Methodological issues with the interpolated twitch technique.

A number of methodological issues in the use of the interpolated twitch technique were investigated for their effect on true maximum force (TMF) and activation (ACT): timing of control (pre- vs post-contraction) and superimposed twitches (first vs second); type of twitch stimulus (primarily magnitude); and the type of extrapolation utilised. On three occasions subjects performed a series of maximal and sub-maximal contractions of the knee extensors, with electrically evoked twitches delivered before, during and after each contraction. The twitch-voluntary force relationship was concave for all types of twitch stimuli, and extrapolation using this relationship typically calculated TMF 39N (7%) higher, and ACT 7% lower than linear extrapolation. The timing of the control (2-4%) and superimposed twitches (approximately 4%) both influenced TMF and ACT. Despite the different twitch stimuli being a range of magnitudes (13-32% maximum voluntary force) they did not affect TMF and ACT. A novel finding was that prior potentiation changed the shape of the twitch-voluntary force relationship. For precise measurement of TMF and ACT it is recommended that: extrapolation is based on the twitch-voluntary force relationship of the experimental model; and post-contraction potentiated twitches be used, as the superimposed twitch on a high level contraction appears to be potentiated.     

Postactivation potentiation in a human muscle: effect on the rate of torque development of tetanic and voluntary isometric contractions.

Postactivation potentiation (PAP), a mechanism by which the torque of a muscle twitch is increased following a conditioning contraction, is well documented in muscular physiology, but little is known about its effect on the maximal rate of torque development and functional significance during voluntary movements. The objective of this study was to investigate the PAP effect on the rate of isometric torque development of electrically induced and voluntary contractions. To that purpose, the electromechanical responses of the thumb adductor muscles to a single electrical stimulus (twitch), a train of 15 pulses at 250 Hz (HFT(250)), and during ballistic (i.e., rapid torque development) voluntary contractions at torque levels ranging from 10 to 75% of maximal voluntary contraction (MVC) were recorded before and after a conditioning 6-s MVC. The results showed that the rate of torque development was significantly (P < 0.001) increased after the conditioning MVC, but the effect was greater for the twitch ( approximately 200%) compared with the HFT(250) ( approximately 17%) or ballistic contractions (range: 9-24%). Although twitch potentiation was maximal immediately after the conditioning MVC, maximal potentiation for HFT(250) and ballistic contractions was delayed to 1 min after the 6-s MVC. Furthermore, the similar degree of potentiation for the rate of isometric torque development between tetanic and voluntary ballistic contractions indicates that PAP is not related to the modality of muscle activation. These observations suggest that PAP may be considered as a mechanism that can influence our contractions during daily tasks and can be utilized to improve muscle performance in explosive sports.     

Postactivation potentiation in a human muscle: effect on the load-velocity relation of tetanic and voluntary shortening contractions.

Recently it was demonstrated that postactivation potentiation (PAP), which refers to the enhancement of the muscle twitch torque as a result of a prior conditioning contraction, increased the maximal rate of torque development of tetanic and voluntary isometric contractions (3). In this study, we investigated the effects of PAP and its decay over time on the load-velocity relation. To that purpose, angular velocity of thumb adduction in response to a single electrical stimulus (twitch), a high-frequency train of 15 pulses at 250 Hz (HFT(250)), and during ballistic voluntary shortening contractions, performed against loads ranging from 10 to 50% of the maximum torque, were recorded before and after a conditioning 6-s maximal voluntary contraction (MVC). The results showed an increase of the peak angular velocity for the different loads tested after the conditioning MVC (P < 0.001), but the effect was greatest for the twitch ( approximately 182%) compared with the HFT(250) or voluntary contractions ( approximately 14% for both contraction types). The maximal potentiation occurred immediately following the conditioning MVC for the twitch, whereas it was reached 1 min later for the tetanic and ballistic voluntary contractions. At that time, the load-velocity relation was significantly shifted upward, and the maximal power of the muscle was increased ( approximately 13%; P < 0.001). Furthermore, the results also indicated that the effect of PAP on shortening contractions was not related to the modality of muscle activation. In conclusion, the findings suggest a functional significance of PAP in human movements by improving muscle performance of voluntary dynamic contractions.     

Twitch contractile properties of plantar flexor muscles in power and endurance trained athletes

This study compared twitch contractile properties of plantar flexor muscles among three groups of 12 subjects each: endurance and power trained athletes and untrained subjects. The posterior tibial nerve was stimulated by supramaximal square wave pulses of 1-ms duration. Power trained athletes had higher twitch maximal force, maximal rates of force development and relaxation and also maximal voluntary contraction (MVC) force. The trained subjects had a smaller twitch maximal force: MVC force ratio and shorter twitch contraction and half-relaxation times than the untrained subjects with no significant differences between the two groups. Thus, the short time for evoked twitches in the athletes compared to the untrained subjects would seem unrelated to the type of training. It is concluded that power training induces a more evident increase of muscle force-generating capacity and speed of contraction and relaxation than endurance training. Accepted: 24 April 1999     

The shape of the force-elbow angle relationship for maximal voluntary contractions and sub-maximal electrically induced contractions in human elbow flexors

The force-length relationship is a basic property of skeletal muscle. Knowledge of this relationship is necessary for most analyses of human movement, and in simulation models predicting movement control strategies. Studies on animal muscles have shown that force-length relationships for sub-maximal contractions are not related through a simple scaling procedure to the relationship for maximal contractions. Furthermore, potentiation might produce a shift of sub-maximal relative to maximal force-length relationships. In this study, we tested the hypothesis that human force-elbow angle relationships for sub-maximal unpotentiated contractions are shifted to larger elbow angles (i.e. larger muscle lengths) compared to the relationship for maximal voluntary contractions (MVC), and that this shift is reduced, or even abolished, for sub-maximal potentiated contractions. Force-elbow angle relationships (48-160 degrees) were obtained from healthy subjects (n=13). At each of nine tested elbow angles, the test set consisted of a single twitch (ST(pre)) and a doublet twitch (DT(pre)) stimulation of m. biceps brachii, followed by an MVC, followed by another single twitch (ST(post)) and a doublet twitch (DT(post)) stimulation. The single and doublet twitches induced sub-maximal contractions. The force-elbow angle relationships for the pre-MVC (unpotentiated) twitch contractions were shifted to larger angles compared to those obtained for MVC. The force-elbow angle relationships for the post-MVC (potentiated) twitch contractions were shifted to smaller angles compared to those obtained for the unpotentiated twitch contractions. These results support the idea that the shift to larger muscle lengths for the sub-maximal, unpotentiated force-length relationships relative to the relationship for maximal contractions may be caused by a length-dependent Ca(2+) sensitivity that may be offset, at least in part, by potentiation.     

Contractile properties of human skeletal muscle in childhood and adolescence

Using a combination of single maximal stimuli and maximum voluntary contractions, a comparison has been made of muscle properties in pre- and post-pubertal male subjects. In the dorsiflexor and plantarflexor muscles of the ankle, the twitch and maximum voluntary torques were approximately twice as large in the older subjects; the mean height and mean weight increased by factors of 1.20 and 1.86 respectively. The only other muscle parameter that changed, as a function of age, was the contraction time of the ankle dorsiflexors; the mean value was significantly longer in the older subjects. In the younger subjects, there were already clear differences between the dorsiflexor and plantarflexor muscles, the former developing smaller torques and having shorter contraction and half-relaxation times, greater post-activation potentiation and more susceptibility to fatigue. Even in the youngest subject, motor unit activation was complete in the ankle dorsiflexors; although this was not always true of the plantarflexors, the difference between the two subject groups was not significant.     

Muscle weakness following sustained and rhythmic isometric contractions in man

The effects of sustained and rhythmically performed isometric contractions on electrically evoked twitch and tetanic force generation of the triceps surae have been investigated in 4 healthy male subjects. The isometric contractions were performed separately and on different occasions at 30%, 60% and 100% of the force of maximal voluntary contraction (MVC). The area under the maximal voluntary contraction (MVC) force/time curve during the rhythmic and sustained contractions was the same for each experiment. The results showed that following rhythmic isometric exercise there was a small decrease in low (10 and 20 Hz) and high (40 Hz) frequency tetanic tension which was associated with % MVC. However, there was no change in the 20/40 ratio of tetanic forces, MVC or the contraction times and force of the maximal twitch. In contrast, following sustained isometric exercise tetanic forces were markedly reduced, particularly at low frequencies of stimulation. The 20/40 ratio decreased and the induced muscle weakness was greater at 30% than 60% or 100% MVC. The performance of sustained isometric contractions also effected a decrease in contraction time of the twitch and MVC. The results are in accord with previous findings for dynamic work (Davies and White 1982), and show that if isometric exercise is performed rhythmically the effect on tetanic tensions is small and there is no evidence of a preferential loss of electrically evoked force at either high or low frequencies of stimulation following the contractions. For sustained contractions, however, the opposite is true, the ratio of 20/40 Hz forces is markedly reduced and following 30% sustained MVC there is a significant (p less than 0.05) change in the time to peak tension (TPT) of the maximal twitch.     

Effects of series compliance on twitches superimposed on voluntary contractions.

The activation of skeletal muscle during voluntary isometric contraction has been assessed by measuring the increase in force caused by a superimposed maximal shock to the motor nerve (the twitch-interpolation technique). When the muscle is held isometric, the increase in force with stimulation (superimposed twitch force) decreases with increasing voluntary force, and a line fit through the data can be extrapolated to maximal voluntary force at the zero twitch force axis. In a previous paper we questioned the applicability of this technique in situations where a high series compliance allows the muscle to shorten during the superimposed twitch. To explore effects of series compliance, we measured force of the adductor pollicis during voluntary isometric contractions with noncompliant and compliant loading devices. With the compliant loading device, superimposed twitch force was systematically less than with the noncompliant device, and the plot of superimposed twitch force vs. voluntary force was often concave upward, preventing easy extrapolation to maximal voluntary force. These findings are consistent with force-velocity characteristics of muscle and suggest that twitch-interpolation data must be interpreted with caution when the muscle is not held isometric during the superimposed twitch.     

Simultaneous potentiation and fatigue in quadriceps after a 60-second maximal voluntary isometric contraction

Potential mechanisms of fatigue (metabolic factors) and potentiation (phosphate incorporation by myosin phosphorylatable light chains) were investigated during recovery from a 60-s maximal voluntary isometric contraction (MVC) in the quadriceps muscle of 12 subjects. On separate days before and for 2 h after the 60-s MVC, either a 1-s MVC or electrically stimulated contractions were used as indexes to test muscle performance. Torque at the end of the 60-s MVC was 57% of the initial level, whereas torques from a 1-s MVC and 50-Hz stimulation were most depressed in the immediate recovery period. At this time, muscle biopsy analyses revealed significant decreases in ATP and phosphocreatine and a 19-fold increase in muscle lactate. Conversely, isometric twitch torque and torque from a 10-Hz stimulus were the least depressed of six contractile indexes and demonstrated potentiation of 25 and 34%, respectively, by 4 min of recovery (P less than 0.05). At this time, muscle lactate concentration was still 16 times greater than at rest. An increased phosphate content of the myosin phosphorylatable light chains (P less than 0.05) was also evident both immediately and 4 min after the 60-s MVC. We conclude that the 60-s MVC produced marked force decreases likely due to metabolic displacement, while the limited decline in the twitch and 10-Hz torques and their significant potentiation suggested that myosin phosphorylation may provide a mechanism to enhance contractile force under conditions of submaximal activation during fatigue.     

Observations on force enhancement in submaximal voluntary contractions of human adductor pollicis muscle.

It has been observed consistently and is well accepted that the steady-state isometric force after active muscle stretch is greater than the corresponding isometric force for electrically stimulated muscles and maximal voluntary contractions. However, this so-called force enhancement has not been studied for submaximal voluntary efforts; therefore, it is not known whether this property affects everyday movements. The purpose of this study was to determine whether there was force enhancement during submaximal voluntary contractions. Human adductor pollicis muscles (n = 17) were studied using a custom-built dynamometer, and both force and activation were measured while muscle activation and force were controlled at a level of 30% of maximal voluntary contraction. The steady-state isometric force and activation after active stretch were compared with the corresponding values obtained during isometric reference contractions. There was consistent and reliable force enhancement in 8 of the 17 subjects, whereas there was no force enhancement in the remaining subjects. Subjects with force enhancement had greater postactivation potentiation and a smaller resistance to fatigue in the adductor pollicis. We conclude from these results that force enhancement exists during submaximal voluntary contractions in a subset of the populations and suggest that it may affect everyday voluntary movements in this subset. On the basis of follow-up testing, it appears that force enhancement during voluntary contractions is linked to potentiation and fatigue resistance and therefore possibly to the fiber-type distribution in the adductor pollicis muscle.     

Changes in motor unit discharge rate are not associated with the amount of twitch potentiation in old men.

This study examined, in nine old men (82 +/- 4 yr), whether there is an association between the magnitude of change in motor unit discharge rate and the amount of twitch potentiation after a conditioning contraction (CC). The evoked twitch force and motor unit discharge rate during isometric ramp-and-hold contractions (10-18 s) of the triceps brachii muscle at 10, 20, and 30% of the maximal voluntary contraction were determined before and 10 s, 2 min, 6 min, and 11 min after a 5-s CC at 75% maximal voluntary contraction. After the CC, there was a potentiation of twitch force (approximately twofold), and the discharge rate of the 47 sampled motor units declined (P < 0.05) an average of 1 Hz 10 s after the CC, compared with the control condition. The CC had no effect on the variability (coefficient of variation) of both force and discharge rate, as well as the electromyographic activity recorded over the triceps brachii and biceps brachii muscles. In contrast to our earlier study of young men (Klein CS, Ivanova TD, Rice CL, and Garland SJ, Neurosci Lett 316: 153-156, 2001), the magnitude of the reduction in discharge rate after the CC was not associated (r = 0.06) with the amount of twitch potentiation. These findings suggest an age-related alteration in the neural strategy for adjusting motor output to a muscle after a CC.     

Muscle contractility in chronic disorders of neuromuscular transmission

Muscle contractility of the thumb was studied in 24 normal subjects, 84 patients with myasthenia and 4 patients with hypothyrosis in response to supramaximal stimulation, on the basis of the time of contraction and semi-relaxation, staircase phenomena, and posttetanic potentiation. During hormonal therapy, the patients with myasthenia and those with hypothyrosis treated by substitution therapy manifested the normalization of the staircase and posttetanic potentiation, reduction of the contraction force and twitch time during single contraction. The comparison of the changes seen in the contraction force, staircase and posttetanic potentiation during examination of the patients over time suggested that the muscle has a regulatory system that determines the force and twitch time of muscle contraction.     

Variability in the interpolated twitch torque for maximal and submaximal voluntary contractions.

The superimposed twitch technique is frequently used to study the degree of motor unit activation during voluntary effort. This technique is one of the preferred methods to determine the activation deficit (AD) in normal, athletic, and patient populations. One of the limitations of the superimposed twitch technique is its variability under given contractile conditions. The objective of this research was to determine the source(s) of variability in the superimposed twitch force (STF) for repeat measurements. We hypothesized that the variability in the AD measurements may be caused by the timing of the twitch force relative to the onset of muscle activation, by force transients during the twitch application, by small variations in the actual force from the nominal target force, and by variations in the resting twitch force. Twenty-eight healthy subjects participated in this study. Sixteen of these subjects participated in a protocol involving contractions at 50% of their maximal voluntary contraction (MVC) effort, whereas the remaining 12 participated in a protocol involving contractions at 100% of their MVC. Doublet-twitch stimuli were superimposed onto the 50 and 100% effort knee extensor muscle contractions, and the resting twitch forces, voluntary knee extensor forces, and STFs were then measured. The mean resting twitch forces obtained before and after 8 s of 50% of MVC were the same. Similarly, the mean STFs determined at 1, 3, 5, and 7 s into the 50% MVC were the same. The variations in twitch force were significantly smaller after accounting for the actual force at twitch application than those calculated from the prescribed forces during the 50% MVC protocol (P < 0.05). Furthermore, the AD and the actual force showed statistically significant negative correlations for the 50% MVC tests. The interpolated twitch torque determined for the maximal effort contractions ranged from 1 to 70%. In contrast to the protocol at 50% of MVC, negative correlations were only observed in 5 of the 12 subjects during the 100% effort contractions. These results suggest that small variations in the actual force from the target force can account for the majority of the variations in the STFs for submaximal but not maximal effort contractions. For the maximal effort contractions, large variations in the STF exist due to undetermined causes.     

Activation-induced force enhancement in human adductor pollicis

It has been known for a long time that the steady-state isometric force after muscle stretch is bigger than the corresponding force obtained in a purely isometric contraction for electrically stimulated and maximal voluntary contractions (MVC). Recent studies using sub-maximal voluntary contractions showed that force enhancement only occurred in a sub-group of subjects suggesting that force enhancement for sub-maximal voluntary contractions has properties different from those of electrically-induced and maximal voluntary contractions. Specifically, force enhancement for sub-maximal voluntary contractions may contain an activation-dependent component that is independent of muscle stretching. To address this hypothesis, we tested for force enhancement using (i) sub-maximal electrically-induced contractions and stretch and (ii) using various activation levels preceding an isometric reference contraction at 30% of MVC (no stretch). All tests were performed on human adductor pollicis muscles. Force enhancement following stretching was found for all subjects (n = 10) and all activation levels (10%, 30%, and 60% of MVC) for electrically-induced contractions. In contrast, force enhancement at 30% of MVC, preceded by 6 s of 10%, 60%, and 100% of MVC was only found in a sub-set of the subjects and only for the 60% and 100% conditions. This result suggests that there is an activation-dependent force enhancement for some subjects for sub-maximal voluntary contractions. This activation-dependent force enhancement was always smaller than the stretch-induced force enhancement obtained at the corresponding activation levels. Active muscle stretching increased the force enhancement in all subjects, independent whether they showed activation dependence or not. It appears that post-activation potentiation, and the associated phosphorylation of the myosin light chains, might account for the stretch-independent force enhancement observed here.     

Activation of human quadriceps femoris during isometric, concentric, and eccentric contractions.

Maximal and submaximal activation level of the right knee-extensor muscle group were studied during isometric and slow isokinetic muscular contractions in eight male subjects. The activation level was quantified by means of the twitch interpolation technique. A single electrical impulse was delivered, whatever the contraction mode, on the femoral nerve at a constant 50 degrees knee flexion (0 degrees = full extension). Concentric, eccentric (both at 20 degrees /s velocity), and isometric voluntary activation levels were then calculated. The mean activation levels during maximal eccentric and maximal concentric contractions were 88.3 and 89.7%, respectively, and were significantly lower (P < 0.05) with respect to maximal isometric contractions (95.2%). The relationship between voluntary activation levels and submaximal torques was linearly fitted (P < 0.01): comparison of slopes indicated lower activation levels during submaximal eccentric compared with isometric or concentric contractions. It is concluded that reduced neural drive is present during 20 degrees /s maximal concentric and both maximal and submaximal eccentric contractions. These results indicate a voluntary activation dependency on both tension levels and type of muscular actions in the human knee-extensor muscle group.     

Reduced strength after passive stretch of the human plantarflexors.

The purpose of this study was to assess strength performance after an acute bout of maximally tolerable passive stretch (PS(max)) in human subjects. Ten young adults (6 men and 4 women) underwent 30 min of cyclical PS(max) (13 stretches of 135 s each over 33 min) and a similar control period (Con) of no stretch of the ankle plantarflexors. Measures of isometric strength (maximal voluntary contraction), with twitch interpolation and electromyography, and twitch characteristics were assessed before (Pre), immediately after (Post), and at 5, 15, 30, 45, and 60 min after PS(max) or Con. Compared with Pre, maximal voluntary contraction was decreased at Post (28%) and at 5 (21%), 15 (13%), 30 (12%), 45 (10%), and 60 (9%) min after PS(max) (P < 0.05). Motor unit activation and electromyogram were significantly depressed after PS(max) but had recovered by 15 min. An additional testing trial confirmed that the torque-joint angle relation may have been temporarily altered, but at Post only. These data indicate that prolonged stretching of a single muscle decreases voluntary strength for up to 1 h after the stretch as a result of impaired activation and contractile force in the early phase of deficit and by impaired contractile force throughout the entire period of deficit.     

Force depression in human quadriceps femoris following voluntary shortening contractions.

The purpose of this study was to investigate whether the isometric muscle force, redeveloped following maximal-effort voluntary shortening contractions in human skeletal muscle, is smaller than the purely isometric muscle force at the corresponding length. Isometric knee extensor moments, surface electromyographic (EMG) signals of quadriceps femoris, and interpolated twitch moments (ITMs) were measured while 10 subjects performed purely isometric knee extensor contractions at a 60 degrees knee angle and isometric knee extensor contractions at a 60 degrees knee angle preceded by maximal-effort voluntary shortening of the quadriceps muscles. It was found that the knee extensor moments were significantly decreased for the isometric-shortening-isometric contractions compared with the isometric contractions for the group as a whole, whereas the corresponding EMG and ITM values were the same. This study is the first to demonstrate force depression following muscle shortening for voluntary contractions. We concluded that force depression following muscle shortening is an actual property of skeletal muscle rather than a stimulation artifact and that force depression during voluntary contraction is not accompanied by systematic changes in muscle activation as evaluated by EMG and ITM.     

Reproducible measurement of voluntary activation of human elbow flexors with motor cortical stimulation.

Voluntary activation of muscle is commonly quantified by comparison of the extra force added by motor nerve stimulation during a contraction [superimposed twitch (SIT)] with that produced at rest by the same stimulus (resting twitch). An inability to achieve 100% voluntary activation implies that failure to produce maximal force output from the muscle must have occurred at a site at or above the level of the motoneurons. We have used cortical stimulation to quantify voluntary activation. Here, incomplete activation implies a failure at or above the level of motor cortical output. With cortical stimulation, it is inappropriate to compare extra force evoked during a contraction with the twitch evoked in resting muscle because motor cortical and spinal cord excitability both increase with activity. However, an appropriate "resting twitch" can be estimated. We previously estimated its amplitude by extrapolation of the linear relation between SIT amplitude and voluntary torque calculated from 35 contractions of >50% maximum (Todd G, Taylor JL, and Gandevia SC. J Physiol 551: 661-671, 2003). In this study, we improved the utility of this method to enable evaluation of voluntary activation when it may be changing over time, such as during the development of fatigue, or in patients who may be unable to perform large numbers of contractions. We have reduced the number of contractions required to only three. Estimation of the resting twitch from three contractions was reliable over time with low variability. Furthermore, its reliability and variability were similar to the resting twitch estimated from 30 contractions and to that evoked by conventional motor nerve stimulation.     

Differences between motor unit firing rate, twitch characteristics and fibre type composition in an agonistic muscle group in man

A comparison was carried out between the motor unit (MU) firing rate and the characteristics of the twitch and the fibre type composition of anconeus and triceps brachii. Fibre type composition (type I, type II) was determined in whole cross-sections of cadaver specimens. The proportion of type I fibre was 60%-67% in anconeus and 32-40% in the lateral head of triceps brachii. Reflecting these histochemical differences, the contraction time of anconeus and triceps was 92 +/- 9 ms and 68 +/- 9 ms respectively. It follows that anconeus can be classified as a slow muscle, as opposed to the lateral head of triceps. The relationship between MU firing rate and isometric force, tested at 90 degrees elbow flexion, differed between the two muscles for force values below 30% of maximal voluntary contraction. No significant increase in MU firing rate was found in anconeus at forces above 5% of maximal voluntary contraction. It is concluded that even within a single agonistic muscle group acting at a single joint there is an adaptation of MU firing rate to the contractile characteristics of each muscle.     

Twitch potentiation after fatiguing exercise in man

Twitch potentiation was studied in the human triceps surae complex before and after intermittent maximal voluntary contractions or electrical stimulation at 20 Hz. Both forms of exercise were conducted with intact circulation for a maximum of 10 min or with circulatory occlusion until force output declined 50%. The relative potentiation was determined when a control twitch was compared to a twitch obtained after 5 s of maximal voluntary plantar flexion. The unpotentiated twitch torque (PT) and potentiated twitch torque (PT*) were reduced most severely after voluntary ischemic exercise (63.2% and 52.5% respectively, (P less than 0.001)). However, the relative potentiation (PT*/PT) immediately after voluntary ischemic exercise increased to 1.65 +/- 0.18 from 1.22 +/- 0.13 at rest. Both PT and PT* recovered quickly after exercise. At rest, twitch contraction time (CT) and one-half relaxation time (1/2 RT) in the unpotentiated twitch were longer than that of contraction (CT*) and one-half relaxation time (1/2 RT*) in the potentiated twitch. Following non-occluded exercise, CT, CT*, 1/2 RT and 1/2 RT* were shortened relative to rest. After ischemic exercise CT and CT* were shortened although 1/2 RT and 1/2 RT* increased relative to rest. Both CT* and 1/2 RT* quickly recovered to pre-exercise values by 5 min post-exercise. Ratios of potentiated/control twitch parameters were not altered after nonoccluded exercise, but were increased after ischemic exercise. These results suggest that the mechanisms of fatigue which depress voluntary torque and twitch and potentiated twitch torques, do not interfere with the extent of potentiation after fatiguing exercise.