Distribution and innervation of short, interdigitated muscle fibers in parallel-fibered muscles of the cat hindlimb

The Structural Adaptations That Mediate Disuse-Induced Atrophy of Skeletal Muscle

The maintenance of skeletal muscle mass plays a fundamental role in health and issues associated with quality of life. Mechanical signals are one of the most potent regulators of muscle mass, with a decrease in mechanical loading leading to a decrease in muscle mass. This concept has been supported by a plethora of human- and animal-based studies over the past 100 years and has resulted in the commonly used term of 'disuse atrophy'. These same studies have also provided a great deal of insight into the structural adaptations that mediate disuse-induced atrophy. For instance, disuse results in radial atrophy of fascicles, and this is driven, at least in part, by radial atrophy of the muscle fibers. However, the ultrastructural adaptations that mediate these changes remain far from defined. Indeed, even the most basic questions, such as whether the radial atrophy of muscle fibers is driven by the radial atrophy of myofibrils and/or myofibril hypoplasia, have yet to be answered. In this review, we thoroughly summarize what is known about the macroscopic, microscopic, and ultrastructural adaptations that mediated disuse-induced atrophy and highlight some of the major gaps in knowledge that need to be filled.

A method to identify, dissect and stain equine neuromuscular junctions for morphological analysis

Morphological study of the neuromuscular junction (NMJ), a specialised peripheral synapse formed between a lower motor neuron and skeletal muscle fibre, has significantly contributed to the understanding of synaptic biology and neuromuscular disease pathogenesis. Rodent NMJs are readily accessible, and research into conditions such as amyotrophic lateral sclerosis (ALS), Charcot–Marie–Tooth disease (CMT), and spinal muscular atrophy (SMA) has relied heavily on experimental work in these small mammals. However, given that nerve length dependency is an important feature of many peripheral neuropathies, these rodent models have clear shortcomings; large animal models might be preferable, but their size presents novel anatomical challenges. Overcoming these constraints to study the NMJ morphology of large mammalian distal limb muscles is of prime importance to increase cross‐species translational neuromuscular research potential, particularly in the study of long motor units. In the past, NMJ phenotype analysis of large muscle bodies within the equine distal pelvic limb, such as the tibialis cranialis, or within muscles of high fibrous content, such as the soleus, has posed a distinct experimental hurdle. We optimised a technique for NMJ location and dissection from equine pelvic limb muscles. Using a quantification method validated in smaller species, we demonstrate their morphology and show that equine NMJs can be reliably dissected, stained and analysed. We reveal that the NMJs within the equine soleus have distinctly different morphologies when compared to the extensor digitorum longus and tibialis cranialis muscles. Overall, we demonstrate that equine distal pelvic limb muscles can be regionally dissected, with samples whole‐mounted and their innervation patterns visualised. These methods will allow the localisation and analysis of neuromuscular junctions within the muscle bodies of large mammals to identify neuroanatomical and neuropathological features.

From fibre to function: are we accurately representing muscle architecture and performance?

The size and arrangement of fibres play a determinate role in the kinetic and energetic performance of muscles. Extrapolations between fibre architecture and performance underpin our understanding of how muscles function and how they are adapted to power specific motions within and across species. Here we provide a synopsis of how this 'fibre to function' paradigm has been applied to understand muscle design, performance and adaptation in animals. Our review highlights the widespread application of the fibre to function paradigm across a diverse breadth of biological disciplines but also reveals a potential and highly prevalent limitation running through past studies. Specifically, we find that quantification of muscle architectural properties is almost universally based on an extremely small number of fibre measurements. Despite the volume of research into muscle properties, across a diverse breadth of research disciplines, the fundamental assumption that a small proportion of fibre measurements can accurately represent the architectural properties of a muscle has never been quantitatively tested. Subsequently, we use a combination of medical imaging, statistical analysis, and physics-based computer simulation to address this issue for the first time. By combining diffusion tensor imaging (DTI) and deterministic fibre tractography we generated a large number of fibre measurements (>3000) rapidly for individual human lower limb muscles. Through statistical subsampling simulations of these measurements, we demonstrate that analysing a small number of fibres (n < 25) typically used in previous studies may lead to extremely large errors in the characterisation of overall muscle architectural properties such as mean fibre length and physiological cross-sectional area. Through dynamic musculoskeletal simulations of human walking and jumping, we demonstrate that recovered errors in fibre architecture characterisation have significant implications for quantitative predictions of in-vivo dynamics and muscle fibre function within a species. Furthermore, by applying data-subsampling simulations to comparisons of muscle function in humans and chimpanzees, we demonstrate that error magnitudes significantly impact both qualitative and quantitative assessment of muscle specialisation, potentially generating highly erroneous conclusions about the absolute and relative adaption of muscles across species and evolutionary transitions. Our findings have profound implications for how a broad diversity of research fields quantify muscle architecture and interpret muscle function.

Great power comes at a high (locomotor) cost: the role of muscle fascicle length in the power versus economy performance trade-off

Muscle design constraints preclude simultaneous specialization of the vertebrate locomotor system for explosive and economical force generation. The resulting performance trade-off between power and economy has been attributed primarily to individual differences in muscle fiber type composition. While certainly crucial for performance specialization, fiber type likely interacts with muscle architectural parameters, such as fascicle length, to produce this trade-off. Longer fascicles composed of more serial sarcomeres can achieve faster shortening velocities, allowing for greater power production. Long fascicles likely reduce economy, however, because more energy-consuming contractile units are activated for a given force production. We hypothesized that longer fascicles are associated with both increased power production and locomotor cost. In 11 power-trained and 13 endurance-trained recreational athletes, we measured (1) muscle fascicle length via ultrasound in the gastrocnemius lateralis, gastrocnemius medialis and vastus lateralis, (2) maximal power during cycling and countermovement jumps, and (3) running cost of transport. We estimated muscle fiber type non-invasively based on the pedaling rate at which maximal cycling power occurred. As predicted, longer gastrocnemius muscle fascicles were correlated with greater lower-body power production and cost of transport. Multiple regression analyses revealed that variability in maximal power was explained by fiber type (46% for cycling, 24% for jumping) and average fascicle length (20% for cycling, 13% for jumping), while average fascicle length accounted for 15% of the variation in cost of transport. These results suggest that, at least for certain muscles, fascicle length plays an important role in the power versus economy performance trade-off.

Development of DTI Based Probabilistic Tractography Methods to Characterize Arm Muscle Architecture in Individuals Post Hemiparetic Stroke

A hemiparetic stroke may lead to changes in muscle structure that further exacerbate motor impairments of the paretic limb. Cadaveric measurements have previously been used to study structural parameters in skeletal muscles but has several limitations, including ex vivo fixation. Here, we present novel application of diffusion tensor imaging (DTI) based probabilistic tractography methods, in comparison to the traditional deterministic approach, with respect to cadaveric dissection to quantify in vivo muscle fascicles in the biceps brachii. Preliminary results show that probabilistic tractography yields longer fascicle lengths that are more consistent with cadaveric measurements, albeit with higher variability, while deterministic tractography identifies shorter fascicle lengths, but with less variability. Results suggest that DTI tractography techniques can capture fascicles consistent with previously published cadaveric measurements and can identify interlimb differences in fascicle lengths in an individual with stroke.Clinical Relevance- The methods proposed here describe a non-invasive way to quantify heterogeneous musculoskeletal parameters such as across upper arm muscles in individuals with hemiparetic stroke. This will expand the current knowledge of macro- and micro-structural muscle changes that occur after stroke and may lead to more effective rehabilitation strategies to prevent such changes in individuals with stroke.

A Physiology-Guided Classification of Active-Stress and Active-Strain Approaches for Continuum-Mechanical Modeling of Skeletal Muscle Tissue

The well-established sliding filament and cross-bridge theory explain the major biophysical mechanism responsible for a skeletal muscle's active behavior on a cellular level. However, the biomechanical function of skeletal muscles on the tissue scale, which is caused by the complex interplay of muscle fibers and extracellular connective tissue, is much less understood. Mathematical models provide one possibility to investigate physiological hypotheses. Continuum-mechanical models have hereby proven themselves to be very suitable to study the biomechanical behavior of whole muscles or entire limbs. Existing continuum-mechanical skeletal muscle models use either an active-stress or an active-strain approach to phenomenologically describe the mechanical behavior of active contractions. While any macroscopic constitutive model can be judged by it's ability to accurately replicate experimental data, the evaluation of muscle-specific material descriptions is difficult as suitable data is, unfortunately, currently not available. Thus, the discussions become more philosophical rather than following rigid methodological criteria. Within this work, we provide a extensive discussion on the underlying modeling assumptions of both the active-stress and the active-strain approach in the context of existing hypotheses of skeletal muscle physiology. We conclude that the active-stress approach resolves an idealized tissue transmitting active stresses through an independent pathway. In contrast, the active-strain approach reflects an idealized tissue employing an indirect, coupled pathway for active stress transmission. Finally the physiological hypothesis that skeletal muscles exhibit redundant pathways of intramuscular stress transmission represents the basis for considering a mixed-active-stress-active-strain constitutive framework.

Muscle-nerve communication and the molecular assessment of human skeletal muscle denervation with aging

Muscle fiber denervation is a major contributor to the decline in physical function observed with aging. Denervation can occur through breakdown of the neuromuscular junctions (NMJ) itself, affecting only that particular fiber, or through the death of a motor neuron, which can lead to a loss of all the muscle fibers in that motor unit. In this review, we discuss the muscle-nerve relationship, where signaling from both the motor neuron and the muscle fiber is required for maximal preservation of neuromuscular function in old age. Physical activity is likely to be the most important single factor that can contribute to this preservation. Furthermore, we propose that inactivity is not an innocent bystander, but plays an active role in denervation through the production of signals hostile to neuron survival. Investigating denervation in human muscle tissue samples is challenging due to the shared protein profile of regenerating and denervated muscle fibers. In this review, we provide a detailed overview of the key traits observed in immunohistochemical preparations of muscle biopsies from healthy, young, and elderly individuals. Overall, a combination of assessing tissue samples, circulating biomarkers, and electrophysiological assessments in humans will prove fruitful in the quest to gain more understanding of denervation of skeletal muscle. In addition, cell culture models represent a valuable tool in the search for key signaling factors exchanged between muscle and nerve, and which exercise has the capacity to alter.

Surgical Mobilization of Skeletal Muscles Changes Functional Properties-Implications for Tendon Transfers

PURPOSE

Tendon transfer surgery restores function by rerouting working muscle-tendon units to replace the function of injured or paralyzed muscles. This procedure requires mobilizing a donor muscle relative to its surrounding myofascial connections, which improves the muscle's new line of action and increases excursion. However, the biomechanical effect of mobilization on a donor muscle's force-generating function has not been previously studied under in vivo conditions. The purpose of this study was to quantify the effect of surgical mobilization on active and passive biomechanical properties of 3 large rabbit hind limb muscles.

METHODS

Myofascial connections were mobilized stepwise from the distal end to the proximal end of muscles (0%, 25%, 50%, and 75% of muscle length) and their active and passive length-tension curves were measured after each degree of mobilization.

RESULTS

Second toe extensor, a short-fibered muscle, exhibited a 30% decline in peak stress and 70% decline in passive stress, whereas extensor digitorum longus, a short-fibered muscle, and tibialis anterior, a long-fibered muscle, both exhibited similar smaller declines in active (about 18%) and passive stress (about 65%).

CONCLUSIONS

The results highlight 3 important points: (1) a trade-off exists between increasing muscle mobility and decreasing force-generating capacity; (2) intermuscular force transmission is important, especially in second toe extensor, because it was able to generate 70% of its premobilization active force although most fibers were freed from their native origin; and (3) muscle architecture is not the major influence on mobilization-induced force impairment.

CLINICAL RELEVANCE

These data demonstrate that surgical mobilization itself alters the passive and active force-generating capacity of skeletal muscles. Thus, surgical mobilization should not be viewed simply as a method to redirect the line of action of a donor muscle because this procedure has an impact on the functional properties of the donor muscle itself.

Do skeletal muscle motor units and microvascular units align to help match blood flow to metabolic demand?

PURPOSE

We explore the motor unit recruitment and control of perfusion of microvascular units in skeletal muscle to determine whether they coordinate to match blood flow to metabolic demand.

METHODS

The PubMed database was searched for historical, current and relevant literature.

RESULTS

A microvascular, or capillary unit consists of 2-20 individual capillaries. Individual capillaries within a capillary unit cannot increase perfusion independently of other capillaries within the unit. Capillary units perfuse a short segment of approx. 12 muscle fibres located beside each other. Motor units consist of muscle fibres that can be dispersed widely within the muscle volume. During a contraction, where not all motor units are recruited, muscle fibre contraction will result in increased perfusion of associated capillaries as well as all capillaries within that capillary unit. Perfusion of the entire capillary unit will result in an increased blood flow delivery to muscle fibres associated with active motor unit plus approximately 11 other inactive muscle fibres within the same region. This will result in an overperfusion of the muscle resulting in blood flow in excess of the muscle fibre needs.

CONCLUSIONS

Given the architecture of the capillary units and the dispersed nature of muscle fibres within a motor unit, during submaximal contractions, where not all motor units are recruited, there will be a greater perfusion to the muscle than that predicted by the number of active muscle fibres. Such overperfusion brings into question if blood flow and metabolic demand are as tightly matched as previously assumed.

Interleaved intramuscular stimulation with minimally overlapping electrodes evokes smooth and fatigue resistant forces

OBJECTIVE

It is known that multi-site interleaved stimulation generates less muscle fatigue compared to single-site synchronous stimulation. However, in the limited number of studies in which intramuscular electrodes were used, the fatigue reduction associated with interleaved stimulation could not consistently be achieved. We hypothesize that this could be due to the inability to place the intramuscular electrodes used in interleaved stimulation in locations that minimize overlap amongst the motor units activated by the electrodes. Our objective in the present study was to use independent intramuscular electrodes to compare fatigue induced by interleaved stimulation with that generated by synchronous stimulation at the same initial force and ripple.

APPROACH

In the medial gastrocnemius muscle of an anesthetized rabbit (n = 3), ten intramuscular hook wire electrodes were inserted at different distances from the nerve entry. Overlap was measured using the refractory technique and only three electrodes were found to be highly independent. After ensuring that forces obtained by both stimulation modalities had the same ripple and magnitude, fatigue induced during interleaved stimulation across three independent distal electrodes was compared to that obtained by synchronously delivering pulses to a single proximal electrode.

MAIN RESULTS

Contractions evoked by interleaved stimulation exhibited less fatigue than those evoked by synchronous stimulation. Twitch force recruitment curves collected from each of the ten intramuscular electrodes showed frequent intermediate plateaus and the force value at these plateaus decreased as the distance between the electrode and nerve entry increased.

SIGNIFICANCE

The results indicate that interleaved intramuscular stimulation is preferred over synchronous intramuscular stimulation when fatigue-resistant and smooth forces are desired. In addition, the results suggest that the large muscle compartments innervated by the primary intramuscular nerve branches give rise to progressively smaller independent compartments in subsequent nerve divisions.

Scaling of inertial delays in terrestrial mammals

As part of its response to a perturbation, an animal often needs to reposition its body. Inertia acts to oppose the corrective motion, delaying the completion of the movement-we refer to this elapsed time as inertial delay. As animal size increases, muscle moment arms also increase, but muscles are proportionally weaker, and limb inertia is proportionally larger. Consequently, the scaling of inertial delays is complex. Our intent is to determine how quickly different sized animals can produce corrective movements when their muscles act at their force capacity, relative to the time within which those movements need to be performed. Here, we quantify inertial delay using two biomechanical models representing common scenarios in animal locomotion: a distributed mass pendulum approximating swing limb repositioning (swing task), and an inverted pendulum approximating whole body posture recovery (posture task). We parameterized the anatomical, muscular, and inertial properties of these models using literature scaling relationships, then determined inertial delay for each task across a large range of movement magnitudes and the full range of terrestrial mammal sizes. We found that inertial delays scaled with an average of M0.28 in the swing task and M0.35 in the posture task across movement magnitudes-larger animals require more absolute time to perform the same movement as small animals. The time available to complete a movement also increases with animal size, but less steeply. Consequently, inertial delays comprise a greater fraction of swing duration and other characteristic movement times in larger animals. We also compared inertial delays to the other component delays within the stimulus-response pathway. As movement magnitude increased, inertial delays exceeded these sensorimotor delays, and this occurred for smaller movements in larger animals. Inertial delays appear to be a challenge for motor control, particularly for bigger movements in larger animals.

Characterization of Electromechanical Delay Based on a Biophysical Multi-Scale Skeletal Muscle Model

Skeletal muscles can be voluntary controlled by the somatic nervous system yielding an active contractile stress response. Thereby, the active muscle stresses are transmitted to the skeleton by a cascade of connective tissue and thus enable motion. In the context of joint perturbations as well as the assessment of the complexity of neural control, the initial phase of the muscle-tendon system's stress response has a particular importance and is analyzed by means of electromechanical delay (EMD). EMD is defined as the time lag between the stimulation of a muscle and a measurable change in force output. While EMD is believed to depend on multiple structures / phenomena, it is hard to separate their contributions experimentally. We employ a physiologically detailed, three-dimensional, multi-scale model of an idealized muscle-tendon system to analyze the influence of (i) muscle and tendon length, (ii) the material behavior of skeletal muscle and tendon tissue, (iii) the chemo-electro-mechanical behavior of the muscle fibers and (iv) neural control on EMD. Comparisons with experimental data show that simulated EMD values are within the physiological range, i.e., between 6.1 and 68.6 ms, and that the model is able to reproduce the characteristic EMD-stretch curve, yielding the minimum EMD at optimal length. Simulating consecutive recruitment of motor units increases EMD by more than 20 ms, indicating that during voluntary contractions neural control is the dominant factor determining EMD. In contrast, the muscle fiber action potential conduction velocity is found to influence EMD even of a 27 cm long muscle by not more than 3.7 ms. We further demonstrate that in conditions where only little pre-stretch is applied to a muscle-tendon system, the mechanical behavior of both muscle and tendon tissue considerably impacts EMD. Predicting EMD for different muscle and tendon lengths indicates that the anatomy of a specific muscle-tendon system is optimized for its function, i.e., shorter tendon lengths are beneficial to minimize the neural control effort for muscles primary acting as motor in concentric contractions.

Multiscale modeling of the neuromuscular system: Coupling neurophysiology and skeletal muscle mechanics

Mathematical models and computer simulations have the great potential to substantially increase our understanding of the biophysical behavior of the neuromuscular system. This, however, requires detailed multiscale, and multiphysics models. Once validated, such models allow systematic in silico investigations that are not necessarily feasible within experiments and, therefore, have the ability to provide valuable insights into the complex interrelations within the healthy system and for pathological conditions. Most of the existing models focus on individual parts of the neuromuscular system and do not consider the neuromuscular system as an integrated physiological system. Hence, the aim of this advanced review is to facilitate the prospective development of detailed biophysical models of the entire neuromuscular system. For this purpose, this review is subdivided into three parts. The first part introduces the key anatomical and physiological aspects of the healthy neuromuscular system necessary for modeling the neuromuscular system. The second part provides an overview on state-of-the-art modeling approaches representing all major components of the neuromuscular system on different time and length scales. Within the last part, a specific multiscale neuromuscular system model is introduced. The integrated system model combines existing models of the motor neuron pool, of the sensory system and of a multiscale model describing the mechanical behavior of skeletal muscles. Since many sub-models are based on strictly biophysical modeling approaches, it closely represents the underlying physiological system and thus could be employed as starting point for further improvements and future developments. This article is categorized under: Physiology > Mammalian Physiology in Health and Disease Analytical and Computational Methods > Computational Methods Models of Systems Properties and Processes > Organ, Tissue, and Physiological Models.

Passive changes in muscle length

This review, the first in a series of minireviews on the passive mechanical properties of skeletal muscles, seeks to summarize what is known about the muscle deformations that allow relaxed muscles to lengthen and shorten. Most obviously, when a muscle lengthens, muscle fascicles elongate, but this is not the only mechanism by which muscles change their length. In pennate muscles, elongation of muscle fascicles is accompanied by changes in pennation and changes in fascicle curvature, both of which may contribute to changes in muscle length. The contributions of these mechanisms to change in muscle length are usually small under passive conditions. In very pennate muscles with long aponeuroses, fascicle shear could contribute substantially to changes in muscle length. Tendons experience moderate axial strains even under passive loads, and, because tendons are often much longer than muscle fibers, even moderate tendon strains may contribute substantially to changes in muscle length. Data obtained with new imaging techniques suggest that muscle fascicle and aponeurosis strains are highly nonuniform, but this is yet to be confirmed. The development, validation, and interpretation of continuum muscle models informed by rigorous measurements of muscle architecture and material properties should provide further insights into the mechanisms that allow relaxed muscles to lengthen and shorten.

Scaling of sensorimotor delays in terrestrial mammals

Whether an animal is trying to escape from a predator, avoid a fall or perform a more mundane task, the effectiveness of its sensory feedback is constrained by sensorimotor delays. Here, we combine electrophysiological experiments, systematic reviews of the literature and biophysical models to determine how delays associated with the fastest locomotor reflex scale with size in terrestrial mammals. Nerve conduction delay is one contributor, and increases strongly with animal size. Sensing, synaptic and neuromuscular junction delays also contribute, and we approximate each as a constant value independent of animal size. Muscle's electromechanical and force generation delays increase more moderately with animal size than nerve conduction delay, but their total contribution exceeds that of the four neural delays. The sum of these six component delays, termed total delay, increases with animal size in proportion to M0.21-large mammals experience total delays 17 times longer than small mammals. The slower movement times of large animals mostly offset their long delays resulting in a more modest, but perhaps still significant, doubling of their total delay relative to movement duration when compared with their smaller counterparts. Irrespective of size, sensorimotor delay is likely a challenge for all mammals, particularly during fast running.

Muscle and Limb Mechanics

Understanding of the musculoskeletal system has evolved from the collection of individual phenomena in highly selected experimental preparations under highly controlled and often unphysiological conditions. At the systems level, it is now possible to construct complete and reasonably accurate models of the kinetics and energetics of realistic muscles and to combine them to understand the dynamics of complete musculoskeletal systems performing natural behaviors. At the reductionist level, it is possible to relate most of the individual phenomena to the anatomical structures and biochemical processes that account for them. Two large challenges remain. At a systems level, neuroscience must now account for how the nervous system learns to exploit the many complex features that evolution has incorporated into muscle and limb mechanics. At a reductionist level, medicine must now account for the many forms of pathology and disability that arise from the many diseases and injuries to which this highly evolved system is inevitably prone. © 2017 American Physiological Society. Compr Physiol 7:429-462, 2017.

Motor units in the human medial gastrocnemius muscle are not spatially localized or functionally grouped

KEY POINTS

Human medial gastrocnemius (MG) motor units (MUs) are thought to occupy small muscle territories or regions, with low-threshold units preferentially located distally. We used intramuscular recordings to measure the territory of muscle fibres from MG MUs and determine whether these MUs are grouped by recruitment threshold or joint action (ankle plantar flexion and knee flexion). The territory of MUs from the MG muscle varied from somewhat localized to highly distributed, with approximately half the MUs spanning at least half the length and width of the muscle. There was also no evidence of regional muscle activity based on MU recruitment thresholds or joint action. The CNS does not have the means to selectively activate regions of the MG muscle based on task requirements.

ABSTRACT

Human medial gastrocnemius (MG) motor units (MUs) are thought to occupy small muscle territories, with low-threshold units preferentially located distally. In this study, subjects (n = 8) performed ramped and sustained isometric contractions (ankle plantar flexion and knee flexion; range: ∼1-40% maximal voluntary contraction) and we measured MU territory size with spike-triggered averages from fine-wire electrodes inserted along the length (seven electrodes) or across the width (five electrodes) of the MG muscle. Of 69 MUs identified along the length of the muscle, 32 spanned at least half the muscle length (≥ 6.9 cm), 11 of which spanned all recording sites (13.6-17.9 cm). Distal fibres had smaller pennation angles (P < 0.05), which were accompanied by larger territories in MUs with fibres located distally (P < 0.05). There was no distal-to-proximal pattern of muscle activation in ramp contraction (P = 0.93). Of 36 MUs identified across the width of the muscle, 24 spanned at least half the muscle width (≥ 4.0 cm), 13 of which spanned all recording sites (8.0-10.8 cm). MUs were not localized (length or width) based on recruitment threshold or contraction type, nor was there a relationship between MU territory size and recruitment threshold (Spearman's rho = -0.20 and 0.13, P > 0.18). MUs in the human MG have larger territories than previously reported and are not localized based on recruitment threshold or joint action. This indicates that the CNS does not have the means to selectively activate regions of the MG muscle based on task requirements.

Diaphragm: A vital respiratory muscle in mammals

The diaphragm is a respiratory muscle that is primarily responsible for the respiratory function in normal individuals. In mammals, the diaphragm muscle has been studied from the early days of zoology, comparative and experimental anatomy, physiology, medicine, physics, and philosophy. However, even with these early advances in knowledge pertaining to the diaphragm, comprehensive morphological data on the diaphragm are still incomplete. In this review, we summarize the beginnings of the morphological description of the diaphragm, and we describe the current status of the known morphological and embryological features. In addition, we correlate how the impairment of the diaphragm muscle in Duchenne muscular dystrophy (DMD) can lead to patient deaths. DMD is the most common X-linked muscle degenerative disease and is caused by a lack of dystrophin protein. Dystrophin is an important muscle protein that links the cellular cytoskeleton with the extracellular matrix. In the absence of dystrophin, the muscle becomes susceptible to damage during muscle contraction. This review allows researchers to obtain an overview of the diaphragm, transcending the morphological data from animals described in conventional literature.

A multiscale chemo-electro-mechanical skeletal muscle model to analyze muscle contraction and force generation for different muscle fiber arrangements

The presented chemo-electro-mechanical skeletal muscle model relies on a continuum-mechanical formulation describing the muscle's deformation and force generation on the macroscopic muscle level. Unlike other three-dimensional models, the description of the activation-induced behavior of the mechanical model is entirely based on chemo-electro-mechanical principles on the microscopic sarcomere level. Yet, the multiscale model reproduces key characteristics of skeletal muscles such as experimental force-length and force-velocity data on the macroscopic whole muscle level. The paper presents the methodological approaches required to obtain such a multiscale model, and demonstrates the feasibility of using such a model to analyze differences in the mechanical behavior of parallel-fibered muscles, in which the muscle fibers either span the entire length of the fascicles or terminate intrafascicularly. The presented results reveal that muscles, in which the fibers span the entire length of the fascicles, show lower peak forces, more dispersed twitches and fusion of twitches at lower stimulation frequencies. In detail, the model predicted twitch rise times of 38.2 and 17.2 ms for a 12 cm long muscle, in which the fibers span the entire length of the fascicles and with twelve fiber compartments in series, respectively. Further, the twelve-compartment model predicted peak twitch forces that were 19% higher than in the single-compartment model. The analysis of sarcomere lengths during fixed-end single twitch contractions at optimal length predicts rather small sarcomere length changes. The observed lengths range from 75 to 111% of the optimal sarcomere length, which corresponds to a region with maximum filament overlap. This result suggests that stability issues resulting from activation-induced stretches of non-activated sarcomeres are unlikely in muscles with passive forces appearing at short muscle length.

Task-dependent inhibition of slow-twitch soleus and excitation of fast-twitch gastrocnemius do not require high movement speed and velocity-dependent sensory feedback

Although individual heads of triceps surae, soleus (SO) and medial gastrocnemius (MG) muscles, are often considered close functional synergists, previous studies have shown distinct activity patterns between them in some motor behaviors. The goal of this study was to test two hypotheses explaining inhibition of slow SO with respect to fast MG: (1) inhibition occurs at high movement velocities and mediated by velocity-dependent sensory feedback and (2) inhibition depends on the ankle-knee joint moment combination and does not require high movement velocities. The hypotheses were tested by comparing the SO EMG/MG EMG ratio during fast and slow motor behaviors (cat paw shake responses vs. back, straight leg load lifting in humans), which had the same ankle extension-knee flexion moment combination; and during fast and slow behaviors with the ankle extension-knee extension moment combination (human vertical jumping and stance phase of walking in cats and leg load lifting in humans). In addition, SO EMG/MG EMG ratio was determined during cat paw shake responses and walking before and after removal of stretch velocity-dependent sensory feedback by self-reinnervating SO and/or gastrocnemius. We found the ratio SO EMG/MG EMG below 1 (p < 0.05) during fast paw shake responses and slow back load lifting, requiring the ankle extension-knee flexion moment combination; whereas the ratio SO EMG/MG EMG was above 1 (p < 0.05) during fast vertical jumping and slow tasks of walking and leg load lifting, requiring ankle extension-knee extension moments. Removal of velocity-dependent sensory feedback did not affect the SO EMG/MG EMG ratio in cats. We concluded that the relative inhibition of SO does not require high muscle velocities, depends on ankle-knee moment combinations, and is mechanically advantageous for allowing a greater MG contribution to ankle extension and knee flexion moments.

Physiological cross-sectional area of the oblique head of the adductor pollicis is greater than its transverse counterpart: implications for functional testing

INTRODUCTION

Despite structural distinction between the transverse and oblique heads of the adductor pollicis, in vivo testing continues to consider the adductor pollicis as functionally simplistic. As a muscle's architecture is a strong indicator of function, in this study we aimed to determine whether the physiological cross-sectional areas (PCSAs) of both heads were uniform.

METHODS

Classical, microdissection, and chemical dissection procedures were conducted on 10 cadaveric left hands to determine structural origin and insertions. Architectural measures of muscle length (Lm ), muscle weight (Wm ), fascicle length (Lf ), sarcomere length (Ls ), and pennation angle (θ) were used to calculate PCSA and fascicle length:muscle length ratio (Lf :Lm ).

RESULTS

The oblique head had greater variation in attachments, significantly greater PCSA (P = 0.008), and smaller Lf :Lm (P = 0.001) than its transverse counterpart.

CONCLUSIONS

Muscle architecture suggests the oblique head has greater potential for force generation, and the transverse has greater potential for joint excursion.

Architectural analysis and predicted functional capability of the human latissimus dorsi muscle

The latissimus dorsi is primarily considered a muscle with actions at the shoulder, despite its widespread attachments at the spine. There is some dispute regarding the potential contribution of this muscle to lumbar spine function. The architectural design of a muscle is one of the most accurate predictors of muscle function; however, detailed architectural data on the latissimus dorsi muscle are limited. Therefore, the aim of this study was to quantify the architectural properties of the latissimus dorsi muscle and model mechanical function in light of these new data. One latissimus dorsi muscle was removed from each of 12 human cadavers, separated into regions, and micro-dissected for quantification of fascicle length, sarcomere length, and physiological cross-sectional area. From these data, sarcomere length operating ranges were modelled to determine the force-length characteristics of latissimus dorsi across the spine and shoulder ranges of motion. The physiological cross-sectional area of latissimus dorsi was 5.6±0.5 cm2 and normalized fascicle length was 26.4±1.0 cm, indicating that this muscle is designed to produce a moderate amount of force over a large range of lengths. Measured sarcomere length in the post-mortem neutral spine posture was nearly optimal at 2.69±0.06 μm. Across spine range of motion, biomechanical modelling predicted latissimus dorsi acts across both the ascending and descending limbs of the force-length curve during lateral bend, and primarily at or near the plateau region (where maximum force generation is possible) during flexion/extension and axial twist. Across shoulder range of motion, latissimus dorsi acts primarily on the plateau region and descending limbs of the force length curve during both flexion/extension and abduction/adduction. These data provide novel insights into the ability of the latissimus dorsi muscle to generate force and change length throughout the spine and shoulder ranges of motion. In addition, these findings provide an improved understanding of the spine and shoulder positions at which the force-generating capacity of this muscle can become jeopardized, and consequently how this may affect its spine-stabilizing ability.

Passive mechanical properties of rat abdominal wall muscles suggest an important role of the extracellular connective tissue matrix

Abdominal wall muscles have a unique morphology suggesting a complex role in generating and transferring force to the spinal column. Studying passive mechanical properties of these muscles may provide insights into their ability to transfer force among structures. Biopsies from rectus abdominis (RA), external oblique (EO), internal oblique (IO), and transverse abdominis (TrA) were harvested from male Sprague-Dawley rats, and single muscle fibers and fiber bundles (4-8 fibers ensheathed in their connective tissue matrix) were isolated and mechanically stretched in a passive state. Slack sarcomere lengths were measured and elastic moduli were calculated from stress-strain data. Titin molecular mass was also measured from single muscle fibers. No significant differences were found among the four abdominal wall muscles in terms of slack sarcomere length or elastic modulus. Interestingly, across all four muscles, slack sarcomere lengths were quite long in individual muscle fibers (>2.4 µm), and demonstrated a significantly longer slack length in comparison to fiber bundles (p < 0.0001). Also, the extracellular connective tissue matrix provided a stiffening effect and enhanced the resistance to lengthening at long muscle lengths. Titin molecular mass was significantly less in TrA compared to each of the other three muscles (p < 0.0009), but this difference did not correspond to hypothesized differences in stiffness.

Neto ACD, Miglino MA, Bombonato PP, Ambrósio CE. Descrição morfológica do diafragma do sagui-de-tufo-branco (Callithrix jacchus)

O músculo diafragma, encontrado apenas nos mamíferos, é o principal músculo no processo respiratório, servindo de fronteira entre as cavidades torácica e abdominal. Sua importância também ganha destaque em pesquisas realizadas no âmbito dos enxertos, empregando-se diversos tipos de membranas biológicas para o reparo de defeitos diafragmáticos, os quais podem gerar hérnias diafragmáticas. Apesar de muitos estudos já conduzidos para com os primatas não humanos, especialmente no que tange a espécie do novo mundo Callithrix jacchus (Sagui-de-tufo-branco), oriundo do nordeste brasileiro, as pesquisas envolvendo o uso do diafragma em tal espécie é inexistente. Deste modo objetivou-se caracterizar a morfologia e a biometria do diafragma na espécie Callithrix jacchus de ambos os sexos, analisando possíveis divergências estruturais entre machos e fêmeas. Para tal foram utilizados quatros animais, 2 machos e 2 fêmeas, adultos, que vieram a óbito por causas naturais, provenientes de um criadouro comercial. Após fixação em solução de formaldeído 10% os animais foram devidamente dissecados para fotodocumentação e em seguida o diafragma coletado para efetuação da biometria (comprimento e largura) com o uso de um paquímetro e para o processamento histológico por meio da coloração de hematoxilina-eosina e tricrômio de masson, da porção muscular. As mensurações feitas permitiram concluir que não houve diferenças signifcativas entre machos e femeas. A topografia e a presença de três aberturas (forame da veia cava caudal, hiato aórtico e esofágico) na extensão do diafragma corroboram com descrições na literatura classica para outros mamíferos. A presença de um centro tendíneo em "V" difere do encontrado para animais como o peixe-boi e porquinho-da-india, mas é similar ao encontrado para o gambá-de-orelhas-brancas e rato albino. No que diz respeito aos achados histológicos conclui-se que as fibras musculares estão dispostas de forma organizada, apresentam diâmetro grande e núcleos basais, tendo, portanto, características similares do músculo estriado esquelético tanto nos animais machos como nas fêmeas.

The arrangement of fascicles in whole muscle

The architecture of the muscle fascicles, here meaning their lengths and their arrangement relative to one another, has important implications for the force a muscle can produce. Therefore, quantifying this architectural arrangement and understanding the implications of the architecture are important for understanding muscle function in vivo. There were two purposes of this study: (1) to assess, via blunt dissection, the number and the length of all the fascicles comprising the First Dorsal Interosseous (FDI) muscle and (2) to visually identify, via magnetic resonance imaging (MRI), the arrangement of the fascicles comprising the FDI. Simple blunt dissection of all the fascicles comprising four FDI muscles and their subsequent measurement demonstrated that the fascicles comprising the whole muscle were not as long as the muscle belly from which they were extracted. Muscle fascicles are surrounded by connective tissue hence the paths of the fascicles in two whole FDI muscles were identified via MRI by tracking the connective tissue surrounding the fascicles. The fascicles had a spiral pattern along the length of each muscle, within both muscles many of the fascicles were arranged in series with other fascicles. These architectural features of the fascicles of the FDI have important implications for the force-length and force-velocity properties of the whole muscle.

INTRA-, EXTRA- AND INTERMUSCULAR MYOFASCIAL FORCE TRANSMISION OF SYNERGISTS AND ANTAGONISTS: EFFECTS OF MUSCLE LENGTH AS WELL AS RELATIVE POSITION

The concepts of intramuscular myofascial force transmission is reintroduced and reviewed on the basis of experiments involving tenotomy and aponeurotomy of dissected rat EDL muscle studied in situ. Results from experiments with measurements of force of EDL muscle, of which the muscle belly was not dissected (i.e. the muscle is surrounded by its natural connective tissue milieu) are discussed. In such experiments, force was measured at proximal as well as distal EDL tendons. Examples of experimental evidence for both extramuscular and intermuscular myofascial force transmission within the rat anterior crural compartment are presented. Evidence is presented also for differential effects of proximal and distal lengthening on myofascial force transmission from EDL, even for the case in which symmetric length changes were imposed on the muscle. It is shown that myofascial force transmission effects are not limited to synergists located within one compartment, but do also play a very substantial role in the interaction between antagonist muscles in neighbouring anterior crural and peroneal compartments.

A morphological comparison of the human lumbar multifidus by chemical dissection

This anatomical study describes the morphology of the human lumbar multifidus muscle through gross and chemical dissection of fresh cadavers. Previous morphological descriptions were analyzed with regard to fascicular divisions and cleavage planes. Gross dissection was performed on the lumbar multifidus of four fresh adult human cadavers and four preserved cadavers. Gradual chemical dissection using nitric or formic acid was used for connective tissue digestion to enhance the documentation of muscle fiber direction. Results revealed four distinct layers of the lumbar multifidus separated by cleavage planes. The superficial layer was more extensive than previously described with bony attachments at both the origin and insertion at several vertebral levels. The attachments of the second through fourth layers differed in that distinct cleavage planes between the various fascicles were not found with chemical dissection. The lumbar multifidus has a multipennate fiber arrangement, and the fascicles between the various layers inter-attach. Inter-fascicle attachment differs with the description by Macintosh et al of distinct cleavage planes between and within the fascicles of each layer. Accurate anatomical knowledge of the fascicles of the lumbar multifidus is integral for defining the actions of this complex lumbar muscle. This study supports the clinical belief that the multifidus has a significant role in control and stabilization of the lumbar spine in multiple planes of action. The multipennate arrangement of this muscle with fascicular inter-attachment supports the clinical premise that the multifidus is activated in a variety of positions and can potentially produce and mediate intersegmental mobility and provide proprioception.

A mathematical model of force transmission from intrafascicularly terminating muscle fibers

Many long skeletal muscles are comprised of fibers that terminate intrafascicularly. Force from terminating fibers can be transmitted through shear within the endomysium that surrounds fibers or through tension within the endomysium that extends from fibers to the tendon; however, it is unclear which pathway dominates in force transmission from terminating fibers. The purpose of this work was to develop mathematical models to (i) compare the efficacy of lateral (through shear) and longitudinal (through tension) force transmission in intrafascicularly terminating fibers, and (ii) determine how force transmission is affected by variations in the structure and properties of fibers and the endomysium. The models demonstrated that even though the amount of force that can be transmitted from an intrafascicularly terminating fiber is dependent on fiber resting length (the unstretched length at which passive stress is zero), endomysium shear modulus, and fiber volume fraction (the fraction of the muscle cross-sectional area that is occupied by fibers), fibers that have values of resting length, shear modulus, and volume fraction within physiologic ranges can transmit nearly all of their peak isometric force laterally through shearing of the endomysium. By contrast, the models predicted only limited force transmission ability through tension within the endomysium that extends from the fiber to the tendon. Moreover, when fiber volume fraction decreases to unhealthy ranges (less than 50%), the force-transmitting potential of terminating fibers through shearing of the endomysium decreases significantly. The models presented here support the hypothesis that lateral force transmission through shearing of the endomysium is an effective mode of force transmission in terminating fibers.

Preliminary observations of muscle fibre cross sectional area of flexor digitorum brevis in cadaver feet with and without claw toes

BACKGROUND

In order to facilitate normal gait, toes require to be in a rectus position during the propulsive phase. This requires a correct balance and sequence of activity of the intrinsic musculature of the feet. Alteration of this balance and sequence may lead to the development of claw toes. Atrophy of the lumbricals occurs in the development of claw toes, but it is not known if changes occur in any other intrinsic muscles, including flexor digitorum brevis. This study set out to investigate whether hypertrophic changes were evident in flexor digitorum brevis in feet with claw toes.

METHODS

Four cadaver feet were investigated, two with rectus toes and two with claw toes. Flexor digitorum brevis was removed from each, and seven anatomically significant tissue sections from each muscle were routinely processed, cut and stained. One hundred and sixty muscle fibre cross sectional areas were measured from each section.

RESULTS

The mean age of the donors was 81.5 years, and three of the four were female. Results showed that the cross sectional area of fibres from feet with claw toes was 417 μg2 significantly greater (p < 0.01) than the cross sectional area of fibres from feet with rectus toes, which was 263 μg2.

CONCLUSIONS

Although this study has several limitations, preliminary observations reveal that flexor digitorum brevis muscle fibre cross sectional area is significantly reduced in feet with claw toes. This would indicate a relationship between muscle fibre atrophy of flexor digitorum brevis and clawing of the lesser toes.

The innervation and organization of motor units in a series-fibered human muscle: the brachioradialis

We studied the innervation and organization of motor units in the brachioradialis muscle of 25 normal human subjects. We recorded intramuscular EMG signals at points separated by 15 mm along the proximodistal muscle axis during moderate isometric contractions, identified from 27 to 61 (mean 39) individual motor units per subject using EMG decomposition, and estimated the locations of the endplates and distal muscle/tendon junctions from the motor-unit action potential (MUAP) propagation patterns and terminal standing waves. In three subjects all the motor units were innervated in a single endplate zone. In the other 22 subjects, the motor units were innervated in 3-6 (mean 4) distinct endplate zones separated by 15-55 mm along the proximodistal axis. One-third of the motor units had fibers innervated in more than one zone. The more distally innervated motor units had distinct terminal waves indicating tendonous termination, while the more proximal motor units lacked terminal waves, indicating intrafascicular termination. Analysis of blocked MUAP components revealed that 19% of the motor units had at least one doubly innervated fiber, i.e., a fiber innervated in two different endplate zones by two different motoneurons, and thus belonging to two different motor units. These results are consistent with the brachioradialis muscle having a series-fibered architecture consisting of multiple, overlapping bands of muscle fibers in most individuals and a simple parallel-fibered architecture in some individuals.

Selfishness examined: Cooperation in the absence of egoistic incentives

Social dilemmas occur when the pursuit of self-interest by individuals in a group leads to less than optimal collective outcomes for everyone in the group. A critical assumption in the human sciences is that people's choices in such dilemmas are individualistic, selfish, and rational. Hence, cooperation in the support of group welfare will only occur if there are selfish incentives that convert the social dilemma into a nondilemma. In recent years, inclusive fitness theories have lent weight to such traditional views of rational selfishness on Darwinian grounds. To show that cooperation is based on selfish incentives, however, one must provide evidence that people do not cooperate without such incentives. In a series of experimental social dilemmas, subjects were instructed to make single, anonymous choices about whether or not to contribute money for a shared “bonus” that would be provided only if enough other people in the group also contributed their money. Noncontributors cited selfish reasons for their choices; contributors did not. If people are allowed to engage in discussion, they will contribute resources at high rates, frequently on irrational grounds, to promote group welfare. These findings are consistent with previous research on ingroup biasing effects that cannot be explained by “economic man” or “selfish gene” theories. An alternative explanation is that sociality was a primary factor shaping the evolution of Homo sapiens. The cognitive and affective mechanisms underlying such choices evolved under selection pressures on small groups for developing and maintaining group membership and for predicting and controlling the behavior of other group members. This sociality hypothesis organizes previously inexplicable and disparate phenomena in a Darwinian framework and makes novel predictions about human choice.