The effect of muscle length on motor-unit recruitment during isometric plantar flexion in humans

The triceps surae muscle group, consisting of the mono-articular soleus (SOL) and bi-articular gastrocnemius (GAS) muscles, primarily generates plantar flexor torque. Since the GAS muscle crosses the knee joint, flexion of the knee reduces the length of this muscle, thus limiting its contribution to torque output. However, it is not clearly understood how the central nervous system activates muscles that are at inefficient or non-optimal force-producing lengths. Therefore, the present study was designed to determine the effect of muscle length on motor-unit recruitment in the medial GAS muscle. Single motor-unit activity was recorded from the medial GAS muscle while electromyographic (EMG) activity was recorded from the SOL muscle in nine male subjects. With the ankle angle held constant at 90 degrees, the knee angle was changed from 180 degrees to 90 degrees, corresponding to a long and short GAS muscle length, respectively. Levels of voluntary plantar flexor torque were produced at a rate of 2 Nm.s-1 until motor-unit activity was detected. A total of 229 motor units were recorded, of which 121 and 108 were obtained at the long and short muscle lengths, respectively. At the short length, onset of motor-unit activity occurred at significantly higher levels of plantar flexor torque and SOL EMG. Onset of motor-unit activity occurred at 2.97 +/- 7.78 Nm and 32.14 +/- 10.25 Nm, corresponding to 0.045 +/- 0.075 mV and 0.231 +/- 0.129 mV of SOL EMG in the long and short positions, respectively. No individual GAS motor unit could be recorded at both muscle lengths. Motor units in the shortened GAS muscle may be influenced by peripheral afferents capable of reducing the excitability of the motoneurone pool. This may also reflect a specific inhibition of motor units having shortened, non-optimal fascicle lengths, and they are thereby incapable of contributing to plantar flexor torque.     

Effect of antagonist muscle fatigue on knee extension torque

The effect of hamstring fatigue on knee extension torque was examined at different knee angles for seven male subjects. Before and after a dynamic flexion fatigue protocol (180° s^–1, until dynamic torque had declined by 50%), maximal voluntary contraction extension torque was measured at four knee flexion angles (90°, 70°, 50° and 30°). Maximal torque generating capacity and voluntary activation of the quadriceps muscle were determined using electrical stimulation. Average rectified EMG of the biceps femoris was determined. Mean dynamic flexion torque declined by 48±11%. Extensor maximal voluntary contraction torque, maximal torque generating capacity, voluntary activation and average rectified EMG at the four knee angles were unaffected by the hamstring fatigue protocol. Only at 50° knee angle was voluntary activation significantly lower (15.7%) after fatigue (P<0.05). In addition, average rectified EMG before fatigue was not significantly influenced by knee angle. It was concluded that a fatigued hamstring muscle did not increase the maximal voluntary contraction extension torque and knee angle did not change coactivation. Three possible mechanisms may explain the results: a potential difference in recruited fibre populations in antagonist activity compared with the fibres which were fatigued in the protocol, a smaller loss in isometric torque generating capacity of the hamstring muscle than was expected from the dynamic measurements and/or a reduction in voluntary activation.     

The force–velocity relationship of the human soleus muscle during submaximal voluntary lengthening actions

In experiments on isolated animal muscle, the force produced during active lengthening contractions can be up to twice the isometric force, whereas in human experiments lengthening force shows only modest, if any, increase in force. The presence of synergist and antagonist muscle activation associated with human experiments in situ may partly account for the difference between animal and human studies. Therefore, this study aimed to quantify the force–velocity relationship of the human soleus muscle and assess the likelihood that co-activation of antagonist muscles was responsible for the inhibition of torque during submaximal voluntary plantar flexor efforts. Seven subjects performed submaximal voluntary lengthening, shortening(at angular, velocities of +5, –5, +15, –15 and +30, and –30° s^–1) and isometric plantar flexor efforts against an ankle torque motor. Angle-specific (90°) measures of plantar flexor torque plus surface and intramuscular electromyography from soleus, medial gastrocnemius and tibialis anterior were made. The level of activation (30% of maximal voluntary isometric effort) was maintained by providing direct visual feedback of the soleus electromyogram to the subject. In an attempt to isolate the contribution of soleus to the resultant plantar flexion torque, activation of the synergist and antagonist muscles were minimised by: (1) flexing the knee of the test limb, thereby minimising the activation of gastrocnemius, and (2) applying an anaesthetic block to the common peroneal nerve to eliminate activation of the primary antagonist muscle, tibialis anterior and the synergist muscles, peroneus longus and peroneus brevis. Plantar flexion torque decreased significantly (P<0.05) after blocking the common peroneal nerve which was likely due to abolishing activation of the peroneal muscles which are synergists for plantar flexion. When normalised to the corresponding isometric value, the force–velocity relationship between pre- and post-block conditions was not different. In both conditions, plantar flexion torques during shortening actions were significantly less than the isometric torque and decreased at faster velocities. During lengthening actions, however, plantar flexion torques were not significantly different from isometric regardless of angular velocity. It was concluded that the apparent inhibition of lengthening torques during voluntary activation is not due to co-activation of antagonist muscles. Results are presented as mean (SEM).     

Activation of agonist and antagonist muscles at different joint angles during maximal isometric efforts

The purpose of this study was to investigate the influence of different angles of the knee joint on the activation level of an agonist (quadriceps femoris muscle) and antagonist (biceps femoris muscle) from electromyographic activities and activation levels (twitch interpolation). Isometric torque measurements were performed on 23 healthy subjects at 10° intervals between 40° and 110° of knee joint flexion. Superimposed twitches at maximal voluntary contraction were applied and the voluntary activation estimated. To quantify the antagonist muscle activity, we normalized its integrated EMG (iEMG) value at each joint angle with respect to its iEMG value at the same angle when acting as an agonist at maximal effort. The activation levels at the knee-flexed position (80–110°) were higher than that at the knee-extended position (40–70°). The co-activation levels at 90, 100, and 110° were significantly higher than that the other knee angle. These results suggest that the activation level of an agonist (quadriceps femoris) muscle and the co-activation level of an antagonist (biceps femoris) muscle were higher in longer muscles than in shorter muscles. It was also concluded that the risk of knee injuries could be reduced by applying these mutual relationships between activation levels of agonist and antagonist muscles.     

Neural activation of the triceps surae is impaired following 2 weeks of immobilization

The purpose of this study was to investigate the effect of 2 weeks of ankle joint immobilization on triceps surae neural activation, with particular emphasis on the potential differences between the monoarticular soleus and the biarticular gastrocnemius muscles. Seventeen male volunteers were divided into the immobilized group (IG, n=8) and the control group (CG, n=9). Elastic adhesive bandages and an ankle stabilization orthosis were used to immobilize the ankle joint only. The plantar flexor torque obtained during maximal voluntary contractions (MVC) and after single, paired and tetanic stimuli applied at rest was measured. The associated EMG activity from the soleus and gastrocnemius muscles was also recorded, and their activation levels were estimated by means of the twitch interpolation technique. After immobilization, triceps surae maximal voluntary torque significantly decreased by 17% (P<0.001). Strength losses were accompanied by a decrement in activation level (–6%, average of the three techniques used) and in maximal 100 Hz tetanic force (–11%). A significant decrease in the soleus (–22%, P<0.05) but not in the gastrocnemius EMG activity, normalized to respective M-waves, was also found. It was concluded that the reduced voluntary torque output after immobilization could be attributed to both muscular and neural alterations. These latter selectively involved the monoarticular soleus muscle, while neural drive to the biarticular gastrocnemii, which had not been immobilized in their function as knee flexors, was preserved.     

Late neural adaptations to electrostimulation resistance training of the plantar flexor muscles

The present study aimed to examine early and late neural adaptations to short-term electrostimulation training of the plantar flexor muscles. Changes in triceps surae muscle activation (twitch interpolation), maximal electromyographic (EMG) activity, H-reflex amplitudes and antagonist coactivation were investigated after electrostimulation training (4 weeks) and after 4 weeks of detraining in a group of ten young healthy men. Maximal voluntary contraction torque was significantly higher (P < 0.01) after training (+19.4%) and detraining (+17.2%) with respect to baseline. Activation level, soleus and lateral gastrocnemius EMG normalized to the maximal M-wave significantly increased as a result of training (P < 0.05), and these gains were preserved after detraining, excepted for soleus EMG. Maximal H reflex to maximal M wave ratio increased significantly between baseline and detraining for both soleus and lateral gastrocnemius muscles (P < 0.05). Tibialis anterior coactivation was unchanged after training but significantly decreased after the detraining period (P < 0.01). Short-term electrostimulation resistance training was accompanied by early (increased muscle activation and EMG activity) and late neural adaptations (increased spinal reflex amplitude and decreased coactivation), likely explaining the increase and then the preservation of the maximal voluntary strength. These effects may help in conceiving and programming effective electrostimulation therapy programs for both healthy and immobilized plantar flexor muscles.     

Use of a Kin-Com dynamometer to study the stretch-shortening cycle during plantar flexion

Summary The aim of this study was to evaluate the Kin-Com II dynamometer in the study of the stretch-shortening cycle (a concentric muscle action preceded by an eccentric muscle action). Measurements were made of plantar flexion at different angular velocities (120° · s^–1 and 240° · s^–1) with the knee at two different angles (0° and 90°). Ten healthy women ranging in age from 22 to 41 years were studied. Torque values were recorded simultaneously with surface electromyograms (EMG); maximal voluntary concentric torque values were recorded and, after a short rest, the torque values of the concentric action which followed immediately after an eccentric action of the same velocity, both with maximal effort. Mean values were taken at different ankle positions and also averaged over different ranges. A concentric action preceded by an eccentric action generated a torque value on an average about 100% larger than a concentric action alone. The EMG activity was lower or unchanged. It was concluded that the present method could be useful in the study of the stretch-shortening cycle in plantar flexion and in the testing of the behaviour of the elastic components in people with disabilities in the lower limbs.     

Tension regulation during lengthening and shortening actions of the human soleus muscle

In the present study we investigated tension regulation in the human soleus (SOL) muscle during controlled lengthening and shortening actions. Eleven subjects performed plantar flexor efforts on an ankle torque motor through 30° of ankle displacement (75°–105° internal ankle angle) at lengthening and shortening velocities of 5, 15 and 30° · s⁻¹. To isolate the SOL from the remainder of the triceps surae, the subject's knee was flexed to 60° during all trials. Voluntary plantar flexor efforts were performed under two test conditions: (1) maximal voluntary activation (MVA) of the SOL, and (2) constant submaximal voluntary activation (SVA) of the SOL. SVA trials were performed with direct visual feedback of the SOL electromyogram (EMG) at a level resulting in a torque output of 30% of isometric maximum. Angle-specific (90° ankle angle) torque and EMG of the SOL, medial gastrocnemius (MG) and tibialis anterior (TA) were recorded. In seven subjects from the initial group, the test protocol was repeated under submaximal percutaneous electrical activation (SEA) of SOL (to 30% isometric maximal effort). Lengthening torques were significantly greater than shortening torques in all test conditions. Lengthening torques in MVA and SVA were independent of velocity and remained at the isometric level, whereas SEA torques were greater than isometric torques and increased at higher lengthening velocities. Shortening torques were lower than the isometric level for all conditions. However, whereas SVA and SEA torques decreased at higher velocities of shortening, MVA torques were independent of velocity. These results indicate velocity- and activation-type-specific tension regulation in the human SOL muscle. Accepted: 11 October 1999     

Neuromuscular performance of paretic versus non-paretic plantar flexors after stroke

The objective of this study was to compare the neuromuscular function of the paretic and non-paretic plantar flexors (i.e. soleus, gastrocnemius medialis, lateralis) in chronic stroke patients. It was hypothesized that the contractile rate of force development (RFD) and neural activation, assessed by electromyogram (EMG) and V-waves normalized to the M-wave, and voluntary activation (twitch interpolation) would be reduced during plantar flexor maximum voluntary isometric contraction and that the evoked muscle twitch properties would be reduced in the paretic limb. Ten chronic stroke survivors completed the study. The main findings were that the paretic side showed deteriorated function compared to the non-paretic leg in terms of (1) RFD in all analyzed time windows from force onset to 250 ms, although relative RFD (i.e. normalized to maximum voluntary force) was similar; (2) fast neural activation (for most analyzed time windows), assessed by EMG activity in time windows from EMG onset to 250 ms; (3) V-wave responses (except for gastrocnemius medialis); (4) voluntary activation; (5) the evoked peak twitch force, although there was no evidence of intrinsic muscle slowing; (6) EMG activity obtained at maximal voluntary force. In conclusion, this study demonstrates considerable neuromuscular asymmetry of the plantar flexors in chronic stroke survivors. Effective rehabilitation regimes should be investigated.     

Comparative and reliability studies of neuromechanical leg muscle performances of volleyball athletes in different divisions

This study compared neural profiles of the leg muscles of volleyball athletes playing in different divisions of Taiwan’s national league to analyse the reliability and correlations between their profiles and biomechanical performances. Twenty-nine athletes including 12 and 17 from the first and second divisions of the league, respectively, were recruited. The outcome measures were compared between the divisions, including soleus H-reflex, first volitional (V) wave, normalised rate of electromyography (EMG) rise (RER) in the triceps surae muscles, and RER ratio for the tibialis anterior and soleus muscles, normalised root mean square (RMS) EMG in the triceps surae muscles, antagonist co-activation of the tibialis anterior muscle, rate of force development (RFD), and maximal plantar flexion torque and jump height. Compared to the results of the second division, the neural profiles of the first division showed greater normalised V waves, normalised RER in the lateral gastrocnemius, and normalised RMS EMG of the soleus and lateral gastrocnemius muscles with less antagonist co-activation of the tibialis anterior. First division volleyball athletes showed greater maximal torque, jump height, absolute RFD at 0–30, 0–100, and 0–200 ms, and less in the normalised RFD at 0–200 ms of plantar flexion when compared to the results of those in the second division. Neural profiles correlated to fast or maximal muscle strength or jump height. There are differences in the descending neural drive and activation strategies in leg muscles during contractions between volleyball athletes competing at different levels. These measures are reliable and correlate to biomechanical performances.     

Differences in the electromyographic activity of the hamstring muscles during maximal eccentric knee flexion

This study investigated the effects of the knee joint angle and angular velocity on hamstring muscles’ activation patterns during maximum eccentric knee flexion contractions. Ten healthy young males (23.4 ± 1.3 years) performed eccentric knee flexion at constant velocities of 10, 60, 180, and 300 deg/s in random order. The eccentric knee flexion torque and the surface electromyographic (EMG) activity of the biceps femoris (BF), semitendinosus (ST), and semimembranosus (SM) muscles were measured. The results of torque during 10 deg/s were lower than the faster velocities. No significant change was found in eccentric torque output and the EMG amplitude with change in the faster test velocities, although those values showed a decreasing tendency as the knee approached extension. Furthermore, the EMG amplitude of the BF decreased significantly as the knee approached extension, although the EMG activity of the ST and SM remained constant. These results suggest that the neural inhibitory mechanism might be involved in decreasing in maximal voluntary force and hamstring muscles activation toward the knee extension during high-velocity eccentric movement and therefore subjects have difficulties to maintain high eccentric force level throughout the motion. Moreover, the possible mechanism reducing the BF muscle activation as the knee approaches extension was architectural differences in the hamstring muscles, which might reflect each muscle’s function.     

Dynamic multivoxel‐localized 31P MRS during plantar flexion exercise with variable knee angle

Exercise studies investigating the metabolic response of calf muscles using ³¹P MRS are usually performed with a single knee angle. However, during natural movement, the distribution of workload between the main contributors to force, gastrocnemius and soleus is influenced by the knee angle. Hence, it is of interest to measure the respective metabolic response of these muscles to exercise as a function of knee angle using localized spectroscopy. Time‐resolved multivoxel ³¹P MRS at 7 T was performed simultaneously in gastrocnemius medialis and soleus during rest, plantar flexion exercise and recovery in 12 healthy volunteers. This experiment was conducted with four different knee angles. PCr depletions correlated negatively with knee angle in gastrocnemius medialis, decreasing from 79±14 % (extended leg) to 35±23 %(～40°), and positively in soleus, increasing from 20±21 % to 36±25 %; differences were significant. Linear correlations were found between knee angle and end‐exercise PCr depletions in gastrocnemius medialis (R²=0.8) and soleus (R²=0.53). PCr recovery times and end‐exercise pH changes that correlated with PCr depletion were consistent with the literature in gastrocnemius medialis and differences between knee angles were significant. These effects were less pronounced in soleus and not significant for comparable PCr depletions. Maximum oxidative capacity calculated for all knee angles was in excellent agreement with the literature and showed no significant changes between different knee angles. In conclusion, these findings confirm that plantar flexion exercise with a straight leg is a suitable paradigm, when data are acquired from gastrocnemius only (using either localized MRS or small surface coils), and that activation of soleus requires the knee to be flexed. The present study comprises a systematic investigation of the effects of the knee angle on metabolic parameters, measured with dynamic multivoxel ³¹P MRS during muscle exercise and recovery, and the findings should be used in future study design.     

Elbow flexion strength curves in untrained men and women and male bodybuilders

Summary The influence of elbow joint angle on voluntary isometric elbow flexion strength was assessed in 15 young women (F), 18 young men (M) and 11 male body-builders (BB). Measurements were made at elbow joint angles of 1.31, 1.57, 1.83, 2.09, 2.36, 2.62 and 2.88 rad (3.14 rad = 180° = full extension). The peak voluntary strength [mean (SE), N.m] in M [69.5 (4.3)] and BB [93.3 (4.8)] occurred at 2.09 rad (120°), but occurred at 1.57 rad (90°) in F [35.4 (2.4)]. Peak torque at 1.31 rad was 20% and 25% lower than at 2.09 rad in M and BB, respectively, but did not differ between these two angles in F. The larger elbow flexor muscle and fibre size in M and BB may have been responsible for their impaired torque production at joint angles corresponding to the shortest muscle lengths.     

Effect of prior isometric muscle action on concentric torque output during plantar flexion

The influence of different levels of prior isometric muscle action on the concentric torque output during plantar flexion was examined at two angular velocities (60°·s^–1 and 120°·s^–1) in ten healthy female subjects. The levels of the prior muscle actions were 25%, 50%, 75% and 100% of the maximal voluntary isometric contraction (MVIC). A KINetic-COMmunicator II dynamometer was used to measure torque output during plantar flexion within a range of motion of 78°-120° of the ankle joint. Simultaneous recordings of electromyograms (low-pass filtered and rectified) were obtained from the gastrocnemius medialis muscle and the soleus muscle. Torque-angle curves were made for the plantar flexions using different prior muscle actions. Up to 75% of MVIC, the torque output in the first part of the range of motion increased with the level of the prior isometric muscle action; at higher levels of MVIC the torque did not appear to increase any further. Later in the range of motion, after 24° in the plantar flexion at a velocity of 60°·s^–1 and 31° at 120°·s^–1, the prior muscle actions had no further influence. No increase was found in the electromyograms, with one exception, during the concentric movements when preceded by higher levels of MVIC. It would seem therefore that the increase in torque output early in the range of motion cannot be explained on the basis of differences in electrical muscle activation in this study.     

Behaviour of the human gastrocnemius muscle architecture during submaximal isometric fatigue

The purpose of this study was to examine whether the human gastrocnemius medialis (GM) fascicle length and pennation angle alter during a sustained submaximal isometric plantar flexion. Fourteen male subjects performed maximal voluntary plantar flexions (MVC) on a dynamometer before and after a fatiguing task. This task consisted of a sustained submaximal isometric fatiguing contraction (40% MVC) until failure to hold the defined moment. Ultrasonography was used to visualise the muscle belly of the GM. Leg kinematics were recorded (120 Hz) to calculate the joint moment using inverse dynamics. The exerted moments and the EMG signals from GM and lateralis, soleus and tibialis anterior were measured at 1,080 Hz. The root mean square (RMS) of the EMG signal of the three triceps surae muscles increased significantly (P0.05) between 17% and 28% with fatigue. Further, the fascicle length of the GM significantly decreased from 47.1±8.0 mm at the beginning to 41.8±6.7 mm at the end of fatigue and the pennation angle increased from 23.5±4.1° to 26.3±2.2° (P0.05). The changes in fascicle length and pennation angle of the GM during the contraction can influence the force potential of the muscle due to the force–length relationship and the force transmission to the tendon. This provides evidence on that an additional mechanical mechanism, namely tendon creep, can contribute to the increase in the EMG activity of the GM during submaximal isometric sustained contractions.     

Console video games, postural activity, and motion sickness during passive restraint

Transcranial magnetic stimulation (TMS) allows an in vivo assessment of the rate of muscle relaxation during a voluntary contraction. It is unknown if this method can be applied to lower limb muscles, and the effect of stimulus intensity on relaxation rate has not been investigated in any muscle group. The present study sought to address these unknowns. A secondary aim was to test the sensitivity of the method to a change in muscle length by comparing the relaxation rate of the plantar flexor muscles with the gastrocnemius at short and long lengths. Seven subjects performed 21 maximal voluntary isometric contractions (MVCs) of the dorsiflexors (DF) and plantar flexors with a knee angle of either 90° or 180° (PF90 and PF180, respectively). TMS intensity ranged from 40 to 100 % stimulator output in intervals of 10 %. Relaxation rates increased with stimulus intensity but were equivalent to maximal output at 50 (DF and PF90) or 60 % (PF180). MVC torque was greater, and the rate of relaxation was faster for PF180 compared to PF90. The main findings are that TMS can be used to measure relaxation rates of lower limb muscles, and these rates are robust provided the stimulus intensity is above a critical threshold. The dependency of plantar flexor relaxation rate on the length of the fast-twitch gastrocnemius fibers reinforces published temperature and fatigue data which show that the method is sensitive to the contractile properties of the muscle fibers which are actively contributing to torque production.     

The medial gastrocnemius muscle attenuates force fluctuations during plantar flexion

Force fluctuations during steady contractions of multiple agonist muscles may be influenced by the relative contribution of force by each muscle. The purpose of the study was to compare force fluctuations during steady contractions performed with the plantar flexor muscles in different knee positions. Nine men (25.8±5.1 years) performed steady contractions of the plantar flexor muscles in the knee-flexed and knee-extended (greater involvement of the gastrocnemii muscles) positions. The maximal voluntary contraction (MVC) force was 32% greater in the knee-extended position compared with the knee-flexed position. The target forces were 2.5–10% MVC force in the respective position. The amplitude of electromyogram in the medial gastrocnemius muscle was greater in the knee-extended position (10.50±9.80%) compared with the knee-flexed position (1.26±1.15%, P<0.01). The amplitude of electromyogram in the soleus muscle was not influenced by the knee position. The amplitude of electromyogram in the lateral gastrocnemius and tibialis anterior muscles was marginal and unaltered with knee position. At the same force (in Newtons), the standard deviation of force was lower in the knee-extended position compared with the knee-flexed position. These results indicate that force fluctuations during plantar flexion are attenuated with greater involvement of the medial gastrocnemius muscle.     

Effect of ankle position fixation on peak torque and electromyographic activity of the knee flexors and extensors

The purpose of this study was to examine the effect of ankle position fixation on peak torque (PT) and electromyographic (EMG) activity of knee-joint muscles during isokinetic testing. Twelve female athletes performed isokinetic knee flexion and extension at 60 degrees and 180 degrees/s under two conditions: with the ankle fixed in a position of plantarflexion and with the ankle fixed in a position of dorsiflexion. Bipolar surface electrodes were placed on the vastus lateralis, vastus medialis, biceps femoris, medial hamstrings, and the lateral head of the gastrocnemius for determination of the root mean square of the EMG (rmsEMG) and the median frequency of the EMG (mfEMG). No significant differences in knee extensor PT were noted in either ankle position for each velocity tested. Significant differences were noted, however, in knee flexor PT (p < 0.05) at both 60 degrees and 180 degrees/s, with the greatest PT observed with the ankle fixed in dorsiflexion. Neither quadriceps, hamstrings, nor gastrocnemius rmsEMG activity was affected by ankle position; however, there was a significant difference in mfEMG for the gastrocnemius, with higher frequencies observed with the ankle fixed in plantarflexion (p < 0.01). These results suggest that ankle position effects knee flexor PT during open chain isokinetic movements. The reason for decreased knee flexor PT with the ankle fixed in plantarflexion is probably due to the gastrocnemius muscle being in a too shortened position, thereby preventing it from effectively producing force at the knee joint.     

A comparison of electrical activity in the triceps surae at maximum isometric contraction with the knee and ankle at various angles

The purpose of this study was to test the endurance of the soleus muscle, and to examine the joint position at which it is most active, while simultaneously suppressing the activity of the gastrocnemius. Ten young males performed maximum isometric contraction of the triceps surae for 100?s, and the endurance and plantar flexion torque of this muscle were measured at various angles of the knee and ankle joints. The electromyogram was measured simultaneously and subsequently converted into integrated electromyogram (IEMG) values. With the knee flexed at 130°, the rate of change in IEMG values for the soleus (0.454%?·?s^?1) with the ankle in a neutral position was significantly higher than that for the medial and lateral gastrocnemius. Both with the ankle dorsiflexed at 10° and in the neutral position, the rate of change in IEMG for the soleus was significantly higher with the knee flexed at 90° and 130° than with the knee fully extended. With the knee flexed at 90° and 130°, the IEMG activity of the soleus during the initial (5–10?s) and final 5?s tended to be higher than those for the medial and lateral gastrocnemius, regardless of the ankle joint position. We conclude that the position in which the soleus acts most selectively during a sustained maximum isometric contraction of the triceps surae is with the ankle in a neutral position and the knee flexed at 130°.     

Soleus- and gastrocnemii-evoked V-wave responses increase after neuromuscular electrical stimulation training

The aim of the study was to use combined longitudinal measurements of soleus (SOL) and gastrocnemii evoked V-wave and H-reflex responses to determine the site of adaptations within the central nervous system induced by 5 wk of neuromuscular electrical stimulation (NMES) training of the plantar flexor muscles. Nineteen healthy males subjects were divided into a neuromuscular electrostimulated group (n = 12) and a control group (n = 7). The training program consisted of 15 sessions of isometric NMES over a 5-wk period. All subjects were tested before and after the 5-wk period. SOL, lateral gastrocnemius (LG), and medial gastrocnemius (MG) maximal H-reflex and M-wave potentials were evoked at rest (i.e., H(max) and M(max), respectively) and during maximal voluntary contraction (MVC) (i.e., H(sup) and M(sup), respectively). During MVC, a supramaximal stimulus was delivered that allowed us to record the V-wave peak-to-peak amplitudes from all three muscles. The SOL, LG, and MG electromyographic (EMG) activity as well as muscle activation (twitch interpolation technique) were also quantified during MVC. After training, plantar flexor MVC increased significantly by 22% (P < 0.001). Torque gains were accompanied by an increase in muscle activation (+11%, P < 0.05), SOL, LG, and MG normalized EMG activity (+51, +54, and +60%, respectively, P < 0.05) and V/M(sup) ratios (+81, +76, and +97%, respectively, P < 0.05). H(max)/M(max) and H(sup)/M(sup) ratios for all three muscles were unchanged after training. In conclusion, the increase in voluntary torque after 5 wk of NMES training could be ascribed to an increased volitional drive from the supraspinal centers and/or adaptations occurring at the spinal level.