Intermuscular Interaction between Synergists in Rat Originates from both Intermuscular and Extramuscular Myofascial Force Transmission

Neuromuscular control: from a biomechanist's perspective

Understanding neural control of movement necessitates a collaborative approach between many disciplines, including biomechanics, neuroscience, and motor control. Biomechanics grounds us to the laws of physics that our musculoskeletal system must obey. Neuroscience reveals the inner workings of our nervous system that functions to control our body. Motor control investigates the coordinated motor behaviours we display when interacting with our environment. The combined efforts across the many disciplines aimed at understanding human movement has resulted in a rich and rapidly growing body of literature overflowing with theories, models, and experimental paradigms. As a result, gathering knowledge and drawing connections between the overlapping but seemingly disparate fields can be an overwhelming endeavour. This review paper evolved as a need for us to learn of the diverse perspectives underlying current understanding of neuromuscular control. The purpose of our review paper is to integrate ideas from biomechanics, neuroscience, and motor control to better understand how we voluntarily control our muscles. As biomechanists, we approach this paper starting from a biomechanical modelling framework. We first define the theoretical solutions (i.e., muscle activity patterns) that an individual could feasibly use to complete a motor task. The theoretical solutions will be compared to experimental findings and reveal that individuals display structured muscle activity patterns that do not span the entire theoretical solution space. Prevalent neuromuscular control theories will be discussed in length, highlighting optimality, probabilistic principles, and neuromechanical constraints, that may guide individuals to families of muscle activity solutions within what is theoretically possible. Our intention is for this paper to serve as a primer for the neuromuscular control scientific community by introducing and integrating many of the ideas common across disciplines today, as well as inspire future work to improve the representation of neural control in biomechanical models.

Force transmission and interactions between synergistic muscles

The classical view of muscles as independent motors has been challenged over the past decades. An alternative view has emerged in which muscles are not isolated but embedded in a three-dimensional connective tissue network that links them to adjacent muscles and other non-muscular structures in the body. Animal studies showing that the forces measured at the distal and proximal ends of a muscle are not equal have provided undisputable evidence that these connective tissue linkages are strong enough to serve as an extra pathway for muscular force transmission. In this historical review, we first introduce the terminology and anatomy related to these pathways of muscle force transmission and provide a definition for the term epimuscular force transmission. We then focus on important experimental evidence indicating mechanical interactions between synergistic muscles that may affect force transmission and/or influence the muscles' force generating capacity. We illustrate that there may exist different expressions of the highly relevant force-length properties depending on whether the force is measured at the proximal or distal tendon and depending on the dynamics of surrounding structures. Changes in length, activation level or disruption of the connective tissue of neighboring muscles, can affect how muscles interact and produce force on the skeleton. While most direct evidence is from animal experiments, studies on humans also suggest functional implications of the connective tissues surrounding muscles. These implications may explain how distant segments, which are not part of the same joint system, affect force generation at a given joint, and, in clinical conditions, explain observations from tendon transfer surgeries, where a muscle transferred to act as an antagonist continues to produce agonistic moments.

Botulinum Toxin Intervention in Cerebral Palsy-Induced Spasticity Management: Projected and Contradictory Effects on Skeletal Muscles

Spasticity, following the neurological disorder of cerebral palsy (CP), describes a pathological condition, the central feature of which is involuntary and prolonged muscle contraction. The persistent resistance of spastic muscles to stretching is often followed by structural and mechanical changes in musculature. This leads to functional limitations at the respective joint. Focal injection of botulinum toxin type-A (BTX-A) is effectively used to manage spasticity and improve the quality of life of the patients. By blocking acetylcholine release at the neuromuscular junction and causing temporary muscle paralysis, BTX-A aims to reduce spasticity and hereby improve joint function. However, recent studies have indicated some contradictory effects such as increased muscle stiffness or a narrower range of active force production. The potential of these toxin- and atrophy-related alterations in worsening the condition of spastic muscles that are already subjected to changes should be further investigated and quantified. By focusing on the effects of BTX-A on muscle biomechanics and overall function in children with CP, this review deals with which of these goals have been achieved and to what extent, and what can await us in the future.

Non-local acute stretching effects on strength performance in healthy young adults

BACKGROUND

Static stretching (SS) can impair performance and increase range of motion of a non-exercised or non-stretched muscle, respectively. An underdeveloped research area is the effect of unilateral stretching on non-local force output.

OBJECTIVE

The objective of this review was to describe the effects of unilateral SS on contralateral, non-stretched, muscle force and identify gaps in the literature.

METHODS

A systematic literature search following preferred reporting items for systematic review and meta-analyses Protocols guidelines was performed according to prescribed inclusion and exclusion criteria. Weighted means and ranges highlighted the non-local force output response to unilateral stretching. The physiotherapy evidence database scale was used to assess study risk of bias and methodological quality.

RESULTS

Unilateral stretching protocols from six studies involved 6.3 ± 2 repetitions of 36.3 ± 7.4 s with 19.3 ± 5.7 s recovery between stretches. The mean stretch-induced force deficits exhibited small magnitude effect sizes for both the stretched (-6.7 ± 7.1%, d = -0.35: 0.01 to -1.8) and contralateral, non-stretched, muscles (-4.0 ± 4.9%, d = , 0.22: 0.08 to 1.1). Control measures exhibited trivial deficits.

CONCLUSION

The limited literature examining non-local effects of prolonged SS revealed that both the stretched and contralateral, non-stretched, limbs of young adults demonstrate small magnitude force deficits. However, the frequency of studies with these effects were similar with three measures demonstrating deficits, and four measures showing trivial changes. These results highlight the possible global (non-local) effects of prolonged SS. Further research should investigate effects of lower intensity stretching, upper versus lower body stretching, different age groups, incorporate full warm-ups, and identify predominant mechanisms among others.

Non-local Acute Passive Stretching Effects on Range of Motion in Healthy Adults: A Systematic Review with Meta-analysis

BACKGROUND

Stretching a muscle not only increases the extensibility or range of motion (ROM) of the stretched muscle or joint but there is growing evidence of increased ROM of contralateral and other non-local muscles and joints.

OBJECTIVE

The objective of this meta-analysis was to quantify crossover or non-local changes in passive ROM following an acute bout of unilateral stretching and to examine potential dose-response relations.

METHODS

Eleven studies involving 14 independent measures met the inclusion criteria. The meta-analysis included moderating variables such as sex, trained state, stretching intensity and duration.

RESULTS

The analysis revealed that unilateral passive static stretching induced moderate magnitude (standard mean difference within studies: SMD: 0.86) increases in passive ROM with non-local, non-stretched joints. Moderating variables such as sex, trained state, stretching intensity, and duration did not moderate the results. Although stretching duration did not present statistically significant differences, greater than 240-s of stretching (SMD: 1.24) exhibited large magnitude increases in non-local ROM compared to moderate magnitude improvements with shorter (< 120-s: SMD: 0.72) durations of stretching.

CONCLUSION

Passive static stretching of one muscle group can induce moderate magnitude, global increases in ROM. Stretching durations greater than 240 s may have larger effects compared with shorter stretching durations.

Fascial or Muscle Stretching? A Narrative Review

Stretching exercises are integral part of the rehabilitation and sport. Despite this, the mechanism behind its proposed effect remains ambiguous. It is assumed that flexibility increases, e.g., action on muscle and tendon, respectively, but this is not always present in the stretching protocol of the exercises used. Recently, the fasciae have increased popularity and seems that they can have a role to define the flexibility and the perception of the limitation of the maximal range of motion (ROM). Deep fascia is also considered a key element to transmit load in parallel bypassing the joints, transmitting around 30% of the force generated during a muscular contraction. So, it seems impossible dividing the action of the muscles from the fasciae, but they have to be considered as a “myofascial unit”. The purpose of this manuscript is to evaluate the mechanical behavior of muscles, tendons, and fasciae to better understand how they can interact during passive stretching. Stress-strain values of muscle, tendon and fascia demonstrate that during passive stretching, the fascia is the first tissue that limit the elongation, suggesting that fascial tissue is probably the major target of static stretching. A better understanding of myofascial force transmission, and the study of the biomechanical behavior of fasciae, with also the thixotropic effect, can help to design a correct plan of stretching.

A Review of the Theoretical Fascial Models: Biotensegrity, Fascintegrity, and Myofascial Chains

The fascial tissue includes solid and liquid fascia (body fluids such as blood and lymph). The fascia's nomenclature is the subject of debate in the academic world, as it is classified starting from different scientific perspectives. This disagreement is not a brake but is, in reality, the real wealth of research, the multidisciplinarity of thought and knowledge that leads to a deeper understanding of the topic. Another topic of discussion is the fascial model to conceptualize the human body, that is, how the fascial tissue fits into the living. Currently, there are some models: biotensegrity, fascintegrity, and myofascial chains. Biotensegrity is a mechanical model, which takes into consideration the solid fascia; fascintegrity considers the solid and the liquid fascia. Myofascial chains converge attention on the movement and transmission of force in the muscle continuum. The article is a reflection on fascial models and how these are theoretical-scientific visions that need to be further investigated.

A comparative multi-site and whole-body assessment of fascia in the horse and dog: a detailed histological investigation

Fascia in the veterinary sciences is drawing attention, such that physiotherapists and animal practitioners are now applying techniques based on the concept of fascia studies in humans. A comprehensive study of fascia is therefore needed in animals to understand the arrangement of the fascial layers in an unguligrade horse and a digitigrade dog. This study has examined the difference between the horse and the dog fascia at specific regions, in terms of histology, and has compared it with the human model. Histological examinations show that in general the fascia tissue of the horse exhibits a tight and dense composition, while in the dog it is looser and has non-dense structure. Indeed, equine fascia appears to be different from both canine fascia and the human fascia model, whilst canine fascia is very comparable to the human model. Although regional variations were observed, the superficial fascia (fascia superficialis) in the horse was found to be trilaminar in the trunk, yet multilayered in the dog. Moreover, crimping of collagen fibers was more visible in the horse than the dog. Blood vessels and nerves were present in the loose areolar tissue of the superficial and the profound compartment of hypodermis. The deep fascia (fascia profunda) in the horse was thick and tightly attached to the underlying muscle, while in the dog the deep fascia was thin and loosely attached to underlying structures. Superficial and deep fascia fused in the extremities. In conclusion, gross dissection and histology have revealed species variations that are related to the absence or presence of the superficial adipose tissue, the retinacula cutis superficialis, the localization and amount of elastic fibers, as well as the ability to slide and glide between the different layers. Further research is now needed to understand in more detail whether these differences have an influence on the biomechanics, movements and proprioception of these animals.

Myofascial chains of the upper limb: A systematic review of anatomical studies

The presence of structural in-series continuity between skeletal muscles has been confirmed in the trunk and lower extremity. However, it is not yet clear whether the same architecture can be found in the upper limb. Therefore, the aim of the present study was to review the available literature considering the existence of myofascial chains in the shoulder-arm region. Two independent investigators performed a systematic literature search using MEDLINE (PubMed) and Google Scholar (each 1900-2019). Peer-reviewed anatomical dissection studies reporting myofascial in-series continuity in the upper extremity were included. The methodological quality of the included studies was assessed by the QUACS scale. Thirteen studies were included in the review. Analysis of these papers led to the identification of three myofascial chains: the ventral arm chain (pectoralis major, brachial fascia/biceps brachii, flexor carpi ulnaris/brachioradialis/supinator, based on five studies); the lateral arm chain (trapezius, deltoideus, lateral intermuscular septum/brachialis, brachioradialis, four studies); and the dorsal arm chain (latissiumus dorsi/teres minor/infraspinatus, triceps brachii, anconeus, extensor carpi ulnaris, six studies). There is good evidence for direct serial tissue continuity extending from the neck and shoulder region to the forearm. Despite this intriguing finding, which could have implications for health professionals and the treatment of musculoskeletal disorders, further research is needed to establish the mechanical relevance of the identified myofascial chains. Clin. Anat. 32:934-940, 2019. © 2019 Wiley Periodicals, Inc.

Significance of epimuscular myofascial force transmission under passive muscle conditions

In the past 20 yr, force transmission via connective tissue linkages at the muscle belly surface, called epimuscular myofascial force transmission, has been studied extensively. In this article, the effects of epimuscular linkages under passive muscle conditions are reviewed. Several animal studies that included direct (invasive) measurements of force transmission have shown that different connective tissue structures serve as an epimuscular pathway and that these tissues have sufficient stiffness, especially at supraphysiological muscle lengths and relative positions, to transmit substantial passive forces (up to 15% of active optimal force). Exact values of lumped tissue stiffness for different connective tissue structures have not yet been estimated. Experiments using various imaging techniques (ultrasound, MRI, shear wave elastography) have yielded some, but weak, evidence of epimuscular myofascial force transmission for passive muscles in humans. At this point, the functional consequences of epimuscular pathways for muscle and joint mechanics in the intact body are still unknown. Potentially, however, these pathways may affect sensory feedback and, thereby, neuromuscular control. In addition, altered epimuscular force transmission in pathological conditions may also contribute to changes in passive range of joint motion.

Timing and extent of finger force enslaving during a dynamic force task cannot be explained by EMG activity patterns

Finger enslaving is defined as the inability of the fingers to move or to produce force independently. Such finger enslaving has predominantly been investigated for isometric force tasks. The aim of this study was to assess whether the extent of force enslaving is dependent on relative finger movements. Ten right-handed subjects (22–30 years) flexed the index finger while counteracting constant resistance forces (4, 6 and 8 N) orthogonal to the fingertip. The other, non-instructed fingers were held in extension. EMG activities of the mm. flexor digitorum superficialis (FDS) and extensor digitorum (ED) in the regions corresponding to the index, middle and ring fingers were measured. Forces exerted by the non-instructed fingers increased substantially (by 0.2 to 1.4 N) with flexion of the index finger, increasing the enslaving effect with respect to the static, pre-movement phase. Such changes in force were found 260–370 ms after the initiation of index flexion. The estimated MCP joint angle of the index finger at which forces exerted by the non-instructed fingers started to increase varied between 4° and 6°. In contrast to the finger forces, no significant changes in EMG activity of the FDS regions corresponding to the non-instructed fingers upon index finger flexion were found. This mismatch between forces and EMG of the non-instructed fingers, as well as the delay in force development are in agreement with connective tissue linkages being slack when the positions of the fingers are similar, but pulled taut when one finger moves relative to the others. Although neural factors cannot be excluded, our results suggest that mechanical connections between muscle-tendon structures were (at least partly) responsible for the observed increase in force enslaving during index finger flexion.

The deep fascia and retinacula of the equine forelimb - structure and innervation

Recent advances in human fascia research have shed new light on the role of the fascial network in movement perception and coordination, transmission of muscle force, and integrative function in body biomechanics. Evolutionary adaptations of equine musculoskeletal apparatus that assure effective terrestrial locomotion are employed in equestrianism, resulting in the wide variety of movements in performing horses, from sophisticated dressage to jumping and high-speed racing. The high importance of horse motion efficiency in the present-day equine industry indicates the significance of scientific knowledge of the structure and physiology of equine fasciae. In this study, we investigated the structure and innervation of the deep fascia of the equine forelimb by means of anatomical dissection, histology and immunohistochemistry. Macroscopically, the deep fascia appears as a dense, glossy and whitish lamina of connective tissue continuous with its fibrous reinforcements represented by extensor and flexor retinacula. According to the results of our histological examination, the general structure of the equine forelimb fascia corresponds to the characteristics of the human deep fasciae of the limbs. Although we did find specific features in all sample types, the general composition of all examined fascial tissues follows roughly the same scheme. It is composed of dense, closely packed collagen fibers organized in layers of thick fibrous bundles with sparse elastic fibers. This compact tissue is covered from both internal and external sides by loosely woven laminae of areolar connective tissue where elastic fibers are mixed with collagen. Numerous blood vessels running within the loose connective tissue contribute to the formation of regular vascular network throughout the compact layer of the deep fascia and retinacula. We found nerve fibers of different calibers in all samples analyzed. The fibers are numerous in the areolar connective tissue and near the blood vessels but scarce in the compact layers of collagen. We did not observe any Ruffini, Pacini or Golgi-Mazzoni corpuscles. In conclusion, the multilayered composition of compact bundles of collagen, sparse elastic fibers in the deep fascia and continuous transition into retinacula probably facilitate resistance to gravitational forces and volume changes during muscle contraction as well as transmission of muscle force during movement. However, further research focused on innervation is needed to clarify whether the deep fascia of the equine forelimb plays a role in proprioception and movement coordination.

Is remote stretching based on myofascial chains as effective as local exercise? A randomised-controlled trial

Lower limb stretching based on myofascial chains has been demonstrated to increase cervical range of motion (ROM) in the sagittal plane. It is, however, unknown whether such remote exercise is as effective as local stretching. To resolve this research deficit, 63 healthy participants (36 ± 13 years, ♂32) were randomly assigned to one of three groups: remote stretching of the lower limb (LLS), local stretching of the cervical spine (CSS) or inactive control (CON). Prior (M1), immediately post (M2) and 5 min following intervention (M3), maximal cervical ROM was assessed. Non-parametric data analysis (Kruskal-Wallis tests and adjusted post hoc Dunn tests) revealed significant differences between the disposed conditions. With one exception (cervical spine rotation after CSS at M2, P > .05), both LLS and CSS increased cervical ROM compared to the control group in all movement planes and at all measurements (P < .05). Between LLS and CSS, no statistical differences were found (P > .05). Lower limb stretching based on myofascial chains induces similar acute improvements in cervical ROM as local exercise. Therapists might consequently consider its use in programme design. However, as the attained effects do not seem to be direction-specific, further research is warranted in order to provide evidence-based recommendations.

Limited mechanical effects of intermuscular myofascial connections within the intact rat anterior crural compartment

Skeletal muscles of the rat anterior crural compartment are mechanically connected by epimuscular myofascial connections, but the relevance for mechanical muscle function within physiological ranges of joint motion is unclear. We evaluated the net effect at the ankle joint of epimuscular myofascial connections between tibialis anterior (TA) and extensor digitorum longus (EDL) muscles in the rat (n=8) and determined which anatomical structures may mediate such epimuscular mechanical interactions. We assessed (1) effects of knee angle (i.e. changes in EDL length and position relative to TA) and interactions of knee angle with fasciotomy and proximal EDL tenotomy on TA ankle moment and (2) the effect of knee angle on TA and EDL ankle moment summation. Knee angle was varied between 60° and 130°. Ankle angle was kept constant (90°). TA and EDL were excited individually and simultaneously (TA&EDL). The mathematical sum of individual TA and EDL moments was compared with the moment exerted by TA&EDL to assess the extent of non-additive ankle moment summation. Magnitude of TA ankle moment was not affected by knee angle, but frontal plane moment direction was. However, dissections indicated that this was not caused by the compartmental fascia or EDL length changes. Moment summation was non-additive in magnitude (+1.1±1.1% mean±s.d.) and frontal plane direction. The latter was affected by knee angle and ranged from +0.2±0.3° at 60° to +1.1±0.6° at 130°. As the net effects found were very limited, we conclude that myofascial connections between muscles in the anterior crural compartment have limited mechanical relevance during normal movement.

Altered mechanical interaction between rat plantar flexors due to changes in intermuscular connectivity

Connective tissue formation following muscle injury and remedial surgery may involve changes in the stiffness and configuration of the connective tissues linking adjacent muscles. We investigated changes in mechanical interaction of muscles by implanting either a tissue-integrating mesh (n = 8) or an adhesion barrier (n = 8) to respectively increase or decrease the intermuscular connectivity between soleus muscle (SO) and the lateral gastrocnemius and plantaris complex (LG+PL) of the rat. As a measure of mechanical interaction, changes in SO tendon forces and proximal-distal LG+PL force differences in response to lengthening LG+PL proximally were assessed 1 and 2 weeks post-surgery. The extent of mechanical interaction was doubled 1 week post-implantation of the tissue-integrating mesh compared to an unaffected compartment (n = 8), and was more than four times higher 2 weeks post-surgery. This was found only for maximally activated muscles, but not when passive. Implanting the adhesion barrier did not result in a reduction of the mechanical interaction between these muscles. Our findings indicate that the ratio of force transmitted via myofascial, rather than myotendinous pathways, can increase substantially when the connectivity between muscles is enhanced. This improves our understanding of the consequences of connective tissue formation at the muscle boundary on skeletal muscle function.

No functionally relevant mechanical effects of epimuscular myofascial connections between rat ankle plantar flexors

Triceps surae muscles are mechanically connected by the shared Achilles tendon and by epimuscular myofascial connections. We aimed to assess effects of proximal lengthening of gastrocnemius (GA) and plantaris muscles, imposed by changes in knee angle, on (i) the magnitude and direction of the 3D ankle moment exerted by the soleus (SO) muscle, and on (ii) mechanical interaction between ankle plantar flexor muscles during co-activation of GA muscle, in the rat (n=9). Ankle angle was kept constant (90°), while knee angle was varied between 60° and 130°. At each knee angle, SO was excited individually as well as simultaneously with GA (SO&GA). The mathematical sum of individual SO and GA ankle moments was compared with the ankle moment exerted by SO&GA to assess nonlinear summation. Knee angle did not affect the magnitude of the SO ankle moment (p=0.695). Moment directions in the transverse (p=0.050) and frontal (p=0.008) planes were affected by knee angle, but dissection indicated that this was not caused by length changes of the two-joint synergistic muscles. Nonlinear summation was found in the magnitude (-1.4±1.9%, mean±s.d., p&lt;0.001) and in the frontal plane vector direction of the ankle moment (0.13±0.23°, p=0.003), however, the extent did not change with knee angle. While SO&GA contraction increased Achilles tendon length compared to rest, this length was not knee angle dependent (p=0.649). Despite that intermuscular force transmission per se cannot be excluded, we conclude that in vivo the mechanical effects of epimuscular myofascial connections between rat ankle plantar flexors are not functionally relevant.

Intermittent stretch reduces force and central drive more than continuous stretch

INTRODUCTION

The relative contributions of central versus peripheral factors to the force loss induced by acute continuous and intermittent plantarflexor stretches were studied.

METHODS

Eighteen healthy young men with no apparent tissue stiffness limitations randomly performed 1) one 5-min stretch (continuous stretch [CS]), 2) five 1-min stretches (intermittent stretch [IS]), and 3) a control condition, on three separate days. The stretches were constant-torque ankle stretches performed on an isokinetic dynamometer. Gastrocnemius medialis oxygenation status was quantified during stretch using near-infrared spectroscopy. Measures of isometric plantarflexor peak torque (Tpeak), voluntary activation (%VA; interpolated twitch technique), EMG amplitude normalized by Mmax (EMG:M), V-wave amplitude, and excitation-contraction (E-C) coupling efficiency (torque ratio between 20- and 80-Hz tetanic stimulations [20:80]) were taken before, immediately, and 15 and 30 min after each condition.

RESULTS

IS caused substantial cyclic variations in tissue oxygenation, but CS resulted in a greater decrease in oxyhemoglobin concentration. Voluntary Tpeak decreased more after IS (-23.8%) than CS (-14.3%) and remained significantly depressed until 30 min after IS only (-5.6%). EMG:M (-27.7%) and %VA (-15.9%) were reduced only after IS. After CS and IS, the magnitude of decrease in Tpeak was correlated with decreases in EMG:M (r = 0.81 and 0.89, respectively), %VA (r = 0.78 and 0.93), and V-wave (r = 0.51, only after IS). Tetanic torque values (20 and 80 Hz) were decreased after IS (-13.1% and -6.4%, respectively) and CS (-10.9% and -6.7%, respectively), but 20:80 was not different from the control group.

CONCLUSION

These results suggest that IS reduced Tpeak more than CS, and these reductions were strongly associated with a depression in central drive.

Tendon and fascial structure contributions to knee muscle excursions and knee joint displacement

BACKGROUND

Semitendinosus and gracilis muscles whose tendons are used in surgical reconstruction of the anterior cruciate ligament maintain their contractile ability, and a limited decrease of hamstring muscles force is observed postoperatively despite important changes. The goal was to quantify the influence of the myofascial structures on excursions and moment arms of knee muscles to attempt explaining the above-mentioned post-surgical observations.

METHODS

Hamstring harvesting procedures were performed by a senior orthopaedic surgeon on seven lower limbs from fresh-frozen specimens. Femoro-tibial kinematics and tendons excursion were simultaneously recorded at each steps of the surgery.

FINDINGS

No significant difference was demonstrated for excursions and moment arms after tenotomies and gracilis tendon harvesting (P≥0.05). The first significant semitendinosus excursion (P<1.17×10(-4)) and moment arm (P<6.88×10(-5)) decrease was observed after semitendinosus tendon harvesting (46% of the initial excursion).

INTERPRETATION

Gracilis and semitendinosus myofascial pathway is crucial for force transmission towards the knee joint.

Muscle-specific changes in length-force characteristics of the calf muscles in the spastic Han-Wistar rat

The purpose of the present study was to investigate muscle mechanical properties and mechanical interaction between muscles in the lower hindlimb of the spastic mutant rat. Length-force characteristics of gastrocnemius (GA), soleus (SO), and plantaris (PL) were assessed in anesthetized spastic and normally developed Han-Wistar rats. In addition, the extent of epimuscular myofascial force transmission between synergistic GA, SO, and PL, as well as between the calf muscles and antagonistic tibialis anterior (TA), was investigated. Active length-force curves of spastic GA and PL were narrower with a reduced maximal active force. In contrast, active length-force characteristics of spastic SO were similar to those of controls. In reference position (90° ankle and knee angle), higher resistance to ankle dorsiflexion and increased passive stiffness was found for the spastic calf muscle group. At optimum length, passive stiffness and passive force of spastic GA were decreased, whereas those of spastic SO were increased. No mechanical interaction between the calf muscles and TA was found. As GA was lengthened, force from SO and PL declined despite a constant muscle-tendon unit length of SO and PL. However, the extent of this interaction was not different in spastic rats. In conclusion, the effects of spasticity on length-force characteristics were muscle specific. The changes observed for GA and PL muscles are consistent with the changes in limb mechanics reported for human patients. Our results indicate that altered mechanics in spastic rats cannot be attributed to differences in mechanical interaction, but originate from individual muscular structures.

Intramuscular connective tissue differences in spastic and control muscle: a mechanical and histological study

Cerebral palsy (CP) of the spastic type is a neurological disorder characterized by a velocity-dependent increase in tonic stretch reflexes with exaggerated tendon jerks. Secondary to the spasticity, muscle adaptation is presumed to contribute to limitations in the passive range of joint motion. However, the mechanisms underlying these limitations are unknown. Using biopsies, we compared mechanical as well as histological properties of flexor carpi ulnaris muscle (FCU) from CP patients (n = 29) and healthy controls (n = 10). The sarcomere slack length (mean 2.5 µm, SEM 0.05) and slope of the normalized sarcomere length-tension characteristics of spastic fascicle segments and single myofibre segments were not different from those of control muscle. Fibre type distribution also showed no significant differences. Fibre size was significantly smaller (1933 µm2, SEM 190) in spastic muscle than in controls (2572 µm2, SEM 322). However, our statistical analyses indicate that the latter difference is likely to be explained by age, rather than by the affliction. Quantities of endomysial and perimysial networks within biopsies of control and spastic muscle were unchanged with one exception: a significant thickening of the tertiary perimysium (3-fold), i.e. the connective tissue reinforcement of neurovascular tissues penetrating the muscle. Note that this thickening in tertiary perimysium was shown in the majority of CP patients, however a small number of patients (n = 4 out of 23) did not have this feature. These results are taken as indications that enhanced myofascial loads on FCU is one among several factors contributing in a major way to the aetiology of limitation of movement at the wrist in CP and the characteristic wrist position of such patients.

Short-term effect of crural fasciotomy on kinematic variability and propulsion during level locomotion

Treadmill locomotion can be characterized by consistent step-to-step kinematics despite the redundant degrees of freedom. The authors investigated the effect of disrupting the crural fascia in decerebrate cats to determine if the crural fascia contributed to kinematic variability and propulsion in the limb. Crural fasciotomy resulted in statistically significant decreases in velocity and acceleration in the joint angles during level walking, before, during, and after paw-off, particularly at the ankle. A further finding was an increase in variance of the limb segment trajectories in the frontal plane. The crural fascia therefore provides force transmission and reduction in kinematic variability to the limb during locomotion.

Dissection of a single rat muscle-tendon complex changes joint moments exerted by neighboring muscles: implications for invasive surgical interventions

The aim of this paper is to investigate mechanical functioning of a single skeletal muscle, active within a group of (previously) synergistic muscles. For this purpose, we assessed wrist angle-active moment characteristics exerted by a group of wrist flexion muscles in the rat for three conditions: (i) after resection of the upper arm skin; (ii) after subsequent distal tenotomy of flexor carpi ulnaris muscle (FCU); and (iii) after subsequent freeing of FCU distal tendon and muscle belly from surrounding tissues (MT dissection). Measurements were performed for a control group and for an experimental group after recovery (5 weeks) from tendon transfer of FCU to extensor carpi radialis (ECR) insertion. To assess if FCU tenotomy and MT dissection affects FCU contributions to wrist moments exclusively or also those of neighboring wrist flexion muscles, these data were compared to wrist angle-moment characteristics of selectively activated FCU. FCU tenotomy and MT dissection decreased wrist moments of the control group at all wrist angles tested, including also angles for which no or minimal wrist moments were measured when activating FCU exclusively. For the tendon transfer group, wrist flexion moment increased after FCU tenotomy, but to a greater extent than can be expected based on wrist extension moments exerted by selectively excited transferred FCU. We conclude that dissection of a single muscle in any surgical treatment does not only affect mechanical characteristics of the target muscle, but also those of other muscles within the same compartment. Our results demonstrate also that even after agonistic-to-antagonistic tendon transfer, mechanical interactions with previously synergistic muscles do remain present.

Contribution of central vs. peripheral factors to the force loss induced by passive stretch of the human plantar flexors

The purpose of the present research was to identify the contribution of central vs. peripheral factors to the force loss after passive muscle stretching. Thirteen men randomly performed both a 5-min constant-torque stretch of the plantar flexors on an isokinetic dynamometer and a resting condition on 2 separate days. The triceps surae electromyogram (EMG) was recorded simultaneously with plantar flexor isometric torque. Measures of central drive, including the EMG amplitude normalized to the muscle compound action potential amplitude (EMG/M), percent voluntary activation and first volitional wave amplitude, and measures of peripheral function, including the twitch peak torque, 20-to-80-Hz tetanic torque ratio and torque during 20-Hz stimulation preceded by a doublet, were taken before and immediately and 15 min after each condition. Peak torque (-15.7%), EMG/M (-8.2%), and both twitch (-9.4%) and 20-Hz peak torques (-11.5%) were reduced immediately after stretch but recovered by 15 min. There were strong correlations between the torque loss and the reductions in central drive parameters (r = 0.65-0.93). Torque recovery was also strongly correlated with the recovery in EMG/M and percent voluntary activation (r = 0.77-0.81). The moderate decreases in measures of peripheral function were not related to the torque loss or recovery. These results suggest that 1) central factors were strongly related to the torque reduction immediately after stretch and during torque recovery; and 2) the muscle's contractile capacity was moderately reduced, although these changes were not associated with the torque reduction, and changes in excitation-contraction coupling efficiency were not observed.

Myofascial force transmission between the human soleus and gastrocnemius muscles during passive knee motion

The plantarflexors of the lower limb are often assumed to act as independent actuators, but the validity of this assumption is the subject of considerable debate. This study aims to determine the degree to which passive changes in gastrocnemius muscle length, induced by knee motion, affect the tension in the adjacent soleus muscle. A second aim is to quantify the magnitude of myofascial passive force transmission between gastrocnemius and adjacent soleus. Fifteen healthy volunteers participated. Simultaneous ultrasound images of the gastrocnemius and soleus muscles were obtained during passive knee flexion (0-90°), while keeping the ankle angle fixed at either 70° or 115°. Image correlation analysis was used to quantify muscle fascicle lengths in both muscles. The data show that the soleus muscle fascicles elongate significantly during gastrocnemius shortening. The approximate change in passive soleus force as a result of the observed change in fascicle length was estimated and appears to be <5 N, but this estimate is sensitive to the assumed slack length of soleus.

Effects of knee joint angle on global and local strains within human triceps surae muscle: MRI analysis indicating in vivo myofascial force transmission between synergistic muscles

Purpose

Mechanical interactions between muscles have been shown for in situ conditions. In vivo data for humans is unavailable. Global and local length changes of calf muscles were studied to test the hypothesis that local strains may occur also within muscle for which global strain equals zero.

Methods

For determination of globally induced strain in m. gastrocnemius in dissected human cadavers several knee joint angles were imposed, while keeping ankle joint angle constant and measuring its muscle–tendon complex length changes. In vivo local strains in both gastrocnemius and soleus muscles were calculated using MRI techniques in healthy human volunteers comparing images taken at static knee angles of 173° and 150°.

Results

Imposed global strains on gastrocnemius were much smaller than local strains. High distributions of strains were encountered, e.g. overall lengthened muscle contains locally lengthened, as well as shortened areas within it. Substantial strains were not limited to gastrocnemius, but were found also in synergistic soleus muscle, despite the latter muscle–tendon complex length remaining isometric (constant ankle angle: i.e. global strain = 0), as it does not cross the knee. Based on results of animal experiments this effect is ascribed to myofascial connections between these synergistic muscles. The most likely pathway is the neurovascular tract within the anterior crural compartment (i.e. the collagen reinforcements of blood vessels, lymphatics and nerves). However, direct intermuscular transmission of force may also occur via the perimysium shared between the two muscles.

Conclusions

Global strains imposed on muscle (joint movement) are not good estimators of in vivo local strains within it: differing in magnitude, as well as direction of length change. Substantial mechanical interaction occurs between calf muscles, which is mediated by myofascial force transmission between these synergistic muscles. This confirms conclusions of previous in situ studies in experimental animals and human patients, for in vivo conditions in healthy human subjects.

Flexor carpi ulnaris tenotomy alone does not eliminate its contribution to wrist torque

BACKGROUND

Flexor carpi ulnaris muscle tenotomy and transfer to the extensor side of the wrist are common procedures used to improve wrist position and dexterity in patients with cerebral palsy. Our aim was to determine whether this muscle still influences wrist torque even after tenotomy of its distal tendon.

METHODS

Intra-operatively, we determined in vivo maximal wrist torque in hemiplegic cerebral palsy patients (n=15, mean age 17 years) in three conditions: 1) with the arm and the muscle intact; 2) after tenotomy of the flexor carpi ulnaris just proximal to the pisiform bone, with complete release from its insertion; and 3) after careful dissection of the belly of the muscle from its fascial surroundings up until approximately halfway its length.

FINDINGS

After tenotomy of the flexor carpi ulnaris muscle, the maximal wrist torque decreased 18% whereas dissection of the muscle resulted in an additional decrease of 18%.

INTERPRETATION

We conclude that despite the tenotomy of its distal tendon, the flexor carpi ulnaris still contributes to the flexion torque at the wrist through myofascial force transmission. Quantification of this phenomenon will help in the study of the effects of fascial dissection on the functional results of tendon transfer surgery.

Force transmission between synergistic skeletal muscles through connective tissue linkages

The classic view of skeletal muscle is that force is generated within its muscle fibers and then directly transmitted in-series, usually via tendon, onto the skeleton. In contrast, recent results suggest that muscles are mechanically connected to surrounding structures and cannot be considered as independent actuators. This article will review experiments on mechanical interactions between muscles mediated by such epimuscular myofascial force transmission in physiological and pathological muscle conditions. In a reduced preparation, involving supraphysiological muscle conditions, it is shown that connective tissues surrounding muscles are capable of transmitting substantial force. In more physiologically relevant conditions of intact muscles, however, it appears that the role of this myofascial pathway is small. In addition, it is hypothesized that connective tissues can serve as a safety net for traumatic events in muscle or tendon. Future studies are needed to investigate the importance of intermuscular force transmission during movement in health and disease.

In situ estimation of tendon material properties: differences between muscles of the feline hindlimb

Recent experiments to characterize the short-range stiffness (SRS)-force relationship in several cat hindlimb muscles suggested that the there are differences in the tendon elastic moduli across muscles [Cui, L., Perreault, E.J., Maas, H., Sandercock, T.G., 2008. Modeling short-range stiffness of feline lower hindlimb muscles. J. Biomech. 41 (9), 1945-1952.]. Those conclusions were inferred from whole muscle experiments and a computational model of SRS. The present study sought to directly measure tendon elasticity, the material property most relevant to SRS, during physiological loading to confirm the previous modeling results. Measurements were made from the medial gastrocnemius (MG), tibialis anterior (TA) and extensor digitorum longus (EDL) muscles during loading. For the latter, the model indicated a substantially different elastic modulus than for MG and TA. For each muscle, the stress-strain relationship of the external tendon was measured in situ during the loading phase of isometric contractions conducted at optimum length. Young's moduli were assessed at equal strain levels (1%, 2% and 3%), as well as at peak strain. The stress-strain relationship was significantly different between EDL and MG/TA, but not between MG and TA. EDL had a more apparent toe region (i.e., lower Young's modulus at 1% strain), followed by a more rapid increase in the slope of the stress-strain curve (i.e., higher Young's modulus at 2% and 3% strain). Young's modulus at peak strain also was significantly higher in EDL compared to MG/TA, whereas no significant difference was found between MG and TA. These results indicate that during natural loading, tendon Young's moduli can vary considerably across muscles. This creates challenges to estimating muscle behavior in biomechanical models for which direct measures of tendon properties are not available.

The pectoral fascia: anatomical and histological study

AIM

Analysis of the pectoral fascia from a macroscopic and histological point of view.

RESULTS

The pectoral fascia appears as a thin collagen layer (mean thickness of 297 microm) formed by undulated collagen fibres and many elastic fibres, within which small nerves are highlighted. Numerous septa detach from its internal surface, creating an intimate connection between the fascia and the pectoralis major muscle.

DISCUSSION

The pectoral fascia and the pectoralis major muscle should be considered together, given that the anatomical base is effectively a myofascial unit, term that defines the muscles and the fascia of a specific region that have a precise functional organization. The capacity of force transmission between the inferior and superior limbs needs to be attributed to this entire myofascial complex. We hypothesize that the superficial, large muscles of the trunk developed inside the superficial layer of the deep fascia to enhance modulation of tension transmission between the different segments of the body.

Are skeletal muscles independent actuators? Force transmission from soleus muscle in the cat

It is unclear if skeletal muscles act mechanically as independent actuators. The purpose of the present study was to investigate force transmission from soleus (SO) muscle for physiological lengths as well as relative positions in the intact cat hindlimb. We hypothesized that force transmission from SO fibers will be affected by length changes of its two-joint synergists. Ankle plantar flexor moment on excitation of the SO was measured for various knee angles (70–140°). This involved substantial length changes of gastrocnemius and plantaris muscles. Ankle angle was kept constant (80°-90°). However, SO ankle moment was not significantly affected by changes in knee angle; neither were half-relaxation time and the maximal rate of relaxation ( P > 0.05). Following tenotomy, SO ankle moment decreased substantially (55 ± 16%) but did not reach zero, indicating force transmission via connective tissues to the Achilles tendon (i.e., epimuscular myofascial force transmission). During contraction SO muscle shortened to a much greater extent than in the intact case (16.0 ± 0.6 vs. 1.0 ± 0.1 mm), which resulted in a major position shift relative to its synergists. If the SO was moved back to its position corresponding to the intact condition, SO ankle moment approached zero, and most muscle force was exerted at the distal SO tendon. Our results also suggested that in vivo the lumped intact tissues linking SO to its synergists are slack or are operating on the toe region of the stress-strain curve. Thus, within the experimental conditions of the present study, the intact cat soleus muscle appears to act mechanically as an independent actuator.

Passive mechanical properties of human gastrocnemius muscle tendon units, muscle fascicles and tendons in vivo

This study provides the first in vivo measures of the passive length-tension properties of relaxed human muscle fascicles and their tendons. A new method was used to derive passive length-tension properties of human gastrocnemius muscle-tendon units from measures of ankle stiffness obtained at a range of knee angles. Passive length-tension curves of the muscle-tendon unit were then combined with ultrasonographic measures of muscle fascicle length and pennation to determine passive length-tension curves of the muscle fascicles and tendons. Mean slack lengths of the fascicles, tendons and whole muscle-tendon units were 3.3+/-0.5 cm, 39.5+/-1.6 cm and 42.3+/-1.5 cm, respectively (means +/- s.d., N=6). On average, the muscle-tendon units were slack (i.e. their passive tension was zero) over the shortest 2.3+/-1.2 cm of their range. With combined changes of knee and ankle angles, the maximal increase in length of the gastrocnemius muscle-tendon unit above slack length was 6.7+/-1.9 cm, of which 52.4+/-11.7% was due to elongation of the tendon. Muscle fascicles and tendons underwent strains of 86.4+/-26.8% and 9.2+/-4.1%, respectively, across the physiological range of lengths. We conclude that the relaxed human gastrocnemius muscle-tendon unit falls slack over about one-quarter of its in vivo length and that muscle fascicle strains are much greater than tendon strains. Nonetheless, because the tendons are much longer than the muscle fascicles, tendons contribute more than half of the total compliance of the muscle-tendon unit.

Epimuscular myofascial force transmission between antagonistic and synergistic muscles can explain movement limitation in spastic paresis

Details and concepts of intramuscular, extramuscular and intermuscular myofascial force transmission are reviewed. Some new experimental data are added regarding myofascial force transmission between antagonistic muscles across the interosseal membrane of the lower hind limb of the rat. Combined with other result presented in this issue, it can be concluded that myofascial force transmission occurs between all muscles within a limb segment. This means that force generated within sarcomeres of an antagonistic muscle may be exerted at the tendon of target muscle or its synergists. Some, in vivo, but initial indications for intersegmental myofascial force transmission are discussed. The concept of myofascial force transmission as an additional load on the muscle proved to be fruitful in the analysis of its muscular effects. In spastic paresis and for healthy muscles distal myofascial loads are often encountered, but cannot fully explain the movement limitations in spastic paresis. Therefore, the concept of simultaneous and opposing myofascial loads is analyzed and used to formulate a hypothesis for explaining the movement limitation: Myofascially transmitted antagonistic force is borne by the spastic muscle, but subsequently transmitted again to distal tendons of synergistic muscles.

Myofascial force transmission between antagonistic rat lower limb muscles: Effects of single muscle or muscle group lengthening

Effects of lengthening of the whole group of anterior crural muscles (tibialis anterior and extensor hallucis longus muscles (TA+EHL) and extensor digitorum longus (EDL)) on myofascial interaction between synergistic EDL and TA+EHL muscles, and on myofascial force transmission between anterior crural and antagonistic peroneal muscles, were investigated. All muscles were either passive or maximally active. Peroneal muscles were kept at a constant muscle tendon complex length. Either EDL or all anterior crural muscles were lengthened so that effects of lengthening of TA+EHL could be analyzed. For both lengthening conditions, a significant difference in proximally and distally measured EDL passive and active forces, indicative of epimuscular myofascial force transmission, was present. However, added lengthening of TA+EHL significantly affected the magnitude of the active and passive load exerted on EDL. For the active condition, the direction of the epimuscular load on EDL was affected; at all muscle lengths a proximally directed load was exerted on EDL, which decreased at higher muscle lengths. Lengthening of anterior crural muscles caused a 26% decrease in peroneal active force. Extramuscular myofascial connections are thought to be the major contributor to the EDL proximo-distal active force difference. For antagonistic peroneal complex, the added distal lengthening of a synergistic muscle increases the effects of extramuscular myofascial force transmission.

Myofascial force transmission also occurs between antagonistic muscles located within opposite compartments of the rat lower hind limb

Force transmission via pathways other than myotendinous ones, is referred to as myofascial force transmission. The present study shows that myofascial force transmission occurs not only between adjacent synergistic muscles or antagonistic muscles in adjacent compartments, but also between most distant antagonistic muscles within a segment. Tibialis anterior (TA), extensor hallucis longus (EHL), extensor digitorum longus (EDL), peroneal muscles (PER) and triceps surae muscles of 7 male anaesthetised Wistar rats were attached to force transducers, while connective tissues at the muscle bellies were left fully intact. The TA+EHL-complex was made to exerted force at different lengths, but the other muscles were held at a constant muscle-tendon complex length. With increasing TA+EHL-complex length, active force of maximally activated EDL, PER and triceps surae decreased by maximally approximately 5%, approximately 32% and approximately 16%, respectively. These decreases are for the largest part explained by myofascial force transmission. Particularly the force decrease in triceps surae muscles is remarkable, because these muscles are located furthest away from the TA+EHL-complex. It is concluded that substantial extramuscular myofascial force transmission occurs between antagonistic muscles even if the length of the path between them is considerable.

Anatomical study of myofascial continuity in the anterior region of the upper limb

Fifteen unembalmed cadavers were dissected in order to study the "anatomical continuity" between the various muscles involved in the movement of flexion of the upper limb. This study demonstrated the existence of specific myofascial expansions, with a nearly constant pattern, which originate from the flexor muscles and extend to the overlying fascia. The clavicular part of the pectoralis major sends a myofascial expansion, with a mean length of 3.6cm, to the anterior region of the brachial fascia, and the costal part sends one to the medial region of the brachial fascia (mean length: 6.8cm). The biceps brachii presents two expansions: the lacertus fibrosus, oriented medially, with a mean height of 4.7cm and a base of 1.9cm, and a second, less evident, longitudinal expansion (mean length: 4.5cm, mean width: 0.7cm). Lastly, the palmaris longus sends an expansion to the fascia overlying the thenar muscles (mean length: 1.6cm, mean width: 0.5cm). During flexion, as these muscles contract, the anterior portion of the brachial and antebrachial fascia is subject to tension. As the fascia is rich in proprioceptive nerve endings, it is hypothesized that this tension activates a specific pattern of receptors, contributing to perception of motor direction. If the muscular fascia is in a non-physiological state, these mechanisms are altered, and the proprioceptors in the fascia may be incorrectly activated, thus giving rise to many types of extra-articular pain.