Progressive surgical dissection for tendon transposition affects length-force characteristics of rat flexor carpi ulnaris muscle

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.

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.

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.

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.

A lumped stiffness model of intermuscular and extramuscular myofascial pathways of force transmission

Mechanical behavior of skeletal muscles is commonly modeled under the assumption of mechanical independence between individual muscles within a muscle group. Epimuscular myofascial force transmission via the connective tissue network surrounding a muscle challenges this assumption as it alters the force distributed to the tendons of individual muscles. This study aimed to derive a lumped estimate of stiffness of the intermuscular and extramuscular connective tissues and to assess changes in such stiffness in response to a manipulation of the interface between adjacent muscles. Based on in situ measurements of force transmission in the rat plantar flexors, before and after resection of their connective tissue network, a nonlinear estimate of epimuscular myofascial stiffness was quantified and included in a multi-muscle model with lumped parameters which allows for force transmission depending on the relative position between the muscles in the group. Such stiffness estimate was assessed for a group with normal intermuscular connective tissues and for a group with increased connectivity, mimicking scar tissue development. The model was able to successfully predict the amount of epimuscular force transmission for different experimental conditions than those used to obtain the model parameters. The proposed nonlinear stiffness estimates of epimuscular pathways could be integrated in larger musculoskeletal models, to provide more accurate predictions of force when effects of mechanical interaction or altered epimuscular connections, e.g. after surgery or injury, are substantial.

Impact of the Femoral Head Position on Moment Arms in Total Hip Arthroplasty: A Parametric Finite Element Study

BACKGROUND

Although the importance of accurate femoral reconstruction to achieve a good functional outcome is well documented, quantitative data on the effects of a displacement of the femoral center of rotation on moment arms are scarce. The purpose of this study was to calculate moment arms after nonanatomical femoral reconstruction.

METHODS

Finite element models of 15 patients including the pelvis, the femur, and the gluteal muscles were developed. Moment arms were calculated within the native anatomy and compared to distinct displacement of the femoral center of rotation (leg lengthening of 10 mm, loss of femoral offset of 20%, anteversion ±10°, and fixed anteversion at 15°). Calculations were performed within the range of motion observed during a normal gait cycle.

RESULTS

Although with all evaluated displacements of the femoral center of rotation, the abductor moment arm remained positive, some fibers initially contributing to extension became antagonists (flexors) and vice versa. A loss of 20% of femoral offset led to an average decrease of 15% of abductor moment. Femoral lengthening and changes in femoral anteversion (±10°, fixed at 15°) led to minimal changes in abductor moment arms (maximum change of 5%). Native femoral anteversion correlated with the changes in moment arms induced by the 5 variations of reconstruction.

CONCLUSION

Accurate reconstruction of offset is important to maintaining abductor moment arms, while changes of femoral rotation had minimal effects. Patients with larger native femoral anteversion appear to be more susceptible to femoral head displacements.

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.

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.

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.

Whole muscle length-tension relationships are accurately modeled as scaled sarcomeres in rabbit hindlimb muscles

An a priori model of the whole active muscle length-tension relationship was constructed utilizing only myofilament length and serial sarcomere number for rabbit tibialis anterior (TA), extensor digitorum longus (EDL), and extensor digitorum II (EDII) muscles. Passive tension was modeled with a two-element Hill-type model. Experimental length-tension relations were then measured for each of these muscles and compared to predictions. The model was able to accurately capture the active-tension characteristics of experimentally-measured data for all muscles (ICC=0.88 ± 0.03). Despite their varied architecture, no differences in predicted versus experimental correlations were observed among muscles. In addition, the model demonstrated that excursion, quantified by full-width-at-half-maximum (FWHM) of the active length-tension relationship, scaled linearly (slope=0.68) with normalized muscle fiber length. Experimental and theoretical FWHM values agreed well with an intraclass correlation coefficient of 0.99 (p<0.001). In contrast to active tension, the passive tension model deviated from experimentally-measured values and thus, was not an accurate predictor of passive tension (ICC=0.70 ± 0.07). These data demonstrate that modeling muscle as a scaled sarcomere provides accurate active functional but not passive functional predictions for rabbit TA, EDL, and EDII muscles and call into question the need for more complex modeling assumptions often proposed.

Synergistic and antagonistic interactions in the rat forelimb: acute effects of coactivation

The goals of the present study were 1) to assess effects of antagonist coactivation on mechanical interactions between synergistic muscles, and 2) to quantify the extent of epimuscular myofascial force transmission between synergistic and antagonistic muscles in the rat forelimb. Connective tissues enveloping the muscle bellies in the antebrachium were left intact. Forces exerted at the distal tendons of flexor carpi ulnaris (FCU), palmaris longus (PL), and extensor carpi ulnaris (ECU) muscles were measured at various FCU lengths for two different stimulation protocols: 1) simultaneous stimulation of ulnar/median nerve complex (exciting all wrist flexors, including synergistic FCU and PL) and radial nerve (exciting all wrist extensors, including antagonistic ECU); and 2) stimulation of the ulnar/median nerve exclusively. PL and ECU were kept at a constant length. In addition, muscle forces were measured during stimulation of one of the indicated nerves, with later addition of stimulation of the second nerve during the maintained tetanic contraction. Coactivation of antagonistic muscles increased FCU isometric forces (on average, by 10% of optimal force) and PL forces (on average, by 13% of maximal force), but mechanical interaction between FCU and PL was unchanged. Changing the length and relative position of FCU significantly affected PL (by 20%) as well as ECU forces (by 8%). In addition, distal tetanic force of FCU kept at a constant high length was determined by the order of nerve stimulation onset. These results indicate effects of myofascial pathways between synergistic and antagonistic muscles in the rat forelimb. Coactivation may enhance the stiffness of connective tissues between muscles, but the present data suggest that activation of all wrist flexors already preloaded the myofascial pathways to the greatest extent. The stimulation order effects were explained by dynamic features of muscle and connective tissues (i.e., length-history dependence and viscoelasticity).

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.

Muscle force and power following tendon repair at altered tendon length

BACKGROUND

While a great deal is known regarding the performance of muscle with intact tendon, little is known about muscle performance when tendon is surgically lengthened or shortened. This knowledge may allow surgeons to more accurately predict functional outcome following tendon repair when correcting a simple tendon laceration or performing a more complex vascularized neuromuscular transfer.

MATERIALS AND METHODS

We studied muscle performance 12 wk following extensor tendon repairs producing altered tendon lengths. Forty male Fischer 344 rats underwent division of the proximal and distal tendons of the extensor digitorum longus muscle. Tendons were immediately repaired producing tendons with increased length, decreased length, or presurgical length (control). Observation confirmed that altered tendon length produced inverse changes in initial resting muscle tension.

RESULTS

Muscle in the decreased tendon length group demonstrated a 15.2% greater muscle mass, 4.9% greater muscle length, 9.6% greater physiological cross-sectional area, 12.6% greater maximum isometric force, and 31.9% greater maximum power relative to the control tendon length group (P < 0.05). The increased tendon length group did not differ significantly from the control tendon length group for any measurement. Histologically, muscles set with a decreased tendon length demonstrated normal appearing hypertrophied fibers, without evidence of detrimental histological effects such as fibrosis, denervation, necrosis, inflammation, fiber type changes, or fiber splitting.

CONCLUSION

These data support the clinical practice of setting muscles with increased passive tension when performing tendon transfer surgeries. Conversely, setting muscles with decreased tension does not necessarily result in a force or power deficit.

Substantial effects of epimuscular myofascial force transmission on muscular mechanics have major implications on spastic muscle and remedial surgery

The specific aim of this paper is to review the effects of epimuscular myofascial force transmission on muscular mechanics and present some new results on finite element modeling of non-isolated aponeurotomized muscle in order to discuss the dependency of mechanics of spastic muscle, as well as surgery for restoration of function on such force transmission. The etiology of the effects of spasticity on muscular mechanics is not fully understood. Clinically, such effects feature typically a limited joint range of motion, which at the muscle level must originate from altered muscle length-force characteristics, in particular a limited muscle length range of force exertion. In studies performed to understand what is different in spastic muscle and what the effects of remedial surgery are, muscle is considered as being independent of its surroundings. Conceivably, this is because the classical approach in muscle mechanics is built on experimenting with dissected muscles. Certainly, such approach allowed improving our understanding of fundamental muscle physiology yet it yielded implicitly a narrow point of view of considering muscle length-force characteristics as a fixed property of the muscle itself. However, within its context of its intact connective tissue surroundings (the in vivo condition) muscle is not an isolated and independent entity. Instead, collagenous linkages between epimysia of adjacent muscles provide direct intermuscular connections, and structures such as the neurovascular tracts provide indirect intermuscular connections. Moreover, compartmental boundaries (e.g., intermuscular septa, interosseal membranes, periost and compartmental fascia) are continuous with neurovascular tracts and connect muscular and non-muscular tissues at several locations additional to the tendon origins and insertions. Epimuscular myofascial force transmission occurring via this integral system of connections has major effects on muscular mechanics including substantial proximo-distal force differences, sizable changes in the determinants of muscle length-force characteristics (e.g. a condition dependent shift in muscle optimum length to a different length or variable muscle optimal force) explained by major serial and parallel distributions of sarcomere lengths. Therefore, due to epimuscular myofascial force transmission, muscle length-force characteristics are variable and muscle length range of force exertion cannot be considered as a fixed property of the muscle. The findings reviewed presently show that acutely, the mechanical mechanisms manipulated in remedial surgery are dominated by epimuscular myofascial force transmission. Conceivably, this is also true for the mechanism of adaptation during and after recovery from surgery. Moreover, stiffened epimuscular connections and therefore a stiffened integral system of intra- and epimuscular myofascial force transmission are indicated to affect the properties of spastic muscle. We suggest that important advancements in our present understanding of such properties, variability in the outcome of surgery and considerable recurrence of the impeded function after recovery cannot be made without taking into account the effects of epimuscular myofascial force transmission.

Myofascial force transmission is increasingly important at lower forces: firing frequency-related length-force characteristics of rat extensor digitorum longus

AIM

Effects of submaximal stimulation frequencies on myofascial force transmission were investigated for rat anterior crural muscles with all motor units activated.

METHODS

Tibialis anterior and extensor hallucis longus (TAEHL) muscles were kept at constant muscle-tendon complex length, but extensor digitorum longus muscle (EDL) was lengthened distally. All muscles were activated simultaneously at 10, 20, 30 and 100 Hz within an intact anterior crural compartment.

RESULTS

At lower frequencies, significant proximo-distal EDL force differences exist. Absolute EDL proximo-distal active force differences were highest at 100 Hz (deltaF(dist-prox) = 0.4 N). However, the normalized difference was highest at 10 Hz (deltaF(dist-prox) = 30%F(dist)). Firing-frequency dependent shifts of the ascending limb of the EDL length-force curve to higher lengths were confirmed for a muscle within an intact compartment, although effects of firing frequency assessed at proximal and distal EDL tendons differed quantitatively. As EDL was lengthened distally, TAEHL distal isometric active force decreased progressively. The absolute decrease was highest for 100 Hz (deltaF(from initial) = -0.25 N). However, the highest normalized decrease was found for 10 Hz stimulation (deltaF(from initial) = -40%).

CONCLUSIONS

At submaximal stimulation frequencies, myofascial force transmission is present and the fraction of force transmitted myofascially increases with progressively lower firing frequencies. Evidently, the stiffness of epimuscular myofascial paths of force transmission decreases less than the stiffness of serial sarcomeres and myotendinous pathways. It is concluded that low firing frequencies as encountered in vivo enhance the relative importance of epimuscular myofascial force transmission with respect to myotendinous force transmission.

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

The purpose of the present study is to investigate the origin of mechanical interactions between the rat extensor digitorum longus (EDL) muscle and the grouped tibialis anterior and extensor hallucis longus muscles (TA+EHL). The proximal and distal tendons of EDL as well as the tied distal tendons of TA+EHL were transected and connected to force transducers. Connective tissues at the muscle bellies of the anterior crural compartment were left intact. Supramaximal stimulation of the common peroneal nerve activated all muscles maximally and simultaneously. Length-isometric force characteristics of distal TA+EHL were assessed. Simultaneously, forces exerted at the proximal and distal tendons of EDL, kept at constant muscle-tendon complex length and position, were measured. Intermuscular interaction was tested in two conditions: (a) after full longitudinal compartmental fasciotomy, and (b) after blunt dissection of the intermuscular connective tissue linkages between EDL and TA+EHL. Note that in the latter condition, intermuscular myofascial pathways were eliminated. In the initial condition, lengthening TA+EHL by 12 mm increased proximal (by 0.14 N, i.e. 9.5%) and decreased distal EDL force (by 0.21 N, i.e. 11.8%), despite the fact that EDL muscle-tendon complex length was kept constant. Blunt dissection decreased TA+EHL and distal EDL forces at low TA+EHL lengths only, while proximal EDL force decreased for all TA+EHL lengths tested. The dissection caused no changes in the TA+EHL length effects on proximal EDL force. In contrast, the amplitude of change in the distal EDL force curve decreased significantly (by 39%) subsequent to blunt dissection. It is concluded that mechanical interaction between synergists originates from both intermuscular as well as extramuscular connective tissues. The highest contribution, however, should be ascribed to the extramuscular pathway.

Adaptation of muscle size and myofascial force transmission: a review and some new experimental results

This paper considers the literature and some new experimental results important for adaptation of muscle fiber cross-sectional area and serial sarcomere number. Two major points emerge: (1) general rules for the regulation of adaptation (for in vivo immobilization, low gravity conditions, synergist ablation, tenotomy and retinaculum trans-section experiments) cannot be derived. As a consequence, paradoxes are reported in the literature. Some paradoxes are resolved by considering the interaction between different levels of organization (e.g. muscle geometrical effects), but others cannot. (2) An inventory of signal transduction pathways affecting rates of muscle protein synthesis and/or degradation reveals controversy concerning the pathways and their relative contributions. A major explanation for the above is not only the inherently limited control of the experimental conditions in vivo, but also of in situ experiments. Culturing of mature single Xenopus muscle fibers at high and low lengths (allowing longitudinal study of adaptation for periods up to 3 months) did not yield major changes in the fiber cross-sectional area or the serial sarcomere number. This is very different from substantial effects (within days) of immobilization in vivo. It is concluded that overall strain does not uniquely regulate muscle fiber size. Force transmission, via pathways other than the myotendinous junctions, may contribute to the discrepancies reported: because of substantial serial heterogeneity of sarcomere lengths within muscle fibers creating local variations in the mechanical stimuli for adaptation. For the single muscle fiber, mechanical signalling is quite different from the in vivo or in vitro condition. Removal of tensile and shear effects of neighboring tissues (even of antagonistic muscle) modifies or removes mechanical stimuli for adaptation. It is concluded that the study of adaptation of muscle size requires an integrative approach taking into account fundamental mechanisms of adaptation, as well as effects of higher levels of organization. More attention should be paid to adaptation of connective tissues within and surrounding the muscle and their effects on muscular properties.

Spastic muscle properties are affected by length changes of adjacent structures

Recent animal experiments have shown that up to 37% of muscle force may be transmitted to adjacent structures rather than reach the insertion of the muscle's tendon, and that the extent of such force transmission depends on the length and relative position of these structures. We tested whether the force-length characteristics of the distally tenotomized human flexor carpi ulnaris muscle (FCU) of nine patients with cerebral palsy varied with the change of relative length of adjacent structures induced by a change of wrist position. In four patients, the FCU exerted up to 40% more active force in a flexed wrist position at short FCU length, whereas the active force was not significantly higher in the other five. In the same manner, passive force-length characteristics of the spastic FCU changed upon changes in wrist position. Variability in myofascial force transmission may partly explain the variability in success of the FCU-transfer.

Overstretching of sarcomeres may not cause cerebral palsy muscle contracture

To answer the question whether the muscle contracture in patients with cerebral palsy is caused by overstretching of in-series sarcomeres we studied the active and passive force-length relationship of the flexor carpi ulnaris muscle (FCU) in relation to its operating length range in 14 such patients with a flexion deformity of the wrist. Force-length relationship was measured intra-operatively using electrical stimulation, a force transducer, and a data-acquisition system. Muscle length was measured in maximally flexed and maximally extended position of the wrist. The spastic FCU was found to exert over 80% of its maximum active force at maximal extension of the wrist and this indicates abundant overlap of the sarcomeres. At maximal wrist extension, FCU passive force corresponded with only 0.7-18% of maximum active force. Both findings imply that the FCU sarcomeres are not overstretched when the wrist is extended. We conclude that the overstretching of in-series sarcomeres appears not to be the cause of contracture of the spastic FCU.

Properties of the two neuromuscular compartments in a split bipennate muscle

Bipennate muscles may be split along their distal aponeurosis, dividing each into two compartments. These sub-muscle units may be used in tendon transfers. This paper presents the contractile properties of the two sub-units of the flexor carpi ulnaris in a macaca fascicularis, after it was split by up to 80% of its length. The sub-muscle units were electrically stimulated and found to have independent isometric contraction, with minimal contraction recorded from the non-stimulated sub-unit. Also, the sum of the forces measured from each unit when stimulated individually, was found to be greater than the force of the whole muscle, given the same isometric conditions. The distal aponeurosis which is common allows force transmission between the compartments. Splitting the muscle along this distal aponeurosis alters this function and the force capacity of the muscle, providing a new potential for using the sub-units as grafts for tendon transfers.

Muscle force is determined also by muscle relative position: isolated effects

Effects on force of changes of the position of extensor digitorum longus muscle (EDL) relative to surrounding tissues were investigated in rat. Connective tissue at the muscle bellies of tibialis anterior (TA), extensor hallucis longus (EHL) and EDL was left intact, to allow myofascial force transmission. The position of EDL muscle was altered, without changing EDL muscle-tendon complex length, and force exerted at proximal and distal tendons of EDL as well as summed force exerted at the distal tendons of TA and EHL muscles (TA+EHL) were measured. Proximal and distal EDL forces as well as distal TA+EHL force changed significantly on repositioning EDL muscle. These muscle position-force characteristics were assessed at two EDL lengths and two TA+EHL lengths. It was shown that changes of muscle force with length changes of a muscle is the result of the length changes per se, as well as of changes of relative position of parts of the muscle. It is concluded that in addition to length, muscle position relative to its surroundings co-determines isometric muscle force.

Muscular Force Transmission Necessitates a Multilevel Integrative Approach to the Analysis of Function of Skeletal Muscle

Muscular force is transmitted not only to tendon but also to other structures. Connections to extramuscular connective tissue of a compartment and to other muscles are stiff enough to transmit force. The concept of myofascial force transmission is reviewed and some functional consequences considered. An approach for analysis is suggested.

Myofascial force transmission: muscle relative position and length determine agonist and synergist muscle force

Equal proximal and distal lengthening of rat extensor digitorum longus (EDL) were studied. Tibialis anterior, extensor hallucis longus, and EDL were active maximally. The connective tissues around these muscle bellies were left intact. Proximal EDL forces differed from distal forces, indicating myofascial force transmission to structures other than the tendons. Higher EDL distal force was exerted (ratio approximately 118%) after distal than after equal proximal lengthening. For proximal force, the reverse occurred (ratio approximately 157%). Passive EDL force exerted at the lengthened end was 7-10 times the force exerted at the nonlengthened end. While kept at constant length, synergists (tibialis anterior + extensor hallucis longus: active muscle force difference approximately -10%) significantly decreased in force by distal EDL lengthening, but not by proximal EDL lengthening. We conclude that force exerted at the tendon at the lengthened end of a muscle is higher because of the extra load imposed by myofascial force transmission on parts of the muscle belly. This is mediated by changes of the relative position of most parts of the lengthened muscle with respect to neighboring muscles and to compartment connective tissues. As a consequence, muscle relative position is a major codeterminant of muscle force for muscle with connectivity of its belly close to in vivo conditions.