Relationship between muscle length and moment arm on EMG activity of human triceps surae muscle.

PURPOSE: The purpose of this experiment was to evaluate the effects of both muscle length and moment arm (MA) on the electromyographic (EMG) and force output of the triceps surae (TS) muscle. RELEVANCE: It is well recognized that changes in muscle length affect both the muscle's force generating capacity as well as its twitch speed. This relationship is well established in animal preparations. Contrary to animal experiments where length can be directly manipulated in isolated muscles, human experiments require that all muscle length changes be secondary to changes in a joint angle. Such experimental manipulations therefore produce changes in not only muscle length, but also in the muscle's MA. The relative effect of muscle length and MA changes on muscle EMG has not been determined in previous experiments. METHODS: This study was executed in two phases. First, using fresh human cadaver lower limbs, data were gathered describing the relationship between knee and ankle angle changes for maintenance of a constant TS muscle length, while its MA at the ankle joint has been changed. In the second phase of the study, results obtained from phase one were applied to 10 healthy adult human subjects to measure the EMG (surface and fine wire) activity of TS at three different conditions: when both length and MA were shortened, when muscle length was decreased given a constant MA and when MA was shortened given a constant muscle length. RESULTS: A significant increase in muscle activity was found as both the length and MA of TS muscle were shortened. A similar pattern of increased muscle activity was observed when the MA was shortened given a constant muscle length. No significant change in TS activity was found when muscle length was shortened, given a constant MA at the ankle joint. CONCLUSIONS: The findings of this study indicate that changes in the Achilles tendon MA predominate over the muscle length variations in determining the level of TS activity when generating plantar flexion torque.     

A model of the human triceps surae muscle-tendon complex applied to jumping

The purpose of this study was to gain more insight into the behavior of the muscle-tendon complex of human m. triceps surae in jumping. During one-legged vertical jumps of ten subjects ground reaction forces as well as cinematographic data were registered, and electromyograms were recorded from m. soleus and m. gastrocnemius. A model was developed of m. triceps surae, incorporating assumptions concerning dimensions, architecture, force-length and force-velocity relationships of muscle fibers, as well as assumptions concerning dimensions and elastic behavior of tendinous tissue in series with the muscle fibers. The velocity with which origin approaches insertion (V OI) was calculated for m. soleus and m. gastrocnemius using cine film data, and served as input of the model. During the last part of the push-off phase EMG-levels were found to be more or less constant, V OI of m. soleus and m. gastrocnemius rapidly increased, and the plantar flexing moment obtained by solving equations concerning a free body diagram of the foot rapidly declined. A similar decline was observed in the plantar flexing moment obtained by multiplying force calculated with help of the model by estimated moment arm at the ankle. As a result of the decline of exerted force tendon length decreases. According to the model the shortening velocity of tendon reaches higher values than that of muscle fibers. The results of a kinetic analysis demonstrate that during the last part of the push-off phase a combination of high angular velocities with relatively large plantar flexing moments is required. It is concluded that without a compliant tendon m. triceps surae would not be able to satisfy this requirement.     

Dynamic structural characteristics of the m. triceps surae in human subjects of various physiques

At examination of 305 boys and girls Russian, Kazakh in Alma-Ata strength of the m. triceps surae (strength of the foot flexion) has been estimated, anatomical diameter of this muscle and strength per 1 sm2 of the anatomical diameter (specific strength) has been calculated. Somatotyping after V. N. Shevkunenko has been performed. At dolichomorphy the specific strength is higher than at brachymorphy. In 24 corpses of 18-38-year-old men typical peculiarities of construction and structure of the m. triceps surae is studied. Correlation of the m. triceps surae parts and degree of its pinnateness do not reveal any definite differences in connection with the complexion. As demonstrate the data of light microscopy and mathematical treatment of the results by means of mixtures method statistically reliable predominance of the connective tissue component in the m. triceps surae and more often occurrence of thick muscle fibers in the m. soleus at dolichomorphy is stated. According to the electron microscopy data, better development of the connective tissue carcass of the m. triceps surae is proved in persons of dolichomorphic complexion.     

Assessment of muscle volume and physiological cross-sectional area of the human triceps surae muscle in vivo

The purpose of the present study was to investigate whether it is possible to predict the individual muscle volumes within the triceps surae (TS) muscle group by means of easily measurable parameters based on a theoretical consideration. A further objective was to verify the use of the available literature data to assess the contribution of each muscle of the group to the entire TS volume or physiological cross-sectional-area (PCSA). Therefore, magnetic resonance images of the right calf of 13 male subjects were acquired and each muscle of the TS was reconstructed. Muscle length (l(m)), the maximum anatomical cross-sectional-area (ACSA(max)) and muscle volume were obtained from the 3D models. To assess the PCSA, fascicle length was determined by ultrasonography. In general, muscle volume can be expressed as a fraction of the product of ACSA(max) and l(m). The size of the fraction depends on muscle shape and its coefficient of variance among the examined population was considerable low (soleus 6%, gastrocnemius 4% and gastrocnemius lateralis 7%) in the present study. The product of ACSA(max) and l(m) was, therefore, suitable to assess muscle volume (root mean squares, RMS 4-7%). Further, the soleus, gastrocnemius medialis and gastrocnemius lateralis accounted on average for about 52+/-3%, 32+/-2% and 16+/-2% of the total TS volume and 62+/-5%, 26+/-3% and 12+/-2% of the entire TS PCSA, respectively. The coefficient of variance of the relative portions were 5-10% for muscle volume and 8-17% for the PCSA.     

Muscle activation and its distribution within human triceps surae muscles.

The purposes of this study were 1) to quantify the volume of activated parts within a whole muscle and 2) to examine activated area distributions along the length of muscle. Seven male subjects performed five sets of 10 repetitions of a single-leg calf-raise exercise with the knee fully extended. Transverse relaxation time (T2)-weighted spin echo images were acquired before and immediately after the exercise. A range of pixels with a T2 greater than the mean +1 SD of the region of interest (ROI) from the preexercise image and pixels with a T2 lower than the mean + SD of the ROI from the postexercise image were defined as "active" muscle. The active muscle images were three dimensionally reconstructed, from which the volume of the activated muscle was determined for individual triceps surae (TS) muscles. Our data indicate that approximately 46% of the medial gastrocnemius (MG) muscle was activated during the exercise, with activation of the lateral gastrocnemius (LG) and soleus (Sol) muscles being approximately 35%. In the MG, distal portions had a greater percentage area of activated muscle than the proximal portions (P < 0.05), which was consistent with the results regarding electromyogram activity. In contrast, regional activation differences were not observed in the LG and Sol. These findings suggest that the amounts of activated muscle and its distribution would be different among TS muscles.     

Influences of experimental factors on spinal stretch reflex latency and amplitude in the human triceps surae.

The spinal stretch reflex (SSR) is commonly assessed via electromyographic (EMG) analysis of joint perturbations inducing changes in muscle length. Previous literature indicates that when large experimental changes in magnitude of agonist background EMG, perturbation velocity, and perturbation amplitude are employed, SSR latency and amplitude are significantly altered. The purpose of this investigation was to evaluate the relative dependence of SSR latency and amplitude on inherent variability in these experimental variables. Soleus SSR latency and amplitude were assessed in 40 healthy subjects following dorsiflexion perturbation under an active state ( approximately 14% MVC). Experimental variables displayed limited variability (means +/- SD): soleus background EMG (13.47 +/- 7.08% MVC), perturbation velocity (96.1 +/- 30 degrees /s), and perturbation amplitude (4 +/- 1 degrees ). SSR latency was not significantly related to soleus background EMG (r = 0.189), perturbation velocity (r = 0.213), or perturbation amplitude (r = 0.202). Similarly, SSR amplitude was not significantly related to soleus background EMG (r = 0.306), perturbation velocity (r = 0.053), or perturbation amplitude (r = 0.056). Variability in experimental variables was much smaller than what has been reported in the literature to significantly impact SSR characteristics. These results suggest that SSR latency and amplitude are independent of agonist background EMG, perturbation velocity, and perturbation amplitude when experimental variability is relatively limited.     

The sensitivity of Renshaw cells to velocity of sinusoidal stretches of the triceps surae muscle.

1. The electrical activity of Renshaw cells monosynaptically excited by ventral root stimulation and disynaptically excited by electric stimulation of the group I afferents in the GS nerve has been recorded and their response to individual sinusoidal stretches of the deefferented GS muscle tested for different amplitudes and durations of the stimulus. 2. The experimental data indicate that the Rensahw cell responses are not only length dependent but also rate dependent. This finding indicates that the same Renshaw cells receive recurrent collaterals of both tonic and phasic motoneurons. 3. The observation that the discharge of Renshaw cells is particularly sensitive to the velocity of stretch suggests that the recurrent collaterals of large phasic motoneurons, which are recruited during high velocity stretches, exert a stronger excitatory action on Renshaw cells than do axon collaterals of the smaller tonic motoneurons, which are selectively stimulated during low velocity stretches.     

Heterogeneity of muscle activation in relation to force direction: A multi-channel surface electromyography study on the triceps surae muscle

Several skeletal muscles can be divided into sub-modules, called neuromuscular compartments (NMCs), which are thought to be controlled independently and to have distinct biomechanical functions. We looked for distinct muscle activation patterns in the triceps surae muscle (TS) using surface electromyography (EMG) during voluntary contraction. Nine subjects performed isometric and isotonic plantar flexions combined with forces along pre-defined directions. Besides the forces under the ball of the foot, multi-channel surface EMG was measured with electrodes homogeneously distributed over the entire TS. Using principal component analysis, common (global) components were omitted from the EMG signals, thereby estimating muscle activity sufficiently accurate to track fine fluctuations of force during an isotonic contraction (r = 0.80 ± 0.09). A subsequent cluster analysis showed a topographical organization of co-activated parts of the muscle that was different between subjects. Low and negative correlations between the EMG activity within clusters were found, indicating a substantial heterogeneity of TS activation. The correlations between cluster time series and forces at the foot in specific directions differed substantially between clusters, showing that the differentially activated parts of the TS had specific biomechanical functions.     

The effects of athletic training and muscle contractile character on the pressor response to isometric exercise of the human triceps surae

In this study, the influence of athletic training status and the contractile character of the active muscle on the magnitude of the pressor response (PR) to voluntary and electrically evoked isometric plantar flexion was investigated. Subjects were 10 sprint-trained athletes (sprint) (100-m, 200-m and 400-m) [mean (SD) age, 21 (2) years], 14 endurance trained athletes (distance) [22 (2) years] and 8 untrained men (control) [23 (3) years]. Twitch time to peak tension (TPT) in the sprint group [108 (7) ms] was significantly less (P<0.001) than that of the distance group [124 (10) ms]. During voluntary contraction, the mean change in systolic blood pressure (SBP), diastolic blood pressure (DBP) and heart rate (fc) was not significantly different between groups. During electrically evoked contractions, mean changes in SBP, DBP and fc were not significantly different between the sprint, distance and control groups. However, division of the sprint group into 400-m (sprint I) and 100/200-m athletes (sprint II) showed that an increase in DBP of 1.6 kPa (12 mm Hg) in sprint I was significantly less (P<0.05) than the 2.5 kPa (19 mm Hg) increase observed for both the distance and control groups. Prediction of the DBP response from our previously published relationship between TPT and DBP showed close agreement in all subject groups except sprint I; in these subjects the observed DBP response was only 55% of that predicted. Attenuation of the PR in the involuntary experiment suggests that some aspect of sprint training, but not endurance training, modifies the muscle afferent input to the PR in man.     

Histological and histochemical studies on the nervous influence on minced muscle regeneration of triceps surae of the rat.

The degree of minced rat muscle regeneration in the absence of nerve fibers was compared with that of normal regenerates between one and 270 days postoperatively. Up to around 30 days, the number of muscle fibers and their morphology were comparable in both normal innervated and denervated regenerates; both showed clear cross striations and peripherally located nuclei. Histochemically, SDH and myofibrillar ATPase (pH=9.4) reactions were positive, but there were no typical signs of fiber types in either case of regeneration. The only consistent difference in the early period was the smaller fiber cross sectional areas in denervated regenerates than in innervated ones. Starting about 40 days, the muscle fibers in innervated regenerates became differentiated into different fiber types (fast-twitch-oxidative-glycolytic, FOG., fast-twitch-glycolytic, FG., slow-twitch-oxidative, SO.) but there were no such activities in denervated regenerates, although their SDH and myofibrillar ATPase reactions remained positive for a long time. Degenerating muscle fibers could no longer be identified in innervated regenerates. In the denervated regenerates, however, muscle fibers underwent atrophic or degenerative changes and were replaced by connective tissue. The complete disappearance of muscle fibers varied with individual regenerates. In some cases, it occurred about 90 days and in others, traces of muscle fibers could still be seen as late as 150 days postoperatively. Thus, nerves seem to be important primarily in the late phase of regeneration; namely, differentiation of fiber types and maintenance of the structural integrity of muscle fibers.     

Neural and mechanical responses of the triceps surae muscle group after 1 h of repeated fast passive stretches.

Experiments were carried out to examine interaction between mechanical changes of the muscle-tendon unit and reduced reflex sensitivity after repeated and prolonged passive muscle stretching (RPS). There is some evidence that this interaction might be relevant also during active stretch-shortening cycle type of fatigue tasks. The results demonstrated a clear deterioration of voluntarily and electrically induced muscle contractions after RPS. Maximal voluntary contraction (MVC), average electromyographic activities of the gastrocnemius and soleus muscles, and maximal twitch contraction decreased on average by 13.8, 10.4, 7.6, and 16.8%, respectively. In addition, there was a 14% lengthening in the total duration of the twitch. MVCs measured at different ankle joint angles revealed a downward and rightward shift in the torque-fascicle length curve after RPS. Interestingly, there was a crossing in the torque-fascicle length curves while measured at different activation levels but at the same joint angle before and after RPS. Even though no changes were observed in the activation level during MVCs, all the reflex parameters showed a clear reduction after RPS. This study presents evidence that repeated and prolonged passive muscle stretching can lead to some modification of material behavior of the aponeurosis-tendon system, such as stress relaxation and/or plastic deformation. In addition, altered material properties seem to affect proprioceptive feedback and, therefore, the motor unit activation in proportion to the contractile failure.     

Mechanical and morphological properties of human quadriceps femoris and triceps surae muscle-tendon unit in relation to aging and running

The purpose of this study was to examine the effects of aging and endurance running on the mechanical and morphological properties of different muscle-tendon units (MTUs) in vivo. The investigation was conducted on 30 elderly and 19 young adult males. For the analysis of possible MTU adaptation in response to endurance running the subjects were divided into two subgroups: non-active vs. endurance-runners. All subjects performed isometric maximal voluntary plantarflexion and knee extension contractions on a dynamometer. The distal aponeurosis of the gastrocnemius medialis (GM) and vastus lateralis (VL) during plantarflexion and knee extensions and the muscle architecture of the GM and VL were visualized by ultrasonography. The maximal knee and ankle joint moment were higher for the young compared to the elderly population (p<0.05). No identifiable differences in muscle architecture between young and elderly subjects were detected in VL and GM. Aging results in a reduced (p<0.05) normalized stiffness of the quadriceps femoris tendon and aponeurosis, which were not identifiable for the triceps surae. In contrast, the properties of both MTUs showed no major differences between endurance-runners and the non-active group (p>0.05). Only pennation angle at the GM were higher for the runners compared to the non-active group (p<0.05). The present results indicate that tendon changes related to aging do not occur proportionally in different MTUs. Furthermore, it seems that the extra stress and load imposed on high-load-bearing MTUs during endurance running may not be sufficient to produce significant adaptative processes in the mechanical parameters analyzed.     

An apparatus and a method for measuring the relationship of triceps surae torques to tibio-tarsal angles in man.

1. The apparatus and method described give the relationship of triceps surae torque to the absolute tibio-tarsal angle. Data of the anatomy of the foot and the axis of the ankle are taken into consideration. They make complementary measurements necessary, which in certain cases enable the angle recorded by the apparatus to be corrected so as to obtain the true angle. 2. The reliability of the apparatus and the technique of linkage of subject to apparatus are judged by examination of the torque-angle plots obtained by passive stretching of the triceps in 7 subjects for whom correction is monor. Good reliability was observed during the session and from one session to another. The plot is exponential. The merits of the method of correcting angles are illustrated in three subjects with the same protocol: when plots are not corrected, they are more dispersed during the session and no longer exponential. These results justify the considerable correction of the angle that we propose in the study of isometric contractions.     

Rescoring docking hit lists for model cavity sites: predictions and experimental testing

Molecular docking computationally screens thousands to millions of organic molecules against protein structures, looking for those with complementary fits. Many approximations are made, often resulting in low “hit rates.” A strategy to overcome these approximations is to rescore top-ranked docked molecules using a better but slower method. One such is afforded by molecular mechanics-generalized Born surface area (MM-GBSA) techniques. These more physically realistic methods have improved models for solvation and electrostatic interactions and conformational change compared to most docking programs. To investigate MM-GBSA rescoring, we re-ranked docking hit lists in three small buried sites: a hydrophobic cavity that binds apolar ligands, a slightly polar cavity that binds aryl and hydrogen-bonding ligands, and an anionic cavity that binds cationic ligands. These sites are simple; consequently, incorrect predictions can be attributed to particular errors in the method, and many likely ligands may actually be tested. In retrospective calculations, MM-GBSA techniques with binding-site minimization better distinguished the known ligands for each cavity from the known decoys compared to the docking calculation alone. This encouraged us to test rescoring prospectively on molecules that ranked poorly by docking but that ranked well when rescored by MM-GBSA. A total of 33 molecules highly ranked by MM-GBSA for the three cavities were tested experimentally. Of these, 23 were observed to bind—these are docking false negatives rescued by rescoring. The 10 remaining molecules are true negatives by docking and false positives by MM-GBSA. X-ray crystal structures were determined for 21 of these 23 molecules. In many cases, the geometry prediction by MM-GBSA improved the initial docking pose and more closely resembled the crystallographic result; yet in several cases, the rescored geometry failed to capture large conformational changes in the protein. Intriguingly, rescoring not only rescued docking false positives, but also introduced several new false positives into the top-ranking molecules. We consider the origins of the successes and failures in MM-GBSA rescoring in these model cavity sites and the prospects for rescoring in biologically relevant targets.     

Regeneration of a tonic avian muscle: transformation to a twitch morphology.

The tonic anterior latissimus dorsi muscle of the pigeon was excised, minced into fine pieces, replaced into its original bed, and allowed to regenerate for periods up to 37 weeks. Although regeneration was asynchronous, regeneration patterns of the muscle fibers suggested the following sequence of fiber development: undifferentiated to tonic to twitch. Fiber types were identified on the basis of Z-line morphology and qualitative development and organization of the sarcotubular system as demonstrated by electron microscopy. Histochemical demonstration of myofibrillar adenosine triphosphatase and succinic dehydrogenase activities corroborated the morphological evidence, suggesting a transformation to a twitch morphology. In addition to the transformation to the twitch morphology, other alterations were observed in these regenerating fibers. Among these were large numbers of closely-packed 60-nm-diameter tubules, thought to be derived from the sarcoplasmic reticulum; mitochondria with intermembraneous dense material; and Z-line streaming. The transformation of the muscle fibers from tonic to twitch morphology is discussed in terms of alterations in nerve impulse activity to the regenerating muscle.     

Immunity to tetanus: tetanus antitoxin and anti-BIIb in human sera

Immunity to tetanus was investigated in 157 individuals aged between 1 and 77 years using, for the evaluation of both tetanus antitoxin and antibody to fragment BIIb (anti-BIIb), an easily performed ELISA that gave reproducible results. Among these people 72% were protected against tetanus (tetanus antitoxin titres greater than 0.06 IU ml-1). Anti-BIIb titres greater than 0.15 U ml-1 were found in 75% of the males vs 57% of the females (P less than 0.02) with marked variations according to age. Furthermore, 13 out of 41 individuals with antitoxin titres less than 0.06 IU ml-1 (not protected against tetanus) were found to have anti-BIIb titres greater than 0.15 U ml-1. These data raise questions as to the significance and protective effect of anti-BIIb against tetanus.     

Physiological basis of muscle functional MRI: predictions using a computer model.

Muscle functional MRI (mfMRI) has been proposed as a tool for noninvasively measuring the metabolic and hemodynamic responses to muscle activation, but its theoretical basis remains unclear. One challenge is that it is difficult to isolate individually those variables affecting the magnitude and temporal pattern of the mfMRI response. Therefore, the purpose of this study was to develop a computer model of how physiological factors altered during exercise affect the mfMRI signal intensity time course and then predict the contributions made by individual factors. A model muscle containing 39,204 fibers was defined. The fiber-type composition and neural activation strategies were designed to represent isometric contractions of the human anterior tibialis muscle, for which published mfMRI data exist. Sustained isometric contractions at 25 and 40% maximum voluntary contraction were modeled, as were the vascular (capillary recruitment, blood oxygen extraction) and metabolic (lactate accumulation, phosphocreatine hydrolysis, pH) responses. The effects on the transverse relaxation of MRI signal were estimated, and the mfMRI signal intensity time course was measured from simulated images. The model data agreed well qualitatively with published experimental data, and at long exercise durations the quantitative agreement was also good. The model was then used to predict that NMR relaxation effects secondary to blood volume and oxygenation changes, plus the creatine kinase reaction, dominate the mfMRI time course at short exercise durations (up to approximately 45 s) and that effects secondary to glycolysis are the main contributors at later times.     

Predictions of mechanical output of the human M. triceps surae on the basis of electromyographic signals: the role of stimulation dynamics

In order to assess the significance of the dynamics of neural control signals for the rise time of muscle moment, simulations of isometric and dynamic plantar flexion contractions were performed using electromyographic signals (EMG signals) of m. triceps surae as input. When excitation dynamics of the muscle model was optimized for an M-wave of the medial head of m. gastrocnemius (GM), the model was able to make reasonable predictions of the rise time of muscle moment during voluntary isometric plantar flexion contractions on the basis of voluntary GM EMG signals. The rise time of muscle moment in the model was for the greater part determined by the amplitude of the first EMG burst. For dynamic jumplike movements of the ankle joint, however, no relationship between rise time of muscle moment in the experiment and muscle moment predicted by the model on the basis of GM EMG signals was found. Since rise time of muscle moment varied over a small range for this movement, it cannot be completely excluded that stimulation dynamics plays a role in control of these simple single-joint movements.