Effects of repeated Achilles tendon vibration on triceps surae force production

Many studies reported benefits of whole-body vibration (WBV) on muscle force production. Therefore, WBV may be an important technique for muscle re-education. However vibrating platforms are heavy tools that cannot be easily used by all patients. Thus, we propose to apply vibrations directly to the Achilles tendon at rest with a portable vibrator. We investigated whether 14 days of such a vibration program would enhance triceps surae force production in healthy subjects. If successful, such a protocol could be utilized to prevent deleterious effects of hypo-activity. Twenty-nine healthy students participated in this study. The electrical evoked twitch and maximal voluntary contraction (MVC) in plantar-flexion, and electromyograms (EMG) were quantified before and at the end of the program. The vibration program consisted of 14 days of daily vibration applied at rest (duration: 1 h; frequency: 50 Hz). After the program, there was an increase in MVC associated with greater EMG of the TS. No sign of hypertrophy were found on the twitch parameters and the EMG–torque relationships. Repeated vibrations of the Achilles tendon lead to an increase in plantar-flexor activation and thus to greater force developed in voluntary conditions whilst the contractile properties assessed by the twitch are not modified. This program could be beneficial to persons with hypo-activity who are not candidates for WBV.     

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.     

An estimation of power output and work done by the human triceps surae muscle-tendon complex in jumping

In explosive movements involving the lower extremity elastic recoil and transportation of power from knee to ankle via m. gastrocnemius allow power output about the ankle to reach values over and above the maximum power output of the plantar flexors. The object of this study was to estimate the relative power and work contributions of these two mechanisms for the push-off phase in one-legged jumping. During jumps of ten subjects ground reaction forces and cinematographic data were recorded. The data were used for a kinematic and kinetic analysis of the jumps yielding, among other variables, the velocity with which origins of m. soleus and m. gastrocnemius approach insertion (V OI), and net power output about the ankle (P A). V OI of m. soleus and m. gastrocnemius were imposed upon a model of the muscle-tendon complex of m. triceps surae, and power contributions of muscle fibers (P fibers), tendinous structures (P tendon), and transportation (P transported) were calculated. During the last 150 ms before toe-off, P A was found to increase rapidly and to attain an average peak value of 1790 W. The curve obtained by summation of P fibers, P tendon and P transported closely resembled that of P A. On the instant that the latter peaked (50 ms before toe-off) P fibers and P tendon of m. triceps surae contributed 27 and 53% respectively, and P transported contributed 20%.(ABSTRACT TRUNCATED AT 250 WORDS)     

Muscle activation and the isokinetic torque-velocity relationship of the human triceps surae

The influence of muscle activation and the time allowed for torque generation on the angle-specific torque-velocity relationship of the triceps surae was studied during plantar flexion using supramaximal electrical stimulation and a release technique on six male subjects [mean (SD) age 25 (4) years]. Torque-velocity data were obtained under different levels of constant muscle activation by varying the stimulus frequency and the time allowed for isometric torque generation prior to release and isokinetic shortening. To eliminate the effects of the frequency response on absolute torque the isokinetic data were normalized to the maximum isometric torque values at 0.44 rad. There were no significant differences in the normalized torques generated at any angular velocity using stimulus frequencies of 20, 50 or 80 Hz. When the muscle was stimulated at 50 Hz the torques obtained after a 400 ms and 1 s pre-release isometric contraction did not differ significantly. However, with no pre-release contraction significantly less torque was generated at all angular velocities beyond 1.05 rad · s^–1 when compared with either the 200, 400 ms or 1 s condition. With a 200 ms pre-release contraction significantly less torque was generated at angular velocities beyond 1.05 rad · s^–1 when compared with the 400 ms or 1 s conditions. It would seem that the major factor governing the shape of the torque-velocity curve at a constant level of muscle activation is the time allowed for torque generation.     

Effects of long-term immobilization and recovery on human triceps surae and collagen turnover in the Achilles tendon in patients with healing ankle fracture.

The aim of the present study was to analyze how human tendon connective tissue responds to an approximately 7-wk period of immobilization and a remobilization period of a similar length, in patients with unilateral ankle fracture, which is currently unknown. Calf muscle cross-sectional area (CSA) decreased by 15% (5,316 to 4,517 mm2) and strength by 54% (239 to 110 N.m) in the immobilized leg after 7 wk. During the 7-wk remobilization, the CSA increased by 9% (to 4,943 mm2) and strength by 37% (to 176 Nm). Achilles tendon CSA did not change significantly during either immobilization or remobilization. Local collagen turnover was measured as the peritendinous concentrations of NH2-terminal propeptide of type I collagen (PINP) and COOH-terminal telopeptide region of type I collagen (ICTP), markers thought to be indexes of type I collagen synthesis and degradation, respectively. Both markers were increased (PINP: 257 vs. 56 ng/ml; ICTP: 9.8 vs. 2.1 microg/l) in the immobilized leg compared with the control leg after the 7 wk of immobilization, and levels decreased again in the immobilized leg during the recovery period (PINP: 103 vs. 44 ng/ml; ICTP: 4.2 vs. 1.9 microg/l). A significant reduction in calf muscle CSA and strength was found in relation to 7 wk of immobilization. Immobilization increased both collagen synthesis and degradation in tendon near tissue. However, it cannot be excluded that the facture of the ankle in close proximity could have affected these data. Remobilization increased muscle size and strength and tendon synthesis and degradation decreased to baseline levels. These dynamic changes in tendon connective tissue turnover were not associated with macroscopic changes in tendon size.     

Triceps surae muscle-tendon unit length changes as a function of ankle joint angles and contraction levels: the effect of foot arch deformation

The purpose of this study was to clarify how foot deformation affects the relationship between triceps surae muscle-tendon unit (MTU) length and ankle joint angle. For six women and six men a series of sagittal magnetic resonance (MR) images of the right foot were taken, and changes in MTU length (the displacement of the calcaneal tuberosity), foot arch angle, and ankle joint angle were measured. In the passive session, each subject's ankle joint was secured at 10° dorsiflexed position, neutral position (NP), and 10° and 20° plantar flexed positions while MR images were acquired. In the active session, each subject was requested to perform submaximal isometric plantar flexions (30%, 60%, and 80% of voluntary maximum) at NP. The changes in MTU length in each trial were estimated by two different formulae reported previously. The changes of the measured MTU length as a function of ankle joint angles observed in all trials of the active session were significantly (p<0.05) larger than corresponding values in the passive session and by the estimation formulae. In the passive session, MTU length changes were significantly smaller than the estimated values when the ankle was plantar flexed. The foot arch angle increased as the contraction level increased from rest (117 ± 4°) to 80% (125 ± 3°), and decreased as the ankle was positioned further into plantar flexion in the passive session (115 ± 3°). These results indicate that foot deformation profoundly affects the triceps surae MTU length-ankle joint angle relationship during plantar flexion.     

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.     

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.     

Spontaneous activity of passive muscle spindles of the cat triceps surae muscle

The hypothesis that one possible cause of the spontaneous discharge of muscle spindles in the totally relaxed and de-efferented triceps surae muscle, with its tendon divided, is the mechanical factor due to extrafusal-intrafusal interaction was tested in experiments on anesthetized cats. Responses of spontaneously active units were similar with respect to many indices to responses of silent receptors capable of generating a long discharge in response to an increase in static length of the muscle. Isotonic contraction of the relaxed muscle during direct or indirect stimulation was accompanied by a pause in spontaneous activity. The prolonged increase in discharge frequency sometimes arising as a result of pressure on the muscle in the region of the receptor also was abolished by weak isotonic contraction of the muscle. If a long posttetanic sensory discharge was induced in a spontaneously active receptor by intensive tetanization of the nerve to the muscle, and it was then abolished by short stretching of the muscle, the initial spontaneous discharge frequency was restored. The dynamic thresholds of spontaneously active receptors were lower than for silent receptors. In some spontaneously active receptors an initial slowing of the discharge was observed during linear or stepwise stretching of the muscle. It is suggested that the ability of sensory endings to generate a long spontaneous discharge is due to initial stretching of the spindle inside the relaxed muscle.I. P. Pavlov Institute of Physiology, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 10, No. 3, pp. 287–294, May–June, 1978.     

Superficial aponeurosis of human gastrocnemius is elongated during contraction: implications for modeling muscle-tendon unit.

Two questions were addressed in this study: (1) how much strain of the superficial aponeurosis of the human medial gastrocnemius muscle (MG) was obtained during voluntary isometric contractions in vivo, (2) whether there existed inhomogeneity of the strain along the superficial aponeurosis. Seven male subjects, whose knees were extended and ankles were flexed at right angle, performed isometric plantar flexion while elongation of superficial aponeurosis of MG was determined from the movements of the intersections made by the superficial aponeurosis and fascicles using ultrasonography. The strain of the superficial aponeurosis at the maximum voluntary contraction, estimated from the elongation and length data, was 5.6+/-1.2%. There was no significant difference in strain between the proximal and distal parts of the superficial aponeurosis. Based on the present result and that of our previous study for the same subjects (J. Appl. Physiol 90 (2001) 1671), a model was formulated for a contracting uni-pennate muscle-tendon unit. This model, which could be applied to isometric contractions at other angles and therefore of wide use, showed that similar strain between superficial and deep aponeuroses of MG contributed to homogeneous fascicle length change within MG during contractions. These findings would contribute to clarifying the functions of the superficial aponeurosis and the effects of the superficial aponeurosis elongation on the whole muscle behavior.     

A comparison of the triceps surae and residual muscle moments at the ankle during cycling

The rigid linked system model and principles of inverse dynamics have been widely used to calculate residual muscle moments during various activities. EMG driven models and optimization algorithms have also been presented in the literature in efforts to estimate skeletal muscle forces and evaluate their possible contribution to the residual muscle moment. Additionally, skeletal muscle-tendon forces have been measured, directly, in both animals and humans. The purpose of this investigation was to calculate the moment produced by the triceps surae muscles and compare it to the residual muscle moment at the ankle during cycling at three power outputs (90, 180 and 270 W). Inferences were made regarding the potential contribution made by each triceps surae component to the tendon force using EMG and muscle-tendon length changes. A buckle-type transducer was surgically implanted on the right Achilles tendon of one male subject. Achilles tendon forces measured in vivo were multiplied by their corresponding moment arms to yield the triceps surae moment during the three working conditions. Moment arm lengths were obtained in a separate experiment using magnetic resonance imaging (MRI). Pedal reaction forces, body segment accelerations (determined from high speed film), and appropriate mass parameters served as input to the inverse solution. The triceps surae moment was temporally in phase with and consistently represented approximately 65% of the residual muscle moment at the ankle. These data demonstrate the feasibility of using implanted transducers in human subjects and provide a greater understanding of musculoskeletal mechanics during normal human movements.     

Influence of knee flexion angle and age on triceps surae muscle activity during heel raises

ABSTRACT: Hébert-Losier, K, Schneiders, AG, García, JA, Sullivan, SJ, and Simoneau, GG. Influence of knee flexion angle and age on triceps surae muscle activity during heel raises. J Strength Cond Res 26(11): 3124-3133, 2012-Triceps surae and Achilles tendon injuries are frequent in sports medicine, particularly in middle-aged adults. Muscle imbalances and weakness are suggested to be involved in the etiology of these conditions, with heel-raise testing often used to assess and treat triceps surae (TS) injuries. Although heel raises are recommended with the knee straight for gastrocnemius and bent for soleus (SOL), the extent of muscle selectivity in these positions is not clear. This study aimed to determine the influence of knee angle and age on TS muscle activity during heel raises. Forty-eight healthy men and women were recruited from a younger-aged (18-25 years) and middle-aged (35-45 years) population. All the subjects performed unilateral heel raises in 0° and 45° knee flexion (KF). Soleus, gastrocnemius medialis (GM) and gastrocnemius lateralis (GL) surface electromyography signals were processed to compute root-mean-square amplitudes, and data were analyzed using mixed-effects models and stepwise regression. The mean TS activity during heel raises was 23% of maximum voluntary isometric contraction when performed in 0° KF and 21% when in 45°. Amplitudes were significantly different between TS muscles (p < 0.001) and KF angles (p < 0.001), with a significant interaction (p < 0.001). However, the age of the population did not influence the results (p = 0.193). The findings demonstrate that SOL activity was 4% greater when tested in 45° compared with 0° KF and 5% lower in the GM and GL. The results are consistent with the recommended use of heel raises in select knee positions for assessing, training, and rehabilitating the SOL and gastrocnemius muscles; however, the 4-5% documented change in activity might not be enough to significantly influence clinical outcome measures or muscle-specific benefits. Contrary to expectations, TS activity did not distinguish between middle-aged and younger-aged adults, despite the higher injury prevalence in middle age.     

Influence of knee flexion angle and age on triceps surae muscle fatigue during heel raises

ABSTRACT: Hébert-Losier, K, Schneiders, AG, García, JA, Sullivan, SJ, and Simoneau, GG. Influence of knee flexion angle and age on triceps surae muscle fatigue during heel raises. J Strength Cond Res 26(11): 3134-3147, 2012-The triceps surae (TS) muscle-tendon unit is 1 of the most commonly injured in elite and recreational athletes, with a high prevalence in middle-aged adults. The performance of maximal numbers of unilateral heel raises is used to assess, train, and rehabilitate TS endurance and conventionally prescribed in 0° knee flexion (KF) for the gastrocnemius and 45° for the soleus (SOL). However, the extent of muscle selectivity conferred through the change in the knee angle is lacking for heel raises performed to volitional fatigue. This study investigated the influence of knee angle on TS muscle fatigue during heel raises and determined whether fatigue differed between middle-aged and younger-aged adults. Forty-eight healthy individuals aged 18-25 and 35-45 years performed maximal numbers of unilateral heel raises in 0° and 45° KF. Median frequencies and linear regression slopes were calculated from the SOL, gastrocnemius medialis (GM), and gastrocnemius lateralis (GL) surface electromyographic signals. Stepwise mixed-effect regressions were used for analysis. The subjects completed an average of 45 and 48 heel raises in 0° and 45° KF, respectively. The results indicated that the 3 muscles fatigued during testing as all median frequencies decreased, and regression slopes were negative. Consistent with muscle physiology and fiber typing, fatigue was greater in the GM and GL than in the SOL (p < 0.001). However, knee angle did not influence TS muscle fatigue parameters (p = 0.814), with similar SOL, GM, and GL fatigue in 0° and 45° KF. These findings are in contrast with the traditionally described clinical use of heel raises in select knee angles for the gastrocnemius and the SOL. Furthermore, no difference in TS fatigue between the 2 age groups was able to be determined, despite the reported higher prevalence of injury in middle-aged individuals.     

Nonlinear force-length relationship in the ADP-induced contraction of skeletal myofibrils

The regulatory mechanism of sarcomeric activity has not been fully clarified yet because of its complex and cooperative nature, which involves both Ca²⁺ and cross-bridge binding to the thin filament. To reveal the mechanism of regulation mediated by the cross-bridges, separately from the effect of Ca²⁺, we investigated the force-sarcomere length (SL) relationship in rabbit skeletal myofibrils (a single myofibril or a thin bundle) at SL > 2.2 μm in the absence of Ca²⁺ at various levels of activation by exogenous MgADP (4-20 mM) in the presence of 1 mM MgATP. The individual SLs were measured by phase-contrast microscopy to confirm the homogeneity of the striation pattern of sarcomeres during activation. We found that at partial activation with 4-8 mM MgADP, the developed force nonlinearly depended on the length of overlap between the thick and the thin filaments; that is, contrary to the maximal activation, the maximal active force was generated at shorter overlap. Besides, the active force became larger, whereas this nonlinearity tended to weaken, with either an increase in [MgADP] or the lateral osmotic compression of the myofilament lattice induced by the addition of a macromolecular compound, dextran T-500. The model analysis, which takes into account the [MgADP]-and the lattice-spacing-dependent probability of cross-bridge formation, was successfully applied to account for the force-SL relationship observed at partial activation. These results strongly suggest that the cross-bridge works as a cooperative activator, the function of which is highly sensitive to as little as ≤1 nm changes in the lattice spacing.     

Passive triceps surae stretch inhibits vasoconstriction in the nonexercised limb during posthandgrip muscle ischemia.

We investigated whether selective muscle mechanoreceptor activation in the lower limb opposes arm muscle metaboreceptor activation-mediated limb vasoconstriction. Seven subjects completed two trials: one control trial and one stretch trial. Both trials included 2 min of handgrip and 2 min of posthandgrip exercise muscle ischemia (PEMI). In the stretch trial, a 2-min sustained triceps surae stretch, by brief passive dorsiflexion of the right foot, was performed simultaneously during PEMI. Mean arterial pressure, heart rate, and forearm blood flow (FBF) in the nonexercised arm and forearm vascular conductance (FVC) in the nonexercised arm were measured. During PEMI in the control trial, mean arterial pressure was significantly greater and FBF and FVC were significantly lower than baseline values (P < 0.05 for each). In contrast, FBF and FVC during PEMI in the stretch trial exhibited different responses than in the control trial. FBF and FVC were significantly greater in the stretch trial than in the control trial (FBF, 5.5 +/- 0.4 vs. 3.8 +/- 0.4 ml x 100 ml(-1) x min(-1); FVC, 0.048 +/- 0.004 vs. 0.033 +/- 0.003 unit, respectively; P < 0.05). These results indicate that passive triceps surae stretch can inhibit vasoconstriction in the nonexercised forearm mediated via muscle metaboreceptor activation in the exercised arm.     

Sway-dependent modulation of the triceps surae H-reflex during standing.

Previous research has shown that changes in spinal excitability occur during the postural sway of quiet standing. In the present study, it was of interest to examine the independent effects of sway position and sway direction on the efficacy of the triceps surae Ia pathway, as reflected by the Hoffman (H)-reflex amplitude, during standing. Eighteen participants, tested under two different experimental protocols, stood quietly on a force platform. Percutaneous electrical stimulation was applied to the posterior tibial nerve when the position and direction of anteroposterior (A-P) center of pressure (COP) signal satisfied the criteria for the various experimental conditions. It was found that, regardless of sway position, a larger amplitude of the triceps surae H-reflex (difference of 9-14%; P = 0.005) occurred when subjects were swaying in the forward compared with the backward direction. The effects of sway position, independent of the sway direction, on spinal excitability exhibited a trend (P = 0.075), with an 8.9 +/- 3.7% increase in the H-reflex amplitude occurring when subjects were in a more forward position. The observed changes to the efficacy of the Ia pathway cannot be attributed to changes in stimulus intensity, as indicated by a constant M-wave amplitude, or to the small changes in the level of background electromyographic activity. One explanation for the changes in reflex excitability with respect to the postural sway of standing is that the neural modulation may be related to the small lengthening and shortening contractions occurring in the muscles of the triceps surae.     

Difference in aftereffects following prolonged Achilles tendon vibration on muscle activity during maximal voluntary contraction among plantar flexor synergists.

It has been suggested that a suppression of maximal voluntary contraction (MVC) induced by prolonged vibration is due to an attenuation of Ia afferent activity. The purpose of the present study was to test the hypothesis that aftereffects following prolonged vibration on muscle activity during MVC differ among plantar flexor synergists owing to a supposed difference in muscle fiber composition. The plantar flexion MVC torque and surface electromyogram (EMG) of the medial head of gastrocnemius (MG), the lateral head of gastrocnemius (LG), and the soleus (Sol) were recorded in 13 subjects before and after prolonged vibration applied to the Achilles tendon at 100 Hz for 30 min. The maximal H reflexes and M waves were also determined from the three muscles, and the ratio between H reflexes and M waves (H/Mmax) was calculated before and after the vibration. The MVC torque was decreased by 16.6 +/- 3.7% after the vibration (P < 0.05; ANOVA). The H/Mmax also decreased for all three muscles, indicating that Ia afferent activity was successfully attenuated by the vibration in all plantar flexors. However, a reduction of EMG during MVC was observed only in MG (12.7 +/- 4.0%) and LG (11.4 +/- 3.9%) (P < 0.05; ANOVA), not in Sol (3.4 +/- 3.0%). These results demonstrated that prolonged vibration-induced MVC suppression was attributable mainly to the reduction of muscle activity in MG and LG, both of which have a larger proportion of fast-twitch muscle fibers than Sol. This finding suggests that Ia-afferent activity that reinforces the recruitment of high-threshold motor units is necessary to enhance force exertion during MVC.     

Variation in the human Achilles tendon moment arm during walking

The Achilles tendon (AT) moment arm is an important determinant of ankle moment and power generation during locomotion. Load and depth-dependent variations in the AT moment arm are generally not considered, but may be relevant given the complex triceps surae architecture. We coupled motion analysis and ultrasound imaging to characterize AT moment arms during walking in 10 subjects. Muscle loading during push-off amplified the AT moment arm by 10% relative to heel strike. AT moment arms also varied by 14% over the tendon thickness. In walking, AT moment arms are not strictly dependent on kinematics, but exhibit important load and spatial dependencies.     

Facilitation of triceps brachii muscle contraction by tendon vibration after chronic cervical spinal cord injury.

One way to improve the weak triceps brachii voluntary forces of people with chronic cervical spinal cord injury may be to excite the paralyzed or submaximally activated fraction of muscle. Here we examined whether elbow extensor force was enhanced by vibration (80 Hz) of the triceps or biceps brachii tendons at rest and during maximum isometric voluntary contractions (MVCs) of the elbow extensors performed by spinal cord-injured subjects. The mean +/- SE elbow extensor MVC force was 22 +/- 17.5 N (range: 0-23% control force, n = 11 muscles). Supramaximal radial nerve stimuli delivered during elbow extensor MVCs evoked force in six muscles that could be stimulated selectively, suggesting potential for force improvement. Biceps vibration at rest always evoked a tonic vibration reflex in biceps, but extension force did not improve with biceps vibration during triceps MVCs. Triceps vibration induced a tonic vibration reflex at rest in one-half of the triceps muscles tested. Elbow extensor MVC force (when >1% of control force) was enhanced by vibration of the triceps tendon in one-half of the muscles. Thus triceps, but not biceps, brachii tendon vibration increases the contraction strength of some partially paralyzed triceps brachii muscles.