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

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.

Three-dimensional interactive graphical model of the hindlimb muscles of the rat

Many questions in human movement sciences are addressed by exploiting the advantages of animal models. However, a 3D model of the musculoskeletal system of the frequently used rat model that includes a sufficient level of detail does not exist. Therefore, the aim of the present work was to develop an freely accessible 3D model of the rat hindlimb. Using the anatomical data of the Wistar rat (Mus norveqicus albinus) published by Green [Greene, 1935], a 3D representation of 34 muscles of the hindlimb was drawn. Two models were created, one using muscle like appearances and one using different colors. Each muscle can be viewed separately or within the context of its synergistic and antagonistic muscles. This model can serve to train new students before starting their experiments, but also for producing illustrations of experimental conditions or results. Further development of the model will be needed to equip it with the same advanced functionalities of some of the human anatomy atlases.

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.

Minimal force transmission between human thumb and index finger muscles under passive conditions

It has been hypothesized that force can be transmitted between adjacent muscles. Intermuscle force transmission violates the assumption that muscles act in mechanical isolation, and implies that predictions from biomechanical models are in error due to mechanical interactions between muscles, but the functional relevance of intermuscle force transmission is unclear. To investigate intermuscle force transmission between human flexor pollicis longus and the index finger part of flexor digitorum profundus, we compared finger flexion force produced by passive thumb flexion after one of three conditioning protocols: passive thumb flexion-extension cycling, thumb flexion maximal voluntary contraction (MVC), and thumb extension stretch. Finger flexion force increased after all three conditions. Compared to passive thumb flexion-extension cycling, change in finger flexion force was less after thumb extension stretch (mean difference 0.028 N, 95% CI 0.005 to 0.051 N), but not after thumb flexion MVC (0.007 N, 95% CI -0.020 to 0.033 N). As muscle conditioning changed finger flexion force produced by passive thumb flexion, the change in force is likely due to intermuscle force transmission. Thus, intermuscle force transmission resulting from passive stretch of an adjacent muscle is probably small enough to be ignored.

Mechanical Coupling Between Muscle-Tendon Units Reduces Peak Stresses

The presence of mechanical linkages between synergistic muscles and their common tendons may distribute forces among the involved structures. We review studies, using humans and other animals, examining muscle and tendon interactions and discuss the hypothesis that connections between muscle bellies and within tendons may serve as a mechanism to distribute forces and mitigate peak stresses.

Evidence of adaptations of locomotor neural drive in response to enhanced intermuscular connectivity between the triceps surae muscles of the rat

The aims of this study were to investigate changes 1) in the coordination of activation of the triceps surae muscle group, and 2) in muscle belly length of soleus (SO) and lateral gastrocnemius (LG) during locomotion (trotting) in response to increased stiffness of intermuscular connective tissues in the rat. We measured muscle activation and muscle belly lengths, as well as hindlimb kinematics, before and after an artificial enhancement of the connectivity between SO and LG muscles obtained by implanting a tissue-integrating surgical mesh at the muscles' interface. We found that SO muscle activation decreased to 62%, while activation of LG and medial gastrocnemius muscles increased to 134 and 125%, respectively, compared with the levels measured preintervention. Although secondary additional or amplified activation bursts were observed with enhanced connectivity, the primary pattern of activation over the stride and the burst duration were not affected by the intervention. Similar muscle length changes after manipulation were observed, suggesting that length feedback from spindle receptors within SO and LG was not affected by the connectivity enhancement. We conclude that peripheral mechanical constraints given by morphological (re)organization of connective tissues linking synergists are taken into account by the central nervous system. The observed shift in activity toward the gastrocnemius muscles after the intervention suggests that these larger muscles are preferentially recruited when the soleus has a similar mechanical disadvantage in that it produces an unwanted flexion moment around the knee.NEW & NOTEWORTHY Connective tissue linkages between muscle-tendon units may act as an additional mechanical constraint on the musculoskeletal system, thereby reducing the spectrum of solutions for performing a motor task. We found that intermuscular coordination changes following intermuscular connectivity enhancement. Besides showing that the extent of such connectivity is taken into account by the central nervous system, our results suggest that recruitment of triceps surae muscles is governed by the moments produced at the ankle-knee joints.

Longitudinal and transversal displacements between triceps surae muscles during locomotion of the rat

The functional consequences of differential muscle activation and contractile behavior between mechanically coupled synergists are still poorly understood. Even though synergistic muscles exert similar mechanical effects at the joint they span, differences in the anatomy, morphology and neural drive may lead to non-uniform contractile conditions. This study aimed to investigate the patterns of activation and contractile behavior of triceps surae muscles, to understand how these contribute to the relative displacement between the one-joint soleus (SO) and two-joint lateral gastrocnemius (LG) muscle bellies and their distal tendons during locomotion in the rat. In seven rats, muscle belly lengths and muscle activation during level and upslope trotting were measured by sonomicrometry crystals and electromyographic electrodes chronically implanted in the SO and LG. Length changes of muscle–tendon units (MTUs) and tendon fascicles were estimated based on joint kinematics and muscle belly lengths. Distances between implanted crystals were further used to assess longitudinal and transversal deformations of the intermuscular volume between the SO and LG. For both slope conditions, we observed differential timing of muscle activation as well as substantial differences in contraction speeds between muscle bellies (maximal relative speed 55.9 mm s−1). Muscle lengths and velocities did not differ significantly between level and upslope locomotion, only EMG amplitude of the LG was affected by slope. Relative displacements between SO and LG MTUs were found in both longitudinal and transversal directions, yielding an estimated maximal length change difference of 2.0 mm between their distal tendons. Such relative displacements may have implications for the force exchanged via intermuscular and intertendinous pathways.

Synergistic Co-activation Increases the Extent of Mechanical Interaction between Rat Ankle Plantar-Flexors

Force transmission between rat ankle plantar-flexors has been found for physiological muscle lengths and relative positions, but only with all muscles maximally activated. The aims of this study were to assess intermuscular mechanical interactions between ankle plantar-flexors during (i) fully passive conditions, (ii) excitation of soleus (SO), (iii) excitation of lateral gastrocnemius (LG), and (iv) during co-activation of SO, and LG (SO&LG). We assessed effects of proximal lengthening of LG and plantaris (PL) muscles (i.e., simulating knee extension) on forces exerted at the distal SO tendon (F_SO) and on the force difference between the proximal and distal LG+PL tendons (ΔF_LG+PL) of the rat. LG+PL lengthening increased F_SO to a larger extent (p = 0.017) during LG excitation (0.0026 N/mm) than during fully passive conditions (0.0009 N/mm). Changes in F_SO in response to LG+PL lengthening were lower (p = 0.002) during SO only excitation (0.0056 N/mm) than during SO&LG excitation (0.0101 N/mm). LG+PL lengthening changed ΔF_LG+PL to a larger extent (p = 0.007) during SO excitation (0.0211 N/mm) than during fully passive conditions (0.0157 N/mm). In contrast, changes in ΔF_LG+PL in response to LG+PL lengthening during LG excitation (0.0331 N/mm) were similar (p = 0.161) to that during SO&LG excitation (0.0370 N/mm). In all conditions, changes of F_SO were lower than those of ΔF_LG+PL. This indicates that muscle forces were transmitted not only between LG+PL and SO, but also between LG+PL and other surrounding structures. In addition, epimuscular myofascial force transmission between rat ankle plantar-flexors was enhanced by muscle activation. However, the magnitude of this interaction was limited.