling behaviour, in the context of a heterogeneous substrate adhesion, elastic stiffness, or growth. We show how the same functional form of heterogeneity in different properties is manifest in a vastly differing post-buckled shape. Finally, a fourth form of heterogeneity, an imperfect foundation, is incorporated and shown to have a more dramatic impact on the buckling instability, a difference that can be qualitatively understood via the weakly nonlinear analysis.

Transverse anisotropy in the deformation of the muscle during dynamic contractions

When pennate muscle fibres shorten, the transverse deformation of fibres results in an increase in pennation angle of fascicles (bundles of fibres) and transverse deformation of muscle belly. Transverse shape changes of a muscle can influence force generation. Recent modelling studies predicted asymmetrical transverse deformations in the muscle fascicles in the gastrocnemii. However, these predictions have not been tested experimentally. As muscle is a 3D entity, it is important to explore the structural changes in a 3D perspective to enhance our understanding of the underlying structural mechanisms that have functional implications. The medial and lateral gastrocnemius muscles from 12 subjects were imaged during plantarflexion movements on a dynamometer. The muscle belly was simultaneously scanned from two orthogonal directions using two ultrasound probes. Fascicle deformations were measured from the two orthogonal ultrasound scans to provide 3D information of muscle geometry. Whilst transverse deformations in the medial gastrocnemius were similar from the two directions, the data for the lateral gastrocnemius confirm that transverse anisotropy can occur in the muscle fascicles. As the lateral gastrocnemius fascicle length shortened, the pennation angle increased and the fascicles bulged transversally in one direction (closest to the typical 2D scanning plane) while thinning in the other orthogonal direction. We suggest that the transverse deformation of the muscle fascicles depends on the stiffness of the aponeuroses, properties of connective tissue structures surrounding muscle, and compressive forces both internal and external to the muscle. These results highlight that muscle fascicles do not bulge uniformly and the implications for this behaviour on muscle function remain largely unexplored.