Electrically evoked isometric and isokinetic properties of the triceps surae in young male subjects

Summary The relationship between electrically evoked isometric and isokinetic properties of the triceps surae have been studied in 11 healthy male subjects. The results showed that the time to peak tension (TPT) and half relaxation time (1/2 RT) of the maximal twitch were 110±11 ms and 82±11 ms respectively, and the peak rates of rise of contraction (P ₅₀, P ₂₀₀) and relaxation (P _R50, P _R200) at 50 and 200 Hz were 0.36±0.07, 0.48±0.08 and 1.27±0.33, 1.25±0.27% Po ms^–1 respectively. The decline in force during a fatigue test was significantly (P<0.02) associated with the decrease in maximal relaxation rate (r=0.79). The TPT was significantly (P<0.05) and inversely related to P ₅₀ and P ₂₀₀. The mean angle specific torque-velocity relationship for the 11 subjects was adequately described by the empirical exponential equation of the form: V=16.5 (e ^–p/30.8–e ^–84.3/30.8) where V=velocity (rads s^–1) P=torque (Nm). The only significant association found between the isometric and isokinetic properties of the muscle was between P _R200 and the torque expressed at a given velocity of 4 rads s^–1. This lack of association between the two variables is difficult to explain with certainty but it is suggested that it may be due to the differential effects of Ca²⁺ release and uptake and cross-bridge turnover rate in the two situations.     

A comparison of voluntary and electrically evoked isokinetic plantar flexor torque in males

Summary The angle-specific isokinetic torque- and power-velocity relationships of the triceps surae were examined in ten male sprint athletes aged [mean (SD)] 22.4 (3.2) years, ten non-trained adult men aged 27.4 (4.8) years and six elderly male subjects aged 68.5 (2.4) years. Normal voluntary contractions were compared with those obtained using maximal tetanic stimulation and a release technique which standardised the level of muscle activation during isokinetic contractions. When the isokinetic data was normalized to the maximum isometric torque the stimulated release contractions at 5.18–5.29 rad · s^–1 produced significantly (P < 0.05) greater torque than the voluntary no-release contractions at the same angular velocity in each group of subjects. The three subject groups generated their peak power at 3.07 rad · s^–1 during the voluntary no-release contractions. However, with the stimulated release contractions, power had still not reached a peak at 5.29 rad · s^–1, the highest angular velocity that could be tested. It appears that at higher angular velocities the triceps surae is capable of greater torque and power generation when contractions are evoked using a stimulated release technique. It is suggested that the stimulated release technique gives a more complete picture of the torque-velocity characteristics of the contractile component of the triceps surae.     

Electrically evoked contractions of the triceps surae during and following 21 days of voluntary leg immobilization

Summary The effects of 21 days voluntary leg (plaster) immobilization on the mechanical properties of the triceps surae have been studied in 11 young female subjects, mean age 19.4 years. The results show that during the period of immobilization the mean time to peak tension (TPT) and half relaxation time (1/2RT) and tension (P_t) of the maximal twitch increased significantly (p<0.001) but the effects were short lived. Maximal tension and contraction times of the twitch recovered within 2–14 days following the removal of the plaster cast. The electrically evoked tetanic tensions at 10 Hz and 20 Hz did not change significantly (p>0.1) during immobilization, but the 50 Hz tetanic tension (P_°50) and maximal voluntary contraction (MVC) were reduced (p<0.05). The fall in P_°50 and MVC was associated with 10% decrease in the estimated muscle (plus bone) cross-sectional area. The relative (%) change in P_°50 and MVC following immobilization was related to the initial physiological status (as indicated by the response of the triceps surae to a standard fatigue test prior to immobilization) of the muscle. The rate of rise and recovery fall of the tetanus were slightly but significantly (p<0.01) reduced on day 7 of immobilization, but thereafter remained constant. The isokinetic properties of the triceps surae as reflected in the measured torque/velocity relation of the muscle in 4 subjects did not change significantly if account was taken of the slight degree of atrophy present following immobilization. It was concluded that short term voluntary leg immobilization produces atrophy and some loss of isometric twitch and tetanic function, but has little effect on the isokinetic properties of the triceps surae. The changes in the twitch characteristics during and immediately following immobilization may be indicative of a prolongation of the active state of the muscle.     

Voluntary activation and mechanical performance of human triceps surae muscle after exhaustive stretch-shortening cycle jumping exercise

The purpose of this study was to examine neuromuscular factors that may contribute to post exercise force loss and subsequent recovery after exhaustive stretch-shortening cycle (SSC) exercise. Six subjects were fatigued on a sledge apparatus by 100 maximal rebound jumps followed by continuous submaximal jumping until complete exhaustion. Exercise-induced changes in neuromuscular performance were followed up to 7 days post exercise. The total number of jumps in the SSC exercise ranged from 336 to 1392. The SSC exercise induced a significant immediate plantarflexion torque decline of 29, 38 and 44% (P<0.05) in maximal voluntary contraction and evoked maximal twitch and low-frequency (LF) stimulation, respectively. The higher the number of jumps in the SSC exercise the larger was the post exercise reduction in voluntary activation as well as in contractile force (r=–0.94, P<0.01, in both). Furthermore, a higher number of jumps augmented a delayed force recovery and late decline in stretch reflex EMG response (r=–0.94, P<0.01). Clear differences were found in central and peripheral adaptation to the exhaustive SSC exercise between the subjects. The magnitude of post exercise contractile and activation failure as well as the delayed recovery of neuromuscular performance may have been augmented in some subjects due to their high number of jumps in the exercise.     

Comparison of the histochemical and contractile properties of human triceps surae

The purpose of the study was to compare the contractile properties determined from an electrically stimulated twitch with histochemically determined fibre type parameters of the human triceps surae. Muscle samples were obtained from the medial head of the gastrocnemius of ten male athletes. Ages ranged from 20 to 29 years. Muscle samples from the belly of the medial gastrocnemius muscle were obtained using the needle biopsy technique. The samples were treated histochemically for myosin ATPase to classify the fibres as either slow twitch (ST) or fast twitch (FT) and to determine fibre areas. Surface electrical stimulation was used to determine muscle twitch parameters. The contractile variables of the muscle twitch were latency (L), time to peak force (TPF), peak force (PF), half-contraction time (1/2 CT) and half-relaxation time (1/2 RT). Backward elimination procedures for dependent variables were used to determine which contractile properties best represented the histochemical profile of the muscles. Prediction formulas were developed for FT and ST percentages (R²=0·98, p<0·001), relative area percentage (R²=0·87, p<0·001), and ST area (R²=0·85, p<0·01). It was concluded that the use of the electrotensiometer (ETM) protocol was a valid testing procedure when studying physiological relationships of histochemical properties in intact human skeletal muscle. Protected by patent no. 4 688 581.     

Effects of 17-day spaceflight on electrically evoked torque and cross-sectional area of the human triceps surae

The effects of spaceflight on triceps surae muscle torque and cross-sectional area (CSA) were investigated on four astronauts using electrically evoked contractions to by-pass neural control. Muscle twitch characteristics, ankle joint angle–twitch torque relation, frequency–torque relation, tetanic torque and fatigability were assessed before, during and after a 17-day Space Shuttle flight (STS-78). Muscle plus bone cross-sectional area (CSAm+b) was evaluated before and after the flight. Whereas no changes in muscle function were observed during the flight, marked alterations were found during the recovery period. Peak twitch (PTw) and tetanic torques at 50 Hz (PT₅₀) continued to fall up to the 8th recovery day (R+8) on which losses in PTw and PT₅₀ were 24.4% (P<0.01) and 22.0% (P<0.01), respectively. The decline in PTw was not joint-angle-specific. Post-flight, especially on R+8, torque decreased at all stimulation frequencies (1, 20, 30 and 50 Hz); however the shape of the frequency–torque curve, normalised for PT₅₀, was not modified. Similarly, no changes in twitch kinetics were observed. Post- flight, an 8% (P<0.01) reduction in CSAm+b was found on R+2. Normalisation of PT₅₀ values for CSAm+b showed a progressive loss in specific torque (PT₅₀/CSAm+b), which was maximal on R+2 (19.5%, P<0.05). Also, fatigability during 2-min intermittent stimulation at 20 Hz increased throughout recovery, reaching a nadir of 16.4% (P<0.01) on R+15. In conclusion, 17 days of spaceflight resulted in significant changes in muscle function during the recovery phase, but not in microgravity. The disproportionate loss of torque compared with that of muscle size suggests the presence of muscle damage due to reloading in 1 g.     

The effects of two forms of isometric training on the mechanical properties of the triceps surae in man

The training effects of rhythmic and sustained isometric contractions on the contractile characteristics of the triceps surae have been investigated in four healthy subjects over a period of 8 weeks. One leg (ST) of each subject was trained by performing repeated daily sustained (1 min) isometric contractions at 30% of maximal voluntary contraction force (MVC), and the other using rhythmic isometric contractions at 100% of MVC. The protocol was so arranged that the total area under the training force/ time curves was the same for each limb. Electrically evoked maximal twitch (P _t) and tetanic (P _o) tensions of the triceps surae were measured weekly on both legs using a standard procedure.The results showed that the rhythmic regime increased the MVC at the rate of 5.5% per week and the sustained training increased MVC at the rate of 3.3% per week. Twitch and tetanic tensions were not altered by either regime. However, only training using sustained contractions produced a progressive increase in endurance as measured by performance in a fatigue test.The results suggest that increases in voluntary strength resulting from short term isometric training are not necessarily associated with a rise in the intrinsic involuntary capacity of muscle fibres to generate force.     

Electrically evoked torque-velocity characteristics and isomyosin composition of the triceps surae in young and elderly men

The electrically evoked isokinetic torque-velocity relationship of the triceps surae of eight elderly and four non-trained young men was examined in relation to the isomyosin composition of the soleus and the gastrocnemius muscles, determined under non-denaturing conditions using pyrophosphate gel electrophoresis. The angle specific torque-velocity properties of the triceps surae were measured using maximal percutaneous electrical stimulation at 50 Hz and a release technique. The elderly subjects generated significantly (P < 0.05) less absolute torque at all angular velocities when compared with the young subjects. When the isokinetic data were normalized to the isometric torque, the lower normalized torques generated by the elderly subjects were not statistically different from the young. The total fast isomyosin (FM) content of the soleus and gastrocnemius in the elderly subjects was 22 ± 13 and 35 ± 18%, respectively. This compared with 29 ± 8 (n.s.) and 44 ± 8% (n.s.) in the young subjects. When the gastrocnemius and soleus muscles were given an equal weighting and considered together to represent the whole triceps surae, the normalized torque at the fixed angular velocity of 5 rad s^-1 was significantly associated with%FM (r = 0.90, P < 0.01), and the isomyosin bands%FM1 (r = 0.90, P < 0.01) and%FM2 (r = 0.93, P < 0.001) when only the elderly subjects were considered. No relationships were observed between contractile characteristics and contractile protein profile when only the young subjects were considered. This was despite the inclusion of a further two sprint and three endurance trained athletes to increase the range of contractile characteristics and differences in muscle composition.     

Effect of electrical stimulation training on the contractile characteristics of the triceps surae muscle

Summary This study aimed to assess the effects of training using electrical stimulation (ES) on the contractile characteristics of the triceps surae muscle. A selection of 12 subjects was divided into two groups (6 control, 6 experimental). The ES sessions were carried out using a stimulator. Flexible elastomer electrodes were used. The current used discharged pulses lasting 200 s at 70 Hz. Contraction time was 5 s and rest time 15 s. The session lasted 10 min for each muscle. Training sessions were three times a week for 4 weeks. Biomechanical tests were performed using an isokinetic ergometer. Subjects performed plantar flexions of the ankle over a concentric range of movement at different angular velocities (60, 120, 180, 240, 300, 360°·s^–1) and held isometric contractions for 5 s at several ankle flexion angles (–30/–15/0/15°–0 corresponded to foot flexion of 90° relative to the leg axis). The force-velocity relationship was seen to shift evenly upwards under the influence of ES (P<0,05). The increased force during the after test was greater (P<0,05) for ankle angle positions of 15° and –30°, which demonstrated a link between the training angle and the gain in strength. No change was noted in the cross-sectional area of the muscle. The results showed that ES allowed the contractile qualities of muscle to be developed in isometric and dynamic conditions. Nervous mechanisms can account for most of these adaptations.     

Muscle stiffness and spinal stretch reflex sensitivity in the triceps surae

CONTEXT: Greater musculotendinous stiffness may enhance spinal stretch reflex sensitivity by improving mechanical coupling of the muscle spindle and the stretch stimulus. This heightened sensitivity would correspond with a shorter latency and higher-amplitude reflex response, potentially enhancing joint stability. OBJECTIVE: To compare spinal stretch reflex latency and amplitude across groups that differed in musculotendinous stiffness. DESIGN: Static group comparisons. SETTING: Research laboratory. PATIENTS OR OTHER PARTICIPANTS: Forty physically active individuals (20 men, 20 women). Intervention(s): We verified a sex difference in musculotendinous stiffness and compared spinal stretch reflex latency and amplitude in high-stiffness (men) and low-stiffness (women) groups. We also evaluated relationships between musculotendinous stiffness and spinal stretch reflex latency and amplitude, respectively. MAIN OUTCOME MEASURE(S): Triceps surae musculotendinous stiffness and soleus spinal stretch reflex latency and amplitude were assessed at 30% of a maximal voluntary isometric plantar-flexion contraction. RESULTS: The high-stiffness group demonstrated significantly greater stiffness (137.41 +/- 26.99 N/cm) than the low-stiffness group did (91.06 +/- 20.10 N/cm). However, reflex latency (high stiffness = 50.11 +/- 2.07 milliseconds, low stiffness = 48.26 +/- 2.40 milliseconds) and amplitude (high stiffness = 0.28% +/- 0.12% maximum motor response, low stiffness = 0.31% +/- 0.16% maximum motor response) did not differ significantly across stiffness groups. Neither reflex latency (r = .053, P = .746) nor amplitude (r = .073, P = .653) was related significantly to musculotendinous stiffness. CONCLUSIONS: A moderate level of pretension (eg, 30%) likely eliminates series elastic slack; thus, a greater change in force per unit-of-length change (ie, heightened stiffness) would have minimal effects on coupling of the muscle spindle and the stretch stimulus and, therefore, on spinal stretch reflex sensitivity. It appears unlikely that differences in musculotendinous stiffness influenced spinal stretch reflex sensitivity when initiated from a moderate level of pretension. Consequently, differences in musculotendinous stiffness did not appear to influence dynamic joint stability with respect to reflexive neuromuscular control.     

等速肌力测试技术在髋关节运动及创伤评估中的应用

目的探讨等速肌力测试技术在髋关节运动及创伤评估中的应用现状。方法在PubMed、万方等数据库上以“髋关节”、“生物力学”、“肌力”、及“等速测试”为关键词检索1990年1月—2014年5月国内外有关等速肌力测试技术在髋关节运动及损伤评估中应用的相关文献,进行分析总结。结果等速肌力测试技术作为一种动态肌力测试技术,不仅能够对运动员髋关节的肌肉功能进行定量测试,指导运动员进行专项训练;同时,能够为髋关节创伤后的功能情况提供客观评价指标,对设计合理的、有针对性的康复训练方案有指导意义。采用等速肌力测试技术可对髋关节运动创伤的发生进行相关性研究,有望探索一套预防运动创伤发生的基本方法。结论等速肌力测试技术对髋关节运动及创伤的评估、治疗及预防有重要意义。     

Contractile properties of the human triceps surae with some observations on the effects of temperature and exercise

Summary The contractile properties of the triceps of five healthy male subjects (mean age 22 years) during electrically stimulated and voluntary isometric muscle contractions were investigated and some observations made on the effects of muscle heating and cooling and dynamic exercise. The times to peak twitch tension (TPT) and half relaxation time (1/2RT) were 111±20 ms and 83±13 ms respectively. Heating and prior exercise decreased, and cooling severely prolonged, TPT and 1/2RT. Exercise and heating had no effect on supramaximal twitch tension (P_t0) but cooling the muscle to a temperature of 24.3‡ C reduced it by 52%. The effects of repetitive stimuli on P_t0 were dependent on frequency; at 0.2 Hz potentiation was observed but at 2 Hz, P_t0 was depressed under control conditions. Heating had no effect on these responses but cooling reversed the 2 Hz and abolished 0.2 Hz response. Post-tetanic potentiation of the twitch was observed under all conditions of measurement. At submaximal stimulation voltages, heating and exercise enhanced twitch and tetanic tensions, but at supramaximal voltages heating reduced tetanic tension (P₀) at 10 Hz (by 115N), though not at 20 Hz. Exercise decreased P₀ at both frequencies of stimulation. Cooling significantly reduced the maximal voluntary contraction and P₀ at 20 Hz. At submaximal voltages, heating enhanced and cooling severely depressed tetanic tensions at high frequency (100 Hz) stimulation. A 2-min fatigue test was unaffected by heating but cooling reduced force generation at the onset of, and the decline of force during, the test. It was concluded that studies of the contractile properties of human muscle in vivo should be based on supramaximal stimulation, and that temperature and prior exercise should be carefully standardised in order to obtain reliable and meaningful results.     

Effects of isokinetic training of the knee extensors on high-intensity exercise performance and skeletal muscle buffering

Twenty-three subjects isokinetically trained the right and left quadriceps femoris, three times per week for 16 weeks; one group (n=13) trained at an angular velocity of 4.19 rad · s^–1 and a second group (n=10), at 1.05 rad · s^–1. A control group (n=10) performed no training. Isometric endurance time at 60% quadriceps maximum voluntary contraction (MVC), mean power output and work done (W) during all-out cycling, and the muscle buffer value (B) and carnosine concentration of biopsy samples from the vastus lateralis, were all assessed before and after training. The two training groups did not differ significantly from each other in their training response to any of these variables (P < 0.05). No significant difference in either 60% MVC endurance time or impulse [(endurance time × force) at 60% MVC] was observed for any group after the 16 week period (P > 0.05). However, the post-training increase (9%) in W during high-intensity cycling was greater in the training group than in the control group (P=0.04). NeitherB nor carnosine concentration showed any significant change following training (P=0.56 andP=0.37, respectively). It is concluded that 16 weeks of isokinetic training of the knee extensors enables subjects to do more work during high-intensity cycling. Although the precise adaptations responsible for the improved performance have yet to be identified, they are unlikely to include an increase inB.     

Use of the iso-inertial force mass relationship in the prediction of dynamic human performance

The purpose of this investigation was to develop a new test of muscle function, termed the isoinertial force-mass relationship, and to determine its relationship to dynamic physical performance in comparison to an isometric test. A group of 13 trained subjects performed an isometric, and a series of iso-inertial maximal upper body tests, in a bench press movement at loads of 30%, 60%, 100% (concentric) and 100%, 130% and 150% (eccentric) of maximum. Vertical forces exerted throughout the movement were recorded by a force plate. In addition, the subjects performed the following three performance tests: a maximal bench press, a seated shotput, and two drop bench-press throws from a height of 0.25 m, with loads of 10 kg and 30% of maximum. Correlation analysis demonstrated that in each instance the iso-inertial force mass tests were the best predictors of performance (r=0.78–0.88) with both contraction type and mass specific effects apparent. Maximal isometric force and rate of force development were significantly related to some performance variables (r=0.22–0.78). However, for all the performance movements assessed, the iso-inertial test modality recorded the highest relationship to performance. The difference in the relationship between performance and iso-inertial and isometric test modalities was particularly evident in the light load dynamic performance of the seated shotput (r=0.86 vsr=0.38, respectively). These results are explained in part by the neural and mechanical differences between iso-inertial and isometric muscle actions and their respective specificity to dynamic physical performance.     

The effects of motor learning on clinical isokinetic performance of postmenopausal women

Objective

To analyze the effects of motor learning on knee extension–flexion isokinetic performance during clinical isokinetic evaluation of postmenopausal women.

Methods

One-hundred and twenty postmenopausal women (60.3 ± 3.2 years; BMI = 27.6 ± 4.7 kg/m²) without knee pain or injury and that never underwent isokinetic testing, were submitted to two bilateral knee extension–flexion (concentric–concentric) isokinetic evaluation (5 repetitions) at 60°/s (Biodex™ Multi-Joint System 3 dynamometer). The tests were first performed in the dominant leg, with a 1-min recovery between them, and after a standardized warm-up that included 3 submaximal isokinetic repetitions. The same procedure was repeated in the non-dominant leg. Peak torque (PTQ) was adjusted for body weight (PTQ/BW), total work (TW), coefficient of variation (CV) and agonist/antagonist (agon/antag) ratio was compared between tests.

Results

Subjects showed greater levels (P < 0.001) of PTQ, PTQ/BW and TW, and lower CV levels (P < 0.01) in test 2 of both legs. Agon/antag ratio did not change significantly between tests.

Conclusions

PTQ, PTQ/BW, TW and CV improved in the second knee extension flexion isokinetic testing of postmenopausal women. The results suggests that performing two tests, even with a short period of recovery between them, could be considered for reducing motor learning effects on clinical isokinetic evaluation of knee joint in postmenopausal women.     

In vivo human triceps surae and quadriceps femoris muscle function in a squat jump and counter movement jump

An optic fibre method was used to measure in humans in vivo Achilles (ATF) and patellar tendon forces (PTF) during submaximal squat jumps (SJ) and counter movement jumps (CMJ). Normal two-legged jumps on a force plate and one-legged jumps on a sledge apparatus were made by four volunteers. Kinetics, kinematics, and muscle activity from seven muscles were recorded. The loading patterns of the tendomuscular system differed among the jumping conditions, but were similar when the jumping height was varied. Peak PTF were greater than ATF in each condition. In contrast to earlier simulation studies it was observed that tendomuscular force could continue to increase during the shortening of muscle-tendon unit in CMJ. The concentric tendomuscular output was related to the force at the end of the stretching phase while the enhancement of the output in CMJ compared to SJ could not be explained by increases in muscle activity. The stretching phase in CMJ was characterised by little or no electromyogram activity. Therefore, the role of active stretch in creating beneficial conditions for the utilisation of elastic energy in muscle was only minor in these submaximal performances. The modelling, as used in the present study, showed, however, that tendon underwent a stretch-shortening cycle, thus having potential for elastic energy storage and utilisation. In general, the interaction between muscle and tendon components may be organised in a manner that takes advantage of the basic properties of muscle at given submaximal and variable activity levels of normal human locomotion. Accepted: 28 June 2000     

Comparison of the histochemical and contractile properties of human triceps surae

Summary Contractile and histochemical properties of the triceps surae were compared in 16 males and 4 females aged 20 to 49 years. Surface electrical stimulation was used to determine twitch, tetanic and fatigue parameters. From these tests, twitch tension (Pt), time to peak tension (TPT), half relaxation time 1/2 RT), tetanic tensions at 10, 20 and 50 Hz and an index of fatigue (FI) were calculated. A maximal voluntary contraction (MVC) was also performed. Muscle samples from the belly of the lateral gastrocnemius were obtained using the needle biopsy technique. The samples were treated histochemically for myosin ATPase and NADH-tetrazolium reductase in order to classify the fibres as either Type I, slow twitch (ST) or Type II, fast twitch (FT) and to determine fibre areas. Correlations were performed between the grouped male and female contractile and histochemical variables. The results demonstrated significant positive relationships between percentage of ST fibres (%ST) and TPT (r=0.49), and %ST and the ratio of tetanic forces at 10 Hz to 50 Hz (Po10/Po50) (r=0.55). No significant relationships were obtained for Pt, 1/2 RT, MVC or FI with any histochemical parameter. The results suggest that fibre type distribution determined using myosin ATPase is related to electrically stimulated isometric contractile speeds and not to voluntary force generation (MVC) or electrically induced fatigue.     

System identification of human triceps surae stretch reflex dynamics

The interpretation of stretch-evoked reflex responses is complicated by the fact that the pattern of response will depend upon both the underlying reflex mechanisms and the time course of the stretch used to evoke the response. The objective of the present study was to use engineering systems analysis techniques to identify the dynamics of the human triceps surae (TS) stretch reflex in terms of its impulse response by deconvolving the position input from the observed response. Five normal subjects were instructed to maintain a tonic contraction of (TS) while subjected to repeated, computer-generated, stochastic perturbations of ankle position. Position, torque and smoothed, rectified surface EMGs were recorded and ensemble averaged over 25 stimulus presentations. Linear impulse response functions describing the dynamic relation between ankle velocity and TS EMG were found to account for a significant amount of the observed EMG variance (mean 60%). However, the impulse responses were noisy and the predicted EMG was systematically smaller than the observed EMG during the dorsiflexing phases of displacement. These findings suggested that a direction dependent nonlinearity might be present. Consequently, impulse responses relating half-wave rectified velocity to TS EMG were computed and found to be less noisy and to account for significantly more variance (mean 74%) than the purely linear model. The undirectional, velocity-sensitive impulse response functions were dominated by a large peak at about 40 ms followed by a smaller period of reduced activity. This is consistent with its mediation by primary spindle afferents. Although the shape of the impulse response remained unchanged, its amplitude, which provides a measure of relative gain, varied systematically with the level of contraction and the displacement amplitude. Multiple regression analysis demonstrated that most of the variation in the impulse response amplitude could be attributed to proportional increases with level of contraction (measured by average EMG) and proportional decreases with displacement amplitude.