Torque potentiation and myosin light-chain phosphorylation in human muscle following a fatiguing contraction

Indices of electrically stimulated and maximal voluntary isometric muscle torgue and the phosphate content of myosin phosphorylatable light chains (P light chains) were studied during recovery following a 60-s maximal voluntary isometric contraction (MVC) in 21 human subjects. Analysis of muscle biopsy samples revealed that immediately after the 60-s MVC there were significant decreases in ATP (-15%) and phosphocreatine (-82%), and lactate concentration increased by 17-fold. All indices of muscle torque production were reduced by the 60-s MVC, but the twitch torque and torque at 10 Hz were relatively less reduced compared with the torque at 20 and 50 Hz or a 1-s MVC. Between 3 and 6 min of recovery, twitch torque and torque at 10 Hz stimulation were significantly potentiated, reaching peak values of 125 and 134%, respectively, compared with rest. Phosphate content of the fast and two slow P light chains was significantly increased over rest levels immediately after and 4 min after the 60-s MVC. These results suggest that myosin P light-chain phosphorylation could provide a mechanism to increase human muscle torque under conditions of submaximal contractile element activation following fatigue.     

Fatigue of intermittent submaximal voluntary contractions: central and peripheral factors

Central and peripheral factors were studied in fatigue of submaximal intermittent isometric contractions of the human quadriceps and soleus muscles. Subjects made repeated 6 s, 50% maximal voluntary contractions (MVC) followed by 4 s rest until the limit of endurance (Tlim). Periodically, a fatigue test was performed. This included a brief MVC, either a single shock or 8 pulses at 50 Hz during a rest period and a shock superimposed on a target force voluntary contraction. At Tlim, the MVC force had declined by 50%, usually in parallel with the force from stimulation at 50 Hz. The twitches superimposed on the target forces declined more rapidly, disappearing entirely at Tlim. In similar experiments on adductor pollicis, no reduction of the evoked M wave was seen. The results suggest that, during fatigue of quadriceps and adductor pollicis induced by this protocol, no central fatigue was apparent, but some was seen in soleus. Thus the reduced force-generating capacity could result mainly or entirely from failure of the muscle contractile apparatus.     

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.     

Fatigue and recovery contractile properties of young and elderly men

The 24 h recovery pattern of contractile properties of the triceps surae muscle, following a period of muscle fatigue, was compared in physically active young (25 years, n = 10) and elderly (66 years, n = 7) men. The fatigue test protocol consisted of 10 min of intermittent submaximal 20 Hz tetani. The maximal twitch (Pt) and tetanic force at 3 frequencies (10, 20 and 50 Hz) were determined at baseline and at 15 min, 1, 4 and 24 h after fatiguing the muscle. Maximal voluntary contraction (MVC) and vertical jump (MVJ) were also assessed. The loss of force during the fatigue test was not significantly different between the young (18 +/- 13%) and elderly (22 +/- 15%). Both groups showed similar and significant reductions of Pt (15%), tetanic force (10 to 35%) and rate of force development (dp/dt) (20%) 15 min and 1 h into recovery. The loss of force was greater at the lower stimulation frequencies of 10 and 20 Hz. Time-to-peak tension was unchanged from baseline during recovery in either group. The average rate of relaxation of twitch force (-dPt/dt) was decreased (p less than 0.05) and half-relaxation time significantly increased at 15 min and 1 h in the elderly but not the young. The findings indicate that after fatiguing contractions, elderly muscle demonstrates a slower return to resting levels of the rate and time course of twitch relaxation compared to the young.     

Sustained contraction at very low forces produces prominent supraspinal fatigue in human elbow flexor muscles.

During sustained maximal voluntary contractions (MVCs), most fatigue occurs within the muscle, but some occurs because voluntary activation of the muscle declines (central fatigue), and some of this reflects suboptimal output from the motor cortex (supraspinal fatigue). This study examines whether supraspinal fatigue occurs during a sustained submaximal contraction of 5% MVC. Eight subjects sustained an isometric elbow flexion of 5% MVC for 70 min. Brief MVCs were performed every 3 min, with stimulation of the motor point, motor cortex, and brachial plexus. Perceived effort and pain, elbow flexion torque, and surface EMGs from biceps and brachioradialis were recorded. During the sustained 5% contraction, perceived effort increased from 0.5 to 3.9 (out of 10), and elbow flexor EMG increased steadily by approximately 60-80%. Torque during brief MVCs fell to 72% of control values, while both the resting twitch and EMG declined progressively. Thus the sustained weak contraction caused fatigue, some of which was due to peripheral mechanisms. Voluntary activation measured by motor point and motor cortex stimulation methods fell to 90% and 80%, respectively. Thus some of the fatigue was central. Calculations based on the fall in voluntary activation measured with cortical stimulation indicate that about two-thirds of the fatigue was due to supraspinal mechanisms. Therefore, sustained performance of a very low-force contraction produces a progressive inability to drive the motor cortex optimally during brief MVCs. The effect of central fatigue on performance of the weak contraction is less clear, but it may contribute to the increase in perceived effort.     

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.     

Post-tetanic potentiation of reciprocal Ia inhibition in human lower limb.

The purpose of this study was to investigate how reciprocal Ia inhibition is changed during muscle fatigue of lower limb muscle, induced with a voluntary contraction or height frequency electrical stimulation. Reciprocal Ia inhibition from ankle flexors to extensors has been investigated in 12 healthy subjects. Hoffmann reflex (H-reflex) in the soleus muscle was used to monitor changes in the amount of reciprocal Ia inhibition from common peroneal nerve as demonstrated during voluntary dorsi or planterflexion and 50 Hz electrical stimulation induced dorsi or planterflexion. The test soleus H-reflex was kept at 20-25% of maximum directly evoked motor response (M response) and the strength of the conditioning common peroneal nerve stimulation was kept at 1.0 x motor threshold. At rest, weak la inhibition was demonstrated in 12 subjects, maximal inhibition from the common peroneal nerve was 28.8%. During voluntary dorsiflexion and 50 Hz electrical stimulation induced dorsiflexion, there absolute amounts of inhibition increased as compared to at rest, and decreased or disappeared during voluntary planterflexion and 50 Hz electrical stimulation induced planterflexion as compared to at rest. During voluntary or electrical stimulation induced agonist muscle fatigue, the inhibition of the soleus H-reflex from the common peroneal nerve was greater during voluntary dorsiflexion (maximal, 11.1%) and 50 Hz (maximal, 6.7%) electrical stimulation induced dorsiflexion than at rest. The inhibition was decreased or disappeared during voluntary planterflexion 50 Hz electrical stimulation induced planterflexion. It was concluded that the results were considered to support the hypothesis that alpha-motoneurones and la inhibitory intemeurones link to antagonist motoneurones in reciprocal inhibition. The diminished reciprocal Ia inhibition of voluntary contraction during muscle fatigue as compared to electrical stimulation, is discussed in relation to its possible contribution to ankle stability.     

The role of motor unit rate modulation versus recruitment in repeated submaximal voluntary contractions performed by control and spinal cord injured subjects.

The relative roles of motor unit firing rate modulation and recruitment were evaluated when individuals with cervical spinal cord injury (SCI) and able-bodied controls performed a brief (6 s), 50% maximal voluntary contraction (50% MVC; target contraction) of triceps brachii every 10 s until it required maximal effort to achieve the target force. Mean (+/-SD) endurance times for SCI and control subjects were 34+/-26 and 15+/-5 min, respectively, at which point significant reductions in maximal triceps force had occurred. Twitch occlusion analysis in controls indicated that force declines resulted largely from peripheral contractile failure. In SCI subjects, triceps surface EMG and motor unit potential amplitude declined in parallel suggesting failure at axon branch points and/or alterations in muscle membrane properties. The force of low threshold units, measured by spike-triggered averaging, declined in SCI but not control subjects, suggesting that higher threshold units fatigued in controls. Central fatigue was also obvious after SCI. Mean (+/-SD) MVC motor unit firing rates declined significantly with fatigue for control (24.6+/-7.1 to 17.3+/-5.1Hz), but not SCI subjects (25.9+/-12.7 to 20.1+/-9.7Hz). Unit firing rates were unchanged during target contractions for each subject group, but with the MVC rate decreases, units of SCI and control subjects were activated intensely at endurance time (88% and 99% MVC rates, respectively). New unit recruitment also maintained the target contractions although it was limited after SCI because many descending inputs to triceps motoneurons were disrupted. This resulted in sparse EMG, even during MVCs, but allowed the same unit to be recorded throughout. These EMG data showed that both unit recruitment and rate modulation were important for maintaining force during repeated submaximal intermittent contractions of triceps brachii muscles performed by SCI subjects. Similar results were found for control subjects. Muscles weakened by SCI may therefore provide a useful model in which to directly study motor unit rate modulation and recruitment during weak or strong voluntary contractions.     

Central and peripheral contributions to muscle fatigue in humans during sustained maximal effort

The purpose of this study was to estimate the relative contributions of central and peripheral factors to the development of human muscle fatigue. Nine healthy subjects [five male, four female; age = 30 (2) years, mean (SE)] sustained a maximum voluntary isometric contraction (MVC) of the ankle dorsiflexor muscles for 4 min. Fatigue was quantitated as the fall in MVC. Three measures of central activation and one measure of peripheral activation (compound muscle action potential, CMAP) were made using electromyography (EMG) and electrical stimulation. Measures of intramuscular metabolism were made using magnetic resonance spectroscopy. After exercise, MVC and electrically stimulated tetanic contraction (50 Hz, 500 ms) forces were 22.2 (3.7)% and 37.3 (7.1)% of pre-exercise values, respectively. The measures of central activation suggested some central fatigue during exercise: (1) the central activation ratio [MVC/(MVC + superimposed tetanic force)] fell from 0.94 (0.03) to 0.78 (0.09), (2) the MVC/tetanic force ratio fell from 2.3 (0.7) to 1.3 (0.7), and (3) the integral of the EMG (iEMG) signal decreased to 72.6 (9.1)% of the initial value, while the CMAP amplitude was unchanged. Intramuscular pH was associated by regression with the decline in MVC force (and therefore fatigue) and iEMG. The results indicate that central factors, which were not associated with altered peripheral excitability, contributed approximately 20% to the muscle fatigue developed, with the remainder being attributable to intramuscular (i.e., metabolic) factors. The association between pH and iEMG is consistent with proton concentration as a feedback mechanism for central motor drive during maximal effort.     

Postactivation potentiation influences differently the nonlinear summation of contractions in young and elderly adults.

The force enhancement of a twitch after a maximal conditioning muscle contraction [i.e., postactivation potentiation (PAP)] is reduced with aging, but its influence on the summation of force in response to repetitive stimulation at different frequencies is not known. The purpose of this work was to compare the electrically evoked mechanical responses of the tibialis anterior muscle between young and elderly adults after a 6-s maximal voluntary contraction (MVC). The results showed that, immediately after the conditioning MVC, twitch torque and its maximal rate of development and relaxation were significantly enhanced in both groups, but the magnitude of potentiation was greater in young (148.0 +/- 14.2, 123.7 +/- 16.5, and 185.4 +/- 36.5%, respectively) compared with elderly adults (87.4 +/- 15.2, 63.8 +/- 9.9, and 62.9 +/- 11.0%, respectively). This age-related difference in potentiation of the twitch disappeared completely 1 min after the conditioning MVC. The potentiation of torque and speed-related parameters in response to two- and three-pulse trains, delivered at a constant interval of 10 ms (100 Hz), was less than for a single pulse for both groups. In young adults, the magnitude of PAP on the successive individual mechanical contributions within a train of stimuli declined progressively such that the third contribution did not differ significantly from the same contribution before the conditioning MVC. In contrast, the second and third contributions did not potentiate (P > 0.05) in elderly adults. Although these contributions did potentiate significantly at a lower frequency of stimulation (20 Hz) in the two groups, the difference in PAP between young and elderly adults still persisted. This overall attenuation of potentiation with aging, however, appears to have a moderate influence on the decrement of the muscular performance.     

Stimulation frequency and force potentiation in the human adductor pollicis muscle

The effect of stimulation frequency on twitch force potentiation was examined in the adductor pollicis muscle of ten normal subjects. The ulnar nerve was supramaximally stimulated at the wrist and isometric twitch force was measured from a 3-Hz train lasting 1 s. Test stimulation frequencies of 5, 10, 20, 25, 30, 40, 50 and 100 Hz were applied for 5 s each in random order (5 min apart) and the twitches (3 Hz) were applied immediately before and after (1 s) the test frequency and at intervals up to 5 min afterwards (10 s, and 1, 2 and 5 min). Poststimulation twitches were expressed as a percentage of the prestimulation twitch. Low frequency fatigue was not induced by the protocol since the 20:50 Hz ratio did not alter within each session. The degree of twitch potentiation was frequency dependent, with potentiation increasing up to 50 Hz [mean 173 (SD 16)%] but the effect was markedly less at 100 Hz [mean 133 (SD 25)%, P less than 0.01] for all subjects. The reduced potentiation at 100 Hz may have occurred due to high frequency fatigue produced by the 100-Hz test stimulation train. The optimal frequency of those examined in the experimental group was 50 Hz but this only produced maximal potentiation in six of the ten subjects and 100 Hz always produced less potentiation. These findings have implications for electrical stimulation of muscle in the clinical setting.     

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.     

A comparison of adductor pollicis fatigue in older men and women

Sex differences in fatigue resistance of the adductor pollicis (AP) muscle were studied in 24 older adults who were divided into three groups: 12 older men (69.8 +/- 4.60 years), 6 older women not on hormone replacement therapy (HRT) (70.2 +/- 4.02 years), and 6 older women on HRT (68.7 +/- 6.47 years). Fatigue in the AP muscle was induced using an intermittent (5 s contraction, 5 s rest) submaximal voluntary contraction (50% of maximal voluntary contraction (MVC)) protocol, which was continued until exhaustion (i.e., when subjects could either no longer maintain a 5-s contraction at 50% MVC or when the MVC was deemed to be lower than the target force). There was no effect of HRT on MVC or time to fatigue (TTF); therefore, the older women were pooled as one subject group. At baseline, men were stronger than women for MVC (75.9 +/- 18.8 N in men vs. 56.8 +/- 10.0 N in women; P < 0.05) and evoked twitch force (7.3 +/- 1.7 N in men vs. 5.2 +/- 0.8 N in women; P < 0.05). There was no difference in TTF between men and women (14.77 +/- 7.06 min in men vs. 11.53 +/- 4.91 min in women; P > 0.20), nor was there a significant relationship between baseline muscle force and TTF (r = 0.14). There was also no difference in the pattern of fatigue and recovery between the men and women. These results suggest that there is no difference in endurance or fatigue characteristics of the AP muscle in men and women over the age of 65 years, and that baseline muscle force does not predict fatigue resistance in this muscle.     

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.     

Effects of caffeine on neuromuscular function.

This double-blind, repeated-measures study examined the effects of caffeine on neuromuscular function. Eleven male volunteers [22.3 +/- 2.4 (SD) yr] came to the laboratory for control, placebo, and caffeine (6 mg/kg dose) trials. Each trial consisted of 10 x 1-ms stimulation of the tibial nerve to elicit maximal H reflexes of the soleus, four attempts at a maximal voluntary contraction (MVC) of the right knee extensors, six brief submaximal contractions, and a 50% MVC held to fatigue. Isometric force and surface electromyographic signals were recorded continuously. The degree of maximal voluntary activation was assessed with the twitch-interpolation technique. Single-unit recordings were made with tungsten microelectrodes during the submaximal contractions. Voluntary activation at MVC increased by 3.50 +/- 1.01 (SE) % (P < 0. 01), but there was no change in H-reflex amplitude, suggesting that caffeine increases maximal voluntary activation at a supraspinal level. Neither the force-EMG relationship nor motor unit firing rates were altered by caffeine. Subjects were able to hold a 50% MVC for an average of 66.1 s in the absence of caffeine. Time to fatigue (T(lim)) increased by 25.80 +/- 16.06% after caffeine administration (P < 0.05). There was no significant change in T(lim) from pretest to posttest in the control or placebo trials. The increase in T(lim) was associated with an attenuated decline in twitch amplitude, which would suggest that the mechanism is, at least in part, peripheral.     

Effects of repeated Achilles tendon vibration on triceps surae force production

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

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     

Neuromuscular Fatigue Is Not Different between Constant and Variable Frequency Stimulation

This study compared fatigue development of the triceps surae induced by two electrical stimulation protocols composed of constant and variable frequency trains (CFTs, VFTs, 450 trains, 30 Hz, 167 ms ON, 500 ms OFF and 146 ms ON, 500 ms OFF respectively). For the VFTs protocol a doublet (100 Hz) was used at the beginning of each train. The intensity used evoked 30% of a maximal voluntary contraction (MVC) and was defined using CFTs. Neuromuscular tests were performed before and after each protocol. Changes in excitation-contraction coupling were assessed by analysing the M-wave [at rest (M_max) and during MVC (M_sup)] and associated peak twitch (Pt). H-reflex [at rest (H_max) and during MVC (H_sup)] and the motor evoked potential (MEP) during MVC were studied to assess spinal and corticospinal excitability of the soleus muscle. MVC decrease was similar between the protocols (−8%, P<0.05). M_max, M_sup and Pt decreased after both protocols (P<0.01). H_max/M_max was decreased (P<0.05), whereas H_sup/M_sup and MEP/M_sup remained unchanged after both protocols. The results indicate that CFTs and VFTs gave rise to equivalent neuromuscular fatigue. This fatigue resulted from alterations taking place at the muscular level. The finding that cortical and spinal excitability remained unchanged during MVC indicates that spinal and/or supraspinal mechanisms were activated to compensate for the loss of spinal excitability at rest.     

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

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 (Pt) of the maximal twitch increased significantly (p less than 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 greater than 0.1) during immobilization, but the 50 Hz tetanic tension (Po50) and maximal voluntary contraction (MVC) were reduced (p less than 0.05). The fall in Po50 and MVC was associated with 10% decrease in the estimated muscle (plus bone) cross-sectional area. The relative (%) change in Po50 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 less than 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.(ABSTRACT TRUNCATED AT 250 WORDS)     

Coexistence of potentiation and low-frequency fatigue during voluntary exercise in human skeletal muscle

The role of muscle potentiation in overcoming low-frequency fatigue (LFF) as it developed during submaximal voluntary exercise was investigated in eight males (age 26.4 +/- 0.7 years, mean +/- SE) performing isometric leg extension at approximately 30% of maximal voluntary contraction for 60 min using a 0.5-duty cycle (1 s contraction, 1 s rest). At 5, 20, 40, and 60 min, exercise was interrupted for 3 min, and the maximum positive rate of force development (+dF/dtmax) and maximal twitch force (Pt) were measured in maximal twitch contractions at 0, 1, 2, and 3 min of rest (R0, R1, R2, R3); they were also measured at 15 min of recovery following the entire 60-min exercise period. These measures were compared with pre-exercise (PRE) as an indicator of potentiation. Force at low frequency (10 Hz) was also measured at R0, R1, R2, and R3, and at 15 min of recovery, while force at high frequency (100 Hz) was measured only at R0 and R3 and in recovery. Voluntary exercise increased twitch +dF/dtmax at R0 following 5, 20, 40, and 60 min of exercise, from 2553 +/- 150 N/s at PRE to 39%, 41%, 42%, and 36% above PRE, respectively (P<0.005). Twitch +dF/dtmax decayed at brief rest (R3) following 20, 40, and 60 min of exercise (P<0.05). Pt at R0 following 5 and 20 min of exercise was above that at PRE (P<0.05), indicating that during the early phase of moderate-intensity repetitive exercise, potentiation occurs in the relative absence of LFF. At 40 and 60 min of exercise, Pt at R0 was unchanged from PRE. The LFF (10 Hz) induced by the protocol was evident at 40 and 60 min (R0-R3; P<0.05) and at 15 min following exercise (P<0.05). High-frequency force was not significantly compromised by the protocol. Since twitch force was maintained, these results suggest that as exercise progresses, LFF develops, which can be compensated for by potentiation.