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Abstract We recently showed that within an intact muscle compartment, changing the length of one muscle affects the firing behaviour of muscle spindles located within a neighbouring muscle. The conditions tested, however, involved muscle lengths and relative positions that were beyond physiological ranges. The aim of the present study was to investigate the effects of simulated knee movements on the firing behaviour of muscle spindles located within rat soleus (SO) muscle. Firing from single muscle spindle afferents in SO was measured intra‐axonally for different lengths (static) and during lengthening (dynamic) of the lateral gastrocnemius and plantaris muscles. Also, the location of the spindle within the muscle was assessed. Changing the length of synergistic ankle plantar flexors (simulating different static knee positions, between 45 and 130°) affected the force threshold, but not the length threshold, of SO muscle spindles. The effects on type II afferents were substantially (four times) higher than those on type IA afferents. Triangular stretch–shortening of synergistic muscles (simulating dynamic knee joint rotations of 15°) caused sudden changes in the firing rate of SO type IA and II afferents. Lengthening decreased and shortening increased the firing rate, independent of spindle location. This supports our prediction that the major point of application of forces exerted by connections between adjacent muscles is at the distal end of SO. We conclude that muscle spindles provide the CNS with information about the condition of adjacent joints that the muscle does not span.

We recently showed that within an intact muscle compartment, changing the length of one muscle affects the firing behaviour of muscle spindles located within a neighbouring muscle. The conditions tested, however, involved muscle lengths and relative positions that were beyond physiological ranges. The aim of the present study was to investigate the effects of simulated knee movements on the firing behaviour of muscle spindles located within rat soleus (SO) muscle. Firing from single muscle spindle afferents in SO was measured intra‐axonally for different lengths (static) and during lengthening (dynamic) of the lateral gastrocnemius and plantaris muscles. Also, the location of the spindle within the muscle was assessed. Changing the length of synergistic ankle plantar flexors (simulating different static knee positions, between 45 and 130°) affected the force threshold, but not the length threshold, of SO muscle spindles. The effects on type II afferents were substantially (four times) higher than those on type IA afferents. Triangular stretch–shortening of synergistic muscles (simulating dynamic knee joint rotations of 15°) caused sudden changes in the firing rate of SO type IA and II afferents. Lengthening decreased and shortening increased the firing rate, independent of spindle location. This supports our prediction that the major point of application of forces exerted by connections between adjacent muscles is at the distal end of SO. We conclude that muscle spindles provide the CNS with information about the condition of adjacent joints that the muscle does not span. What is the central question of this study?Do length changes of bi‐articular neighbouring muscles affect the firing behaviour of a mono‐articular ankle plantar flexor?What is the main finding and its importance?Simulating knee movements by changing the length of synergistic ankle plantar flexors caused sudden changes in the firing rate of type IA and II afferents. This implies that muscle spindles provide the CNS with information about the condition of adjacent joints that the muscle does not span. [ABSTRACT FROM AUTHOR]

The fascial system not only enables the body to operate in an integrated manner but modifies its tension in response to the stress on it. Recent animal, cadaveric and in-vitro trials have shown that "myofascial force transmission" (MFT) can play a major role in homeostasis, musculoskeletal function and pain. Human evidence for the in-vivo existence of MFT is scarce. This scoping review attempts to gather and interpret the available evidence of the in-vivo existence of MFT in humans, its role in homeostasis, and musculoskeletal function. A search of major databases using the keywords 'myofascial force transmission' and 'epimuscular force transmission' yielded 247 articles as of November 2021. For the final analysis, only original in-vivo human studies were considered. In-vitro human studies, cadaveric or animal studies, reviews, and similar studies were excluded. A qualitative analysis of the studies was conducted after rating it with the Oxford's Center for Evidence –based Medicine (CEBM) scale. Twenty studies ranging from randomized controlled trials (RCTs) to case studies covering 405 patients have been included in this review. The analysed trials were highly heterogeneous and of lower methodological quality meddling with the quantitative analysis. The majority of the appraised studies demonstrated a higher probability of MFT existence, while two studies revealed a lower probability. Our search for proof of the in vivo existence of MFT in humans has led us to support such an existence, albeit prudently. Previous research on animals and human cadavers reinforces our finding. We are optimistic that the forthcoming studies on the topic will pave the way for the unraveling of several musculoskeletal riddles that are currently unknown or less well-known. [ABSTRACT FROM AUTHOR]

Purpose. Posterior layer of thoracolumbar fascia (PTLF) is the part of the deep fascia of back of the trunk which connects the trunk, upper and lower limb muscles. PTLF is the major myofascial linkage where myofascial force transmission can take place between the muscles attached to it. The present study evaluates the force transmission through PTLF to the right and left latissimus dorsi, and right and left lower trapezius muscles during isometric contraction of right and left gluteus maximus. Materials and methods. Present descriptive observational study was conducted on 40 male adult healthy volunteers aged between 30 to 45 years. The Root mean square (RMS) value of EMG signal of the bilateral gluteus maximus, latissimus dorsi and lower trapezius muscles was collected using surface Electromyographic sensors. The Electromyographic assessment was carried out at normal contraction, at minimum resistance (load of 1 kg) and at maximum voluntary contraction with resistance (2-5.5 kg) of gluteus maximus. Results. When left gluteus maximus contracts normally (without any resistance), the mean RMS value was found to be 16.28 ± 3.15 µV and on maximum voluntary contraction (MVC) with resistance it was 28.95 ± 4.89 µV. When right gluteus maximus contracts normally without any resistance, the mean RMS value was found to be 25.83 ± 4.48 µV and MVC with resistance was 43.70 ± 2.60 µV. This showed around 30-70% increase in RMS value from normal contraction to MVC with resistance. The relative activation at different isometric contractions of right and left gluteal muscles were compared to the right and left latissimus dorsi and right and left lower trapezius muscles. A significant (p < 0.001) Pearson's correlation was observed between the muscles. Conclusions. Epimuscular myofascial force is transferred from the gluteus maximus muscles to other muscles attached to PTLF directly or indirectly to the same side as well as to the opposite side. The connection via PTLF to the muscles attached to it may affect the sensory feedback and thereby the neuromuscular control. In pathological conditions, same myofascial linkage may contribute to altered biomechanics of the back of the trunk. [ABSTRACT FROM AUTHOR]

Skeletal muscles embed multiple tendon organs, both at the proximal and distal ends of muscle fibers. One of the functions of such spatial distribution may be to provide locally unique force feedback, which may become more important when stresses are distributed non-uniformly within the muscle. Forces exerted by connections between adjacent muscles (i.e. epimuscular myofascial forces) may cause such local differences in force. The aim of this exploratory study was to investigate the effects of mechanical interactions between adjacent muscles on sensory encoding by tendon organs. Action potentials from single afferents were recorded intra-axonally in response to ramp-hold release (RHR) stretches of a passive agonistic muscle at different lengths or relative positions of its passive synergist. The tendons of gastrocnemius (GAS), plantaris (PL) and soleus (SO) muscles were cut from the skeleton for attachment to servomotors. Connective tissues among these muscles were kept intact. Lengthening GAS + PL decreased the force threshold of SO tendon organs (p = 0.035). The force threshold of lateral gastrocnemius (LG) tendon organs was not affected by SO length (p = 0.371). Also displacing LG + PL, kept at a constant muscle–tendon unit length, from a proximal to a more distal position resulted in a decrease in force threshold of LG tendon organs (p = 0.007). These results indicate that tendon organ firing is affected by changes in length and/or relative position of adjacent synergistic muscles. We conclude that tendon organs can provide the central nervous system with information about local stresses caused by epimuscular myofascial forces. [ABSTRACT FROM AUTHOR]

Purpose: Semitendinosus and gracilis muscles whose tendons are used in surgical reconstruction of the anterior cruciate ligament maintain their contractile ability, but the peak torque angle of hamstring muscles shifted to a shallow angle postoperatively. The goal was to quantify the influence of the myofascial structures on instantaneous 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 step of the surgery. Results: No significant difference was demonstrated for instantaneous moment arm of gracilis during anterior cruciate ligament surgery (84% of the maximum intact values; P ≥ 0.05). The first significant semitendinosus moment arm decrease was observed after tendon harvesting (61% of the maximum intact values; p ≤ 0.005). After hamstring harvesting, the maximum and minimum moment arm (both gracilis and semi tendinosus) shifted to a shallow angle and 90°, respectively. Conclusion: Moment arm modifications by paratenons and the loose connective tissue release are essential to understand the peak torque obtained to a shallow angle. Level of evidence: Basic science study, biomechanics. [ABSTRACT FROM AUTHOR]

Purpose: The aim of this study was to assess the effects of experimentally induced photothrombotic stroke on structural and mechanical properties of rat m. flexor carpi ulnaris. Methods: Two groups of Young-adult male Sprague–Dawley rats were measured: stroke (n = 9) and control (n = 7). Photothrombotic stroke was induced in the forelimb region of the primary sensorimotor cortex. Four weeks later, muscle–tendon unit and muscle belly length–force characteristics of the m. flexor carpi ulnaris, mechanical interaction with the neighbouring m. palmaris longus, the number of sarcomeres in series within muscle fibres, and the physiological cross-sectional area were measured. Results: Stroke resulted in higher force and stiffness of the m. flexor carpi ulnaris at optimum muscle–tendon unit length, but only for the passive conditions. Stroke did not alter the length–force characteristics of m. flexor carpi ulnaris muscle belly, morphological characteristics, and the extent of mechanical interaction with m. palmaris longus muscle. Conclusion: The higher passive force and passive stiffness at the muscle–tendon unit level in the absence of changes in structural and mechanical characteristics of the muscle belly indicates that the experimentally induced stroke resulted in an increased stiffness of the tendon. [ABSTRACT FROM AUTHOR]

Experiments in cadavers have demonstrated significant mechanical interactions between constituents of myofascial chains. However, evidence for such force transmission effects is scarce under in vivo conditions. The purpose of this trial was to examine the impact of ankle motion on soft tissue displacement of the dorsal thigh. Eleven healthy active individuals (26.8 ± 4.3 years, six males), in prone position and with the knee extended, underwent passive calf stretches (ankle dorsal extension) imposed by an isokinetic dynamometer. High-resolution ultrasound was used to simultaneously capture the displacement of the semimembranosus muscle, which was quantified by means of cross-correlation analysis. Inactivity of the leg muscles was controlled using surface electromyography (EMG). One participant had to be excluded due to major EMG activity during the experiment. According to a one-sample t test testing the difference to the neutral zero position, ankle dorsal extension induced substantial caudal muscle displacements (5.76 ± 2.67 mm, p < 0.0001). Correlation analysis (Spearman), furthermore, revealed a strong association between maximal dorsal extension and semimembranosus motion (rho = 0.76, p = 0.02). In conclusion, the present trial provides initial in vivo evidence for a mechanical force transmission between serially connected skeletal muscles. This means that local alterations of the mechanical tissue properties may modify flexibility in neighboring (superior or inferior) joints.

Experiments in cadavers have demonstrated significant mechanical interactions between constituents of myofascial chains. However, evidence for such force transmission effects is scarce under in vivo conditions. The purpose of this trial was to examine the impact of ankle motion on soft tissue displacement of the dorsal thigh. Eleven healthy active individuals (26.8 ± 4.3 years, six males), in prone position and with the knee extended, underwent passive calf stretches (ankle dorsal extension) imposed by an isokinetic dynamometer. High-resolution ultrasound was used to simultaneously capture the displacement of the semimembranosus muscle, which was quantified by means of cross-correlation analysis. Inactivity of the leg muscles was controlled using surface electromyography (EMG). One participant had to be excluded due to major EMG activity during the experiment. According to a one-sample t test testing the difference to the neutral zero position, ankle dorsal extension induced substantial caudal muscle displacements (5.76 ± 2.67 mm, p < 0.0001). Correlation analysis (Spearman), furthermore, revealed a strong association between maximal dorsal extension and semimembranosus motion (rho = 0.76, p = 0.02). In conclusion, the present trial provides initial in vivo evidence for a mechanical force transmission between serially connected skeletal muscles. This means that local alterations of the mechanical tissue properties may modify flexibility in neighboring (superior or inferior) joints. [ABSTRACT FROM AUTHOR]

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Anatomical studies have shown structural continuity between the lumbopelvic region and the lower limb. The present study aimed to verify how simultaneous changes on knee/hip positions modify the ankle’s resting position and passive torque. Thirty-seven subjects underwent an isokinetic assessment of ankle passive torque. The relationship between the absolute values of ankle passive resistance torque and the ankle angular position was used to calculate the dependent variables: ankle resting position (position in which the passive resistance torque is zero); and ankle passive torque at 0° (torque at the neutral position of the ankle in the sagittal plane). These measures were carried out under three test conditions: 0° at knee and 0° at hip (0°/0°); 90° at knee and 90° at hip (90°/90°); and, 135° at knee and 120° at hip (135°/120°). The results demonstrated that the ankle resting position shifted towards dorsiflexion when knee/hip position changed from 0°/0° to 90°/90° and shifted towards plantar flexion when knee/hip position changed from 90°/90° to 135°/120°, achieving values close to the ones at the position 0°/0°. Similarly, passive torque reduced when knee/hip position changed from 0°/0° to 90°/90°, but it increased when knee/hip position changed from 90°/90° to 135°/120°. The unexpected changes observed in ankle passive torque and resting position due to changes in knee and hip from 90°/90° to 135°/120°, cannot be explained exclusively by forces related to tissues crossing the knee and ankle. This result supports the existence of myofascial force transmission among lower limb joints. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Lateral connections between muscles provide pathways for myofascial force transmission. To elucidate whether these pathways have functional roles in vivo, we examined whether activation could alter the shear between the soleus ( SOL) and lateral gastrocnemius ( LG) muscles. We hypothesized that selective activation of LG would decrease the stretch-induced shear between LG and SOL. Eleven volunteers underwent a series of knee joint manipulations where plantar flexion force, LG, and SOL muscle fascicle lengths and relative displacement of aponeuroses between the muscles were obtained. Data during a passive full range of motion were recorded, followed by 20° knee extension stretches in both passive conditions and with selective electrical stimulation of LG. During active stretch, plantar flexion force was 22% greater ( P < 0.05) and relative displacement of aponeuroses was smaller than during passive stretch ( P < 0.05). Soleus fascicle length changes did not differ between passive and active stretches but LG fascicles stretched less in the active than passive condition when the stretch began at angles of 70° and 90° of knee flexion ( P < 0.05). The activity-induced decrease in the relative displacement of SOL and LG suggests stronger (stiffer) connectivity between the two muscles, at least at flexed knee joint angles, which may serve to facilitate myofascial force transmission. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Skeletal muscle force can be transmitted to the skeleton, not only via its tendons of origin and insertion but also through connective tissues linking the muscle belly to surrounding structures. Through such epimuscular myofascial connections, length changes of a muscle may cause length changes within an adjacent muscle and hence, affect muscle spindles. The aim of the present study was to investigate the effects of epimuscular myofascial forces on feedback from muscle spindles in triceps surae muscles of the rat. We hypothesized that within an intact muscle compartment, muscle spindles not only signal length changes of the muscle in which they are located but can also sense length changes that occur as a result of changing the length of synergistic muscles. Action potentials from single afferents were measured intra-axonally in response to ramp-hold release (RHR) stretches of an agonistic muscle at different lengths of its synergist, as well as in response to synergist RHRs. A decrease in force threshold was found for both soleus (SO) and lateral gastrocnemius afferents, along with an increase in length threshold for SO afferents. In addition, muscle spindle firing could be evoked by RHRs of the synergistic muscle. We conclude that muscle spindles not only signal length changes of the muscle in which they are located but also local length changes that occur as a result of changing the length and relative position of synergistic muscles. [ABSTRACT FROM AUTHOR]

BTX effects on muscular mechanics are highly important, but their mechanism and variability in due treatment course is not well understood. Recent modeling shows that partial muscle paralysis per se causes restricted sarcomere shortening due to muscle fiber–extracellular matrix (ECM) mechanical interactions. This leads to two notable acute-BTX effects compared to pre-BTX treatment condition: (1) enhanced potential of active force production of the non-paralyzed muscle parts, and (2) decreased muscle length range of force exertion ( ℓ range ). Recent experiments also indicate increased ECM stiffness of BTX treated muscle. Hence, altered muscle fiber–ECM interactions and BTX effects are plausible in due treatment course. Using finite element modeling, the aim was to test the following hypotheses: acute-BTX treatment effects elevate with increased ECM stiffness in the long-term, and are also persistent post-BTX treatment. Model results confirm these hypotheses and show that restricted sarcomere shortening effect becomes more pronounced in the long-term and is persistent or reversed (for longer muscle lengths) post-BTX treatment. Consequently, force production capacity of activated sarcomeres gets further enhanced in the long-term. Remarkably, such enhanced capacity becomes permanent for the entire muscle post-treatment. Shift of muscle optimum length to a shorter length is more pronounced in the long-term, some of which remains permanent post-treatment. Compared to Pre-BTX treatment, a narrower ℓ range (20.3%, 27.1% and 3.4%, acute, long-term and post-BTX treatment, respectively) is a consistent finding. We conclude that ECM adaptations can affect muscular mechanics adversely both during spasticity management and post-BTX treatment. Therefore, this issue deserves major future attention. [ABSTRACT FROM AUTHOR]

Tendon transfer surgery to a new extensor insertion was performed for musculus flexor carpi ulnaris ( FCU) of young adult rats, after which animals were allowed to recover. Mechanical properties and adaptive effects on body mass, bone growth, serial number of sarcomeres, and muscle physiological cross-sectional area were studied. Between the transfer and control groups, no differences were found for body mass and forearm length growth. In contrast, transferred muscles had a 19% smaller physiological cross-sectional area and 25% fewer sarcomeres in series within its muscle fibers than control muscles, i.e., a deficit in muscle belly growth is present. Our present results confirm our the length of previous work showing a limited capability of changing the adapted transferred FCU muscle belly, as the muscle-tendon complex is stretched, so that most of the acute FCU length change must originate from the tendon. This should most likely be attributed to surgery-related additional and/or altered connective tissue linkages at the muscle-tendon boundary. The substantially increased FCU tendon length found, after recovery from surgery and adaptation to the conditions of the transferred position, is likely to be related to such enhanced stretching of the FCU tendon. [ABSTRACT FROM AUTHOR]

Changes in fascicle length and tension of the soleus (SOL) muscle have been observed in humans using B-mode ultrasound to examine the knee from different angles. An alternative technique of assessing muscle and tendon stiffness is myometry, which is non-invasive, accessible, and easy to use. This study aimed to estimate the compressive stiffness of the distal SOL and Achilles tendon (AT) using myometry in various knee and ankle joint positions. Twenty-six healthy young males were recruited. The Myoton-PRO device was used to measure the compressive stiffness of the distal SOL and AT in the dominant leg. The knee was measured in two positions (90° of flexion and 0° of flexion) and the ankle joint in three positions (10° of dorsiflexion, neutral position, and 30° of plantar flexion) in random order. A three-way repeated-measures ANOVA test was performed. Significant interactions were found for structure × ankle position, structure × knee position, and structure × ankle position × knee position (p < 0.05). The AT and SOL showed significant increases in compressive stiffness with knee extension over knee flexion for all tested ankle positions (p < 0.05). Changes in stiffness relating to knee positioning were larger in the SOL than in the AT (p < 0.05). These results indicate that knee extension increases the compressive stiffness of the distal SOL and AT under various ankle joint positions, with a greater degree of change observed for the SOL. This study highlights the relevance of knee position in passive stiffness of the SOL and AT. [ABSTRACT FROM AUTHOR]

In situ studies involving supraphysiological muscle lengths and relative positions have shown that connective tissue linkages connecting adjacent muscles can transmit substantial forces, but the physiological significance is still subject to debate. The present study investigates effects of such epimuscular myofascial force transmission in the rat calf muscles. Unlike previous approaches, we quantified the mechanical interaction between the soleus (SO) and the lateral gastrocnemius and plantaris complex (LG+PL) applying a set of muscle lengths and relative positions corresponding to the range of knee and ankle angles occurring during normal movements. In nine deeply anesthetized Wistar rats, the superficial posterior crural compartment was exposed, and distal and proximal tendons of LG+PL and the distal SO tendon were severed and connected to force transducers. The target muscles were excited simultaneously. We found that SO active and passive tendon force was substantially affected by proximally lengthening of LG+PL mimicking knee extension (10% and 0.8% of maximal active SO force, respectively; P < 0.05). Moreover, SO relative position significantly changed the LG+PL length-force relationship, resulting in nonunique values for passive slack-length and optimum-length estimates. We conclude that also, for physiological muscle conditions, isometric force of rat triceps surae muscles is determined by its muscle-tendon unit length as well as by the length and relative position of its synergists. This has implications for understanding the neuromechanics of skeletal muscle in normal and pathological conditions, as well as for studies relying on the assumption that muscles act as independent force actuators. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Recent experiments involving muscle force measurements over a range of muscle lengths show that effects of botulinum toxin (BTX) are complex e.g., force reduction varies as a function of muscle length. We hypothesized that altered conditions of sarcomeres within active parts of partially paralyzed muscle is responsible for this effect. Using finite element modeling, the aim was to test this hypothesis and to study principles of how partial activation as a consequence of BTX affects muscle mechanics. In order to model the paralyzing effect of BTX, only 50% of the fascicles (most proximal, or middle, or most distal) of the modeled muscle were activated. For all muscle lengths, a vast majority of sarcomeres of these BTX-cases were at higher lengths than identical sarcomeres of the BTX-free muscle. Due to such "longer sarcomere effect", activated muscle parts show an enhanced potential of active force exertion (up to 14.5%). Therefore, a muscle force reduction originating exclusively from the paralyzed muscle fiber populations, is compromised by the changes of active sarcomeres leading to a smaller net force reduction. Moreover, such "compromise to force reduction" varies as a function of muscle length and is a key determinant of muscle length dependence of force reduction caused by BTX. Due to longer sarcomere effect, muscle optimum length tends to shift to a lower muscle length. Muscle fiber-extracellular matrix interactions occurring via their mutual connections along full peripheral fiber lengths (i.e., myofascial force transmission) are central to these effects. Our results may help improving our understanding of mechanisms of how the toxin secondarily affects the muscle mechanically. [ABSTRACT FROM AUTHOR]

There are extensive connections between the latissimus dorsi (LD) and gluteus maximus (GMax) muscles and the thoracolumbar fascia (TLF), which suggests a possible pathway for myofascial force transmission. The present study was designed to provide empirical evidence of myofascial force transmission from LD to contralateral GMax through TFL in vivo. To accomplish this goal, we evaluated whether active or passive tensioning of the LD results in increased passive tension of the contralateral GMax, indexed by changes in the hip resting position (RP) or passive stiffness. The hip RP was defined as the angular position in which the passive joint torque equals zero, and passive hip stiffness was calculated as the change in passive torque per change in joint angle. Thirty-seven subjects underwent an assessment of their passive hip torque against medial rotation by means of an isokinetic dynamometer. These measures were carried out under three test conditions: (1) control, (2) passive LD tensioning and (3) active LD tensioning. Electromyography was used to monitor the activity of the hip muscles and the LD under all conditions. Repeated measures analyses of variance demonstrated that passive LD tensioning shifted the hip RP towards lateral rotation (p=0.009) but did not change the passive hip stiffness (p >0.05). Active LD tensioning shifted the hip RP towards lateral rotation (p < 0.001) and increased the passive hip stiffness (p r 0.004). The results demonstrated that manipulation of the LD tension modified the passive hip variables, providing evidence of myofascial force transmission in vivo. [ABSTRACT FROM AUTHOR]

The goal was to assess the effects of multiple aponeurotomy on mechanics of muscle with extramuscular myofascial connections. Using finite element modelling, effects of combinations of the intervention carried out at a proximal (P), an intermediate (I) and a distal (D) location were studied: (1) Case P, (2) Case P-I, (3) Case P-D and (4) Case P-I-D. Compared to Case P, the effects of multiple interventions on muscle geometry and sarcomere lengths were sizable for the distal population of muscle fibres: e.g. at high muscle length (1) summed gap lengths between the cut ends of aponeurosis increased by 16, 25 and 27% for Cases P-I, P-D and P-I-D, respectively, (2) characteristic substantial sarcomere shortening became more pronounced (mean shortening was 26, 29, 30 and 31% for Cases P, P-I, P-D and P-I-D, respectively) and (3) fibre stresses decreased (mean stress equalled 0.49, 0.39, 0.38 and 0.33 for Cases P, P-I, P-D and P-I-D, respectively). In contrast, no appreciable effects were shown for the proximal population. The overall change in sarcomere length heterogeneity was limited. Consequently, the effects of multiple aponeurotomy on muscle length–force characteristics were marginal: (1) a limited reduction in active muscle force (maximal ‘muscle weakening effect’ remained between 5 and 11%) and (2) an even less pronounced change in slack to optimum length range of force exertion (maximal ‘muscle lengthening effect’ distally was 0.2% for Case P-I-D) were shown. The intended effects of the intervention were dominated by the one intervention carried out closer to the tendon suggesting that aponeurotomies done additionally to that may counter-indicated. [ABSTRACT FROM AUTHOR]

Abstract: The aim of the present study was to quantify to what extent the scar tissue formation following the transfer of flexor carpi ulnaris (FCU) to the distal tendon of extensor carpi radialis (ECR) affects the force transmission from transferred FCU in the rat. Five weeks after recovery from surgery (tendon transfer group) and in a control group, isometric length–force characteristics of FCU were assessed for progressive stages of dissection: (i) with minimally disrupted connective tissues, (ii) after full dissection of FCU distal tendon exclusively, and (iii) after additional partial dissection of FCU muscle belly. Total and passive length–force characteristics of transferred and control FCU changed significantly by progressive stages of dissection. In both groups, tendon dissection decreased passive FCU force exerted at the distal tendon, as well as the slope of the length–force curve. However, force and slope changes were more pronounced for transferred FCU compared to controls. No additional changes occurred after muscle belly dissection. In contrast, total force increased in transferred FCU following both tendon and muscle belly dissection at all lengths studied, while dissection decreased total force of control FCU. In addition, after tendon and muscle belly dissection, we found decreased muscle belly lengths at equal muscle–tendon complex lengths of transferred FCU. We conclude that scar tissue limits the force transmission from transferred FCU muscle via the tendon of insertion to the skeleton, but that some myofascial connectivity of the muscle should be classified as physiological. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]

Abstract: 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 17years) 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. [Copyright &y& Elsevier]

Summary: This paper reviews the major intramuscular extracellular matrix (IM-ECM) structures (endomysium, perimysium and epimysium) and their possible mechanical contributions to muscle functions. The endomysium appears to provide an efficient mechanism for transmission of contractile forces from adjacent muscle fibres within fascicles. This coordinates forces and deformations within the fascicle, protects damaged areas of fibres against over-extension, and provides a mechanism whereby myofibrils can be interrupted to add new sarcomeres during muscle growth without loss of contractile functionality of the whole column. Good experimental evidence shows that perimysium and epimysium are capable in some circumstances to act as pathways for myofascial force transmission. However, an alternative role for perimysium is reviewed, which involves the definition of slip planes between muscle fascicles which can slide past each other to allow large shear displacements due to shape changes in the whole muscle during contraction. As IM-ECM is continually remodelled so as to be mechanically adapted for its roles in developing and growing muscles, control of the processes governing IM-ECM turnover and repair may be an important avenue to explore in the reduction of fibrosis following muscle injury. [Copyright &y& Elsevier]

Abstract: Elements of what we call myofascial force transmission today have been on peoples mind for a long time, usually implicitly, sometimes quite explicitly. A lot is there to be learned from the history of our knowledge on muscle and movement. There is little doubt about the presence and effectiveness of the mechanism and pathways of epimuscular myofascial force transmission. However, we should learn much more about the exact conditions at which such transmission is not only of fundamental biomechanical interest, but also quantitatively so important that it has to be considered for its effects in health and disease. Even if the quantitative effects in terms of force would prove small, one should realize that this mechanism will change the principles of muscular function drastically. A new vision on functional anatomy, as well as the application of imaging techniques and 3-D reconstruction of in vivo muscle, will aid that process of increased quantitative understanding, despite usual limitations regarding the mechanics in such experiments. I expect it is fair to say that without understanding myofascial force transmission we will never be able to understand muscular function completely. [Copyright &y& Elsevier]

Most often muscles (as organs) are viewed as independent actuators. To test if this is true for antagonistic muscles, force was measured simultaneously at: (1) the proximal and distal tendons of the extensor digitorum muscle (EDL) to quantify any proximo-distal force differences, as an indicator of myofascial force transmission, (2) at the distal tendons of the whole antagonistic peroneal muscle group (PER) to test if effects of EDL length changes are present and (3) at the proximal end of the tibia to test if myofascially transmitted force is exerted there. EDL length was manipulated either at the proximal or distal tendons. This way equal EDL lengths are attained at two different positions of the muscle with respect to the tibia and antagonistic muscles. Despite its relatively small size, lengthening of the EDL changed forces exerted on the tibia and forces exerted by its antagonistic muscle group. Apart from its extramuscular myofascial connections, EDL has no connections to either the tibia or these antagonistic muscles. Proximal EDL lengthening increased distal muscular forces (active PER ΔF &thkap; +1.7%), but decreased tibial forces (passive from 0.3 to 0 N; active ΔF &thkap; –5%). Therefore, it is concluded that these antagonistic muscles do not act independently, because of myofascial force transmission between them. Such a decrease in tibial force indicates release of pre-strained connections. Distal EDL lengthening had opposite effects (tripling passive force exerted on tibia; active PER force ΔF &thkap; –3.6%). It is concluded that the length and relative position of the EDL is a co-determinant of passive and active force exerted at tendons of nearby antagonistic muscle groups. These results necessitate a new view of the locomotor apparatus, which needs to take into account the high interdependence of muscles and muscle fibres as force generators, as well as proximo-distal force differences and serial and parallel distributions of sarcomere lengths that are consequences of such interaction. If this is done properly, the effects of integrating a muscle fibre, muscle or muscle group into higher levels of organisation of the body will be evident. Copyright © 2008 S. Karger AG, Basel [ABSTRACT FROM AUTHOR]

Effects of stimulation frequency on myofascial force transmission between rat peroneal and triceps surae and antagonistic anterior crural muscles, and between extensor digitorum longus (EDL) and tibialis anterior and extensor hallucis longus (TA + EHL) muscles were investigated for lengthening of all anterior crural muscles. Muscles contracted isometrically at firing rates of 10, 20, 30 and 100 Hz. EDL and TA + EHL were distally lengthened. Peroneal and triceps surae muscles attained a constant muscle-tendon complex length. Peroneal and triceps surae distal active force decreased significantly as a function of anterior crural muscle length, also at submaximal activation. The absolute decrease was highest for 100 Hz (peroneal muscles -0.87 N; triceps surae muscles -0.92 N), but the highest normalized decrease occurred at 10 Hz stimulation (peroneal muscles -34%; triceps surae muscles -18%). At all muscle lengths, a negative proximo-distal difference in EDL active force was present which decreased with lower firing frequencies (from -0.4 N at 100 Hz to -0.03 N at 10 Hz). The passive proximo-distal force difference attained positive values. EDL and TA + EHL length-force characteristics agree with effects of firing frequency, except for 10 Hz stimulation, where active force was higher than expected and optimum length shifted to lower muscle lengths. It is concluded that also at submaximal stimulation frequencies, extramuscular myofascial force transmission between peroneal and triceps surae muscles and antagonistic anterior crural muscles is substantial. Although lengthening of submaximally active anterior crural muscles decreases the net myofascially transmitted load on EDL, myofascial force transmission significantly alters effects of firing frequency on length-force characteristics. [ABSTRACT FROM AUTHOR]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: 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 ∼5%, ∼32% and ∼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. [Copyright &y& Elsevier]

Abstract: The purpose of the present study was to test the hypothesis that myofascial force transmission may not be limited by compartmental boundaries of a muscle group to synergists. Muscles of the anterior tibial compartment in rat hindlimb as well as of the neighbouring peroneal compartment (antagonistic muscles) were excited maximally. Length–force data, based on proximal lengthening, of EDL, as well as distal lengthening of the tibial muscles (TA+EHL) and the peroneal muscle group (PER) were collected independently, while keeping the other two muscle groups at a constant muscle–tendon complex length. Simultaneously measured, distal and proximal EDL active forces were found to differ significantly throughout the experiment. The magnitude of this difference and its sign was affected after proximal lengthening of EDL itself, but also of the tibial muscle complex and of the peroneal muscle complex. Proximal lengthening of EDL predominantly affected its synergistic muscles within the anterior crural compartment (force decrease <4%). Lengthening of either TA or PER caused a decrease in distal EDL isometric force (by 5–6% of initial force). It is concluded also that mechanisms for mechanical intermuscular interaction extend beyond the limits of muscle compartments in the rat hindlimb. Even antagonistic muscles should not be considered fully independent units of muscular function. Particular, strong mechanical interaction was found between antagonistic tibial anterior muscle and peroneal muscle complexes: Lengthening of the peroneal complex caused tibial complex force to decrease by approximately 25%, whereas for the reverse a 30% force decrease was found. [Copyright &y& Elsevier]

Abstract: The current rationale of clinical practice in spastic tendon transfer surgery is based on four assumptions: (1) changes in muscle fiber length (serial number of sarcomeres) determine the available length range and joint excursion, (2) muscle cross-sectional area determines the maximal force output, (3) fiber length and muscle force are invariable functions of muscle length, (4) there is an invariable relation between the elastic force and the active force exerted by the sarcomeres. The validity of these assumptions is discussed. Additionally, some new perspectives in muscle research are discussed and myofascial force transmission is introduced as a co-determinant for the outcome of tendon transfer by presenting some exploratory observations. [Copyright &y& Elsevier]

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 (Δ Fdist-prox = 0.4 N). However, the normalized difference was highest at 10 Hz (Δ Fdist-prox = 30% Fdist). 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 (Δ Ffrom initial = −0.25 N). However, the highest normalized decrease was found for 10 Hz stimulation (Δ Ffrom 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. [ABSTRACT FROM AUTHOR]

This study investigated the effects of myofascial force transmission during dynamic shortening of head III of rat extensor digitorum longus muscle (EDL III). The anterior crural compartment was left intact. Force was measured simultaneously at the distal EDL III tendon, the proximal EDL tendon and the distal tendons of tibialis anterior and extensor hallucis longus muscles (TA+EHL). Two types of distal shortening of EDL III were studied: (1) sinusoidal shortening (2 mm) and (2) isokinetic shortening (8 mm). Sinusoidal shortening of EDL III caused a decrease in force exerted at the distal tendon of EDL III: from 0.58 (0.08) N to 0.26 (0.04) N. In contrast, hardly any changes in proximal EDL force and distal TA+EHL force were found. Maximal concentric force exerted at the distal tendon of EDL III was higher than maximal isometric force expected on the basis of the physiological cross-sectional area of EDL III muscle fibers (Maas et al. 2003). Therefore, a substantial fraction of this force must originate from sources other than muscle fibers of EDL III. Isokinetic shortening of EDL III caused high changes in EDL III force from 0.97 (0.15) N to zero. In contrast, changes in proximal EDL force were much smaller: from 2.44 (0.25) N to 1.99 (0.19) N. No effects on TA+EHL force could be shown. These results are explained in terms of force transmission between the muscle belly of EDL III and adjacent tissues. Thus, also in dynamic muscle conditions, muscle fiber force is transmitted via myofascial pathways. [ABSTRACT FROM AUTHOR]

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. [Copyright &y& Elsevier]

The specific purpose of the present study was to show that extramuscular myofascial force transmission exclusively has substantial effects on muscular mechanics. Muscle forces exerted at proximal and distal tendons of the rat extensor digitorium longus (EDL) were measured simultaneously, in two conditions (1) with intact extramuscular connections (2) after dissecting the muscles' extramuscular connections to a maximum extent without endangering circulation and innervation (as in most in situ muscle experiments). A finite element model of EDL including the muscles' extramuscular connections was used to assess the effects of extramuscular myofascial force transmission on muscular mechanics, primarily to test if such effects lead to distribution of length of sarcomeres within muscle fibers. In condition (1), EDL isometric forces measured at the distal and proximal tendons were significantly different (F dist > F prox , ΔF approximates maximally 40% of the proximal force). The model results show that extramuscular myofascial force transmission causes distributions of strain in the fiber direction (shortening in the proximal, lengthening in the distal ends of fibers) at higher lengths. This indicates significant length distributions of sarcomeres arranged in series within muscle fibers. Stress distributions found are in agreement with the higher distal force measured, meaning that the muscle fiber is no longer the unit exerting equal forces at both ends. Experimental results obtained in condition (2) showed no significant changes in the length-force characteristics (i.e., proximo-distal force differences were maintained). This shows that a muscle in situ has to be distinguished from a muscle that is truly isolated in which case the force difference has to be zero. We conclude that extramuscular myofascial force transmission has major effects on muscle functioning. [ABSTRACT FROM AUTHOR]

Force is transmitted from muscle fiber to bone via several pathways: (1) via the tendons (i.e. myotendinous force transmission), (2) via intermuscular connective tissue to adjacent muscles (i.e. intermuscular myofascial force transmission), (3) via structures other than muscles (i.e. extramuscular myofascial force transmission). In vivo, the position of a muscle relative to adjacent muscles changes due to differences in moment arm between synergists as well as due to the fact that some muscles span only one joint and other muscles more than one joint. The position of a muscle relative to non-muscular structures within a compartment is altered with each change of the length of the muscle. The aim of this article is to describe recent experimental results, as well as some new experimental data, that have elucidated the role of muscle relative position on force transmission from muscle. Furthermore, relevant literature is discussed, taking into consideration these new insights of muscle functioning. It is concluded that the position of a muscle relative to surrounding tissues is a major co-determinant of isometric muscle force. For muscles operating within their in vivo context of connective tissue, such position effects should be taken into account. [ABSTRACT FROM AUTHOR]

Intramuscular connective tissues are continuous to extramuscular connective tissues. If force is transmitted there, differences should be present between force at proximal and distal attachments of muscles. Extensor digitorum longus (EDL), tibialis anterior (TA), and extensor hallucis longus muscles (EHL) were excited simultaneously and maximally. Only EDL length was changed, exclusively by moving the position of its proximal tendon. Distal force exerted by TA + EHL complex was not affected significantly. Proximal and distal EDL isometric force were not equal for most EDL lengths: Fprox – Fdist ranged from 0 to ≈+22.7% of Fprox at higher lengths and from 0 to ≈–24.5% at the lowest lengths. It is concluded that extramuscular connections transmit force from muscle. Significant proximo-distal differences of EDL force remained after repeated measurements, regardless of length order, although their length dependence was altered. Measurements of both proximal and distal EDL force were highly reproducible, if EDL did not attain higher lengths than target length. After being active at high lengths, proximal and distal length–force curves were altered at low lengths but not for the highest length range. Extensor digitorum longus length–active force hysteresis was present for proximal as well as distal EDL measurements with increasing and decreasing isometric length order. Further isolating EDL removed the proximo-distal difference for active EDL force. However a decreased difference for passive EDL force remained, which was ascribed to remaining extramuscular connective tissue linkages. It is concluded that extramuscular myofascial force transmission is an important feature of muscle that is not isolated from its surrounding tissues. [ABSTRACT FROM AUTHOR]

Muscles within the anterior tibial compartment (extensor digitorum longus: EDL, tibialis anterior: TA, and extensor hallucis longus muscles: EHL) and within the peroneal compartment were excited simultaneously and maximally. The ankle joint was fixed kept at 90°. For EDL length force characteristics were determined. This was performed first with the anterior tibial compartment intact (1), and subsequently after: (2) blunt dissection of the anterior and lateral interface of EDL and TA. (3) Full longitudinal lateral fasciotomy of the anterior tibial compartment. (4) Full removal of TA and EHL muscles. Length-force characteristics were changed significantly by these interventions. Blunt dissection caused a force decrease of approximately 10% at all lengths, i.e., without changing EDL optimum or active slack lengths. This indicates that intermuscular connective tissue mediates significant interactions between adjacent muscles. Indications of its relatively stiff mechanical properties were found both in the physiological part of the present study, as well as the anatomical survey of connective tissue. Full lateral compartmental fasciotomy increased optimum length and decreased active slack length, leading to an increase of length range (by ≈47%), while decreasing optimal force. As a consequence an increase in force for the lower length range was found. Such changes of length force characteristics are compatible with an increased distribution of fiber mean sarcomere length. On the basis of these results, it is concluded that extramuscular connective tissue has a sufficiently stiff connection to intramuscular connective tissue to be able to play a role in force transmission. Therefore, in addition to intramuscular myofascial force transmission, extramuscular force transmission has to be considered within intact compartments of limbs. A survey of connective tissue structures within the compartment indicated sheet-like neuro-vascular tracts to be major components of extramuscular connective tissue with connections to intramuscular connective tissue stroma. Removal of TA and EHL yielded yet another decrease of force (mean for optimal force ≈10%). No significant changes of optimum and active slack lengths could be shown in this case. It is concluded that myofascial force transmission should be taken into account when considering muscular function and its coordination, and in clinical decisions regarding fasciotomy and repetitive strain injury. [ABSTRACT FROM AUTHOR]

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. 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. 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%). 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. 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. [ABSTRACT FROM AUTHOR]