Neuromuscular fatigue during repeated exhaustive submaximal static contractions of knee extensor muscles in endurance-trained,power-trained and untrained men

The neural and muscular changes during fatigue produced in repeated submaximal static contractions of knee extensors were measured. Three groups of differently adapted male subjects (power-trained, endurance-trained and untrained, 15 in each) performed the exercise that consisted of 10 trials of submaximal static contractions at the level of 40% of maximal voluntary contraction (MVC) force till exhaustion with the inter-trial rest intervals of 1 min. MVC force, reaction time and patellar reflex time components before and after the fatiguing exercise and following 5, 10 and 15 min of recovery were recorded. Endurance-trained athletes had a significantly longer holding times for all the 10 trials compared with power-trained athletes and untrained subjects. However, no significant differences in static endurance between power-trained athletes and untrained subjects were noted. The fatigue test significantly prolonged the time between onset of electrical and mechanical activity (electromechanical delay) in voluntary and reflex contractions. The electromechanical delay in voluntary contraction condition for power-trained and untrained subjects and in reflex condition for endurance-trained subjects had not recovered 15 min after cessation of exercise. No significant changes in the central component of visual reaction time (premotor time of MVC) and latency of patellar reflex were noted after fatiguing static exercise. It is concluded, that in this type of exercise the fatigue development may be largely owing to muscle contractile failure.     

Changes in soleus motoneuron pool reflex excitability and surface EMG parameters during fatiguing low- vs. high-intensity isometric contractions

The fatigue-related changes in soleus motoneuron pool reflex excitability and surface electromyography (EMG) parameters, and maximal voluntary contraction (MVC) force of the plantarflexor (PF) muscles during repeatedly sustained low- (30% MVC) vs. high-intensity (70% MVC) isometric contractions were evaluated Twelve young men with mean (+/- SE) age of 22.4 +/- 0.3 years participated in two fatigue tasks on separate days with at least 1-week interval. The fatigue task consisted of three sustained isometric contractions of PF muscles at a target force level until exhaustion separated with 2-min pause between contractions. M-wave (muscle compound action potential) amplitude (M(max)), Hoffmann reflex maximal amplitude (H(max)) to M-wave amplitude ratio (H(max)/M(max)), and root mean square amplitude (RMS) and median frequency (MF) of EMG power spectrum were recorded from the soleus muscle. The M(max) remained constant immediately post-fatigue and during recovery for low- and high-intensity fatigue tasks, whereas H(max)/M(max) was significantly (p < 0.05) reduced only after high-intensity fatigue task. The increase in RMS and decrease in MF during isometric contractions, and reduction in MVC force immediately after the exercise was greater (p < 0.05) for low-intensity fatigue task. We conclude that low-intensity isometric contractions, repeatedly sustained to fatigue, resulted in a marked increase in the EMG amplitude and spectral compression without a significant post-fatigue reflex inhibition of soleus motoneuron pool. High-intensity contractions, however, resulted in post-fatigue reflex inhibition of soleus motoneuron pool and less pronounced EMG spectral compression during fatiguing contractions. A failure of neuromuscular transmission-propagation was not evident after repetitive fatiguing isometric contractions.     

Reflex regulation during sustained and intermittent submaximal contractions in humans

To investigate whether the intensity and duration of a sustained contraction influences reflex regulation, we compared sustained fatiguing contractions at 25 % and 50 % of maximal voluntary contraction (MVC) force in the human abductor pollicis brevis (APB) muscle. Because the activation of motoneurones during fatigue may be reflexively controlled by the metabolic status of the muscle, we also compared reflex activities during sustained and intermittent (6 s contraction, 4 s rest) contractions at 25 % MVC for an identical duration. The short-latency Hoffmann(H) reflex and the long-latency reflex (LLR) were recorded during voluntary contractions, before, during and after the fatigue tests, with each response normalised to the compound muscle action potential (M-wave). The results showed that fatigue during sustained contractions was inversely related to the intensity, and hence the duration, of the effort. The MVC force and associated surface electromyogram (EMG) declined by 26.2 % and 35.2 %, respectively, after the sustained contraction at 50 % MVC, and by 34.2 % and 44.2 % after the sustained contraction at 25 % MVC. Although the average EMG increased progressively with time during the two sustained fatiguing contractions, the amplitudes of the H and LLR reflexes decreased significantly. Combined with previous data ( Duchateau & Hainaut, 1993 ), the results show that the effect on the H reflex is independent of the intensity of the sustained contraction, whereas the decline in the LLR is closely related to the duration of the contraction. Because there were no changes in the intermittent test at 25 % MVC, the results indicate that the net excitatory spinal and supraspinal reflex-mediated input to the motoneurone pool is reduced. This decline in excitation to the motoneurones, however, can be temporarily compensated by an enhancement of the central drive.     

Fatiguing handgrip exercise alters maximal force-generating capacity of plantar-flexors

Exercise-induced fatigue causes changes within the central nervous system that decrease force production capacity in fatigued muscles. The impact on unrelated, non-exercised muscle performance is still unclear. The primary aim of this study was to examine the impact of a bilateral forearm muscle contraction on the motor function of the distal and unrelated ankle plantar-flexor muscles. The secondary aim was to compare the impact of maximal and submaximal forearm contractions on the non-fatigued ankle plantar-flexor muscles. Maximal voluntary contractions (MVC) of the forearm and ankle plantar-flexor muscles as well as voluntary activation (VA) and twitch torque of the ankle plantar-flexor muscles were assessed pre-fatigue and throughout a 10-min recovery period. Maximal (100 % MVC) and submaximal (30 % MVC) sustained isometric handgrip contractions caused a decreased handgrip MVC (to 49.3 ± 15.4 and 45.4 ± 11.4 % of the initial MVC for maximal and submaximal contraction, respectively) that remained throughout the 10-min recovery period. The fatigue protocols also caused a decreased ankle plantar-flexor MVC (to 77 ± 8.3 and 92.4 ± 6.2 % of pre-fatigue MVC for maximal and submaximal contraction, respectively) and VA (to 84.3 ± 15.7 and 97.7 ± 16.1 % of pre-fatigue VA for maximal and submaximal contraction, respectively). These results suggest central fatigue created by the fatiguing handgrip contraction translated to the performance of the non-exercised ankle muscles. Our results also show that the maximal fatigue protocol affected ankle plantar-flexor MVC and VA more severely than the submaximal protocol, highlighting the task-specificity of neuromuscular fatigue.     

Central fatigue during a long-lasting submaximal contraction of the triceps surae

Our purpose was to study central fatigue and its dependence on peripheral reflex inhibition during a sustained submaximal contraction of the triceps surae. In 11 healthy subjects, superimposed twitches, surface electromyograms (EMG) from the medial head of the gastrocnemius (MG) and soleus (SOL) muscles, maximal compound motor action potentials (M_max), tracking error and tremor were recorded during sustained fatiguing contractions at a torque level corresponding to 30% of maximal voluntary contraction (MVC). When the endurance limit (401±91 s) of the voluntary contraction (VC-I) was reached, the triceps surae could be electrically stimulated to the same torque level for an additional 1 min in 10 of the 11 subjects. These subjects were then able to continue the contraction voluntarily (voluntary contraction II, VC-II) for another 85±48 s. At the endurance limit of VC-I, the superimposed twitch was larger than during the unfatigued MVC, while there was no significant difference between the twitch at the endurance limit of VC-II and MVC. The EMG amplitude of both MG and SOL at the endurance limit of VC-I was significantly less than that during the MVC. While the EMG amplitude of MG increased further during VC-II, SOL EMG remained unchanged, neither muscle reaching their unfatigued MVC values. This difference was diminished for SOL by taking into account its decrease in M_max found during VC-II, and relative EMG levels approached their MVC values. These results clearly indicate that a higher voluntary muscle activation was achievable after 1 min of electrical muscle stimulation, which continued metabolic stress and contractile fatigue processes but allowed for supraspinal, muscle spindle and/or motoneuronal recovery. It is concluded that peripheral reflex inhibition of -motoneurons via small-diameter muscle afferents is of minor significance for the development of the central fatigue that was found to occur during the first voluntary contraction.     

Electrical stimulation-induced changes in performance and fiber type proportion of human knee extensor muscles

The purpose of the present study was to look at the changes in the performance of human knee extensor muscles (KEM) induced by 6 weeks of low-frequency (8 Hz) electrical stimulation (LFES). KEM performance of 20 sedentary (before and after stimulation), ten active, and five endurance-trained subjects was evaluated during 25 consecutive 10-s isometric contractions, each separated by a rest period of 5 s. The mean force maintained during six consecutive 10-s contractions was expressed as a relative percentage of that of the first contraction. The mean performance of the first series of six contractions was not altered in response to stimulation, whereas that of the other four series was significantly increased. No significant difference was noted among the three groups in terms of KEM performance during the first series of six contractions. However, after the first series of six contractions, KEM performance of endurance-trained subjects was better in comparison to the other groups. Citrate synthase (CS) activity, capillary number per type IIA and IIB fibers, and the percentage of type IIA muscle fibers determined from vastus lateralis samples were significantly increased in response to the stimulation protocol. No significant change was observed in the proportion or capillary number of type I fibers, or in the areas of type I, IIA, and IIB fibers. The present study provides evidence that resistance to fatigue of human skeletal muscle can be significantly altered in response to 6 weeks of transcutaneous low-frequency electrical stimulation. The improvement in KEM resistance to fatigue of the sedentary subjects was such that, at the end of the stimulation protocol, resistance to fatigue was similar to that of active subjects. However, the ability of endurance-trained subjects to withstand fatigue was still superior compared to that of the other untrained or active subjects.     

Effects of fatiguing isometric exercise upon achilles tendon reflex and plantar flexion reaction time components in man

Effects of three different fatiguing local muscular exercises upon plantar flexion reaction time and achilles tendon reflex time have been studied in 24 normal males. The Exercise Conditions, each involving a series of 30 maximal voluntary isometric contractions (MVC) of the plantar flexors, differed by allowing either 5, 10 or 20 sec rest interval between each MVC. Decrements in strength ranged from 15% to 34% MVC. Trend analysis of the fatigue patterns revealed that a cubic orthogonal polynomial equation was sufficient to describe the profile of MVC decrement for all conditions (Table 3). Following the fatiguing exercise, simple visual reaction time (plantar flexion), and its two components, premotor and motor time, failed to demonstrate any change from Pre Exercise Conditions. Achilles Tendon Reflex Times, however, demonstrated a marked augmentation, as manifest in reduced total reflex times, contraction times and half relaxation times. These results may suggest the differential fatigue of motor units employed in the three motor tasks,viz. MVC, voluntary reaction and achilles tendon reflex. A plausible explanation for the augmentation of the reflex contraction resides in the known potentiating effect of elevated intramuscular temperature. Alternatively, one might postulate a neurally mediated increase in gain of the stretch servomechanism. The possibility of both mechanisms being operative is not excluded.     

Stimulated and voluntary fatiguing contractions of quadriceps femoris differently disturb postural control

Muscle fatigue affects muscle strength and postural control. However, it is not known whether impaired postural control after fatiguing muscular exercise depends on the nature of the muscle contraction. To answer this question, the present study analyzes changes in postural control after two fatiguing exercises of equal duration and intensity but that induced different magnitudes of strength loss. The effects of fatiguing contractions of the femoris quadriceps were compared for voluntary muscular contraction (VOL) and neuromuscular electrical stimulation (ES) on muscle strength and postural control. Seventeen subjects completed these two fatiguing exercises. Maximal voluntary contraction (MVC) and postural control were recorded using an isokinetic dynamometer and a force platform that recorded the center of foot pressure. Recordings were performed before and after the completion of both fatiguing tasks. Results indicate that, after a fatiguing exercise, the ES exercise affected MVC more than the VOL exercise. Inversely, postural control was disturbed more after VOL exercise than after ES exercise. In conclusion, postural control disturbance is influenced by the nature of the muscular contraction (voluntary vs. non-voluntary) and the type of the motor unit solicited (tonic vs. phasic) rather than by the magnitude of strength loss.     

Mechanomyogram from the different heads of the quadriceps muscle during incremental knee extension

To investigate the time- and frequency-domain responses of mechanomyograms (MMGs) during the progressive fatigue induced by intermittent incremental contractions, a surface MMG was obtained from the three muscle heads of the quadriceps muscle in seven subjects while they performed isometric knee extensions lasting 7.6 min. Isometric intermittent incremental contractions started at 1% of the maximal voluntary contraction (MVC) for 3 s, with a 3-s relaxation period in between each contraction, and the contraction level was increased by 1% of MVC for every contraction (by 10% of MVC per min) up to exhaustion. Separate contractions with sufficient rest periods were also conducted to serve for the MMG characteristics without fatigue. The integrated MMG (iMMG) was linearly related to force in all of the muscles when fatigue was not involved. With regard to the incremental contractions, the relationship exhibited an ascending-descending shape, but the behavior was not the same for the individual muscle heads, especially for the rectus femoris muscle. A steep increase in the median frequency of MMG from around 60% of MVC corresponded to a decrease in iMMG. These results suggest that analysis of MMG in the time- and frequency-domain during an incremental protocol is a useful way of characterizing the motor unit recruitment strategy and fatigue properties of individual muscles.     

Localized muscle pain causes prolonged recovery after fatiguing isometric contractions

The purpose of this study was to investigate the force and electromyographic (EMG) signal recorded from the muscles immediately after a sustained fatiguing contraction with or without muscle pain. Ten subjects performed sustained dorsi- and plantarflexions at two contraction levels (50 and 80% of maximum voluntary contraction) until exhaustion with or without muscle pain induced by injection of 6% hypertonic saline in one of the torque producing muscles. The muscle pain intensity was scored on a visual analogue scale (VAS, 0–10 cm). The root mean square (RMS) of the surface EMG signal from plantarflexors and dorsiflexors were estimated during maximum voluntary contractions (MVC) and ramp contractions before and after the fatiguing task at 0, 5, 10 and 15 min during the recovery phase. VAS scores immediately after the contractions with hypertonic saline (on average 3.2 ± 1.1 cm) progressively decreased during recovery and no pain was experienced 15 min after the contraction. After the painful contraction the RMS-EMG during MVC was on average decreased (23.4 ± 7.4%) compared to the non-painful condition both in muscles where pain was previously induced and in non-painful synergists. During recovery, the slope of the torque–EMG curve during ramp contraction was significantly decreased (28.4 ± 8.1%) after the painful contraction compared to the control contraction both for the muscle previously exposed to pain and also the other active synergists. The decreased EMG during recovery after painful contractions compared with control was not accompanied by significant reductions in force during MVC indicating a change in the strategy for motor unit recruitment. This study shows that localized muscle pain inhibits muscle activation and increases the effects of fatigue on EMG recovery curves both for painful and non-painful synergists probably by a central effect. These effects can modify the normal patterns of synergistic activation and can also generate overload problems in muscle pain patients if compensatory motor control strategies are applied.     

Modulation of motor unit discharge rate and H-reflex amplitude during submaximal fatigue of the human soleus muscle

Declining motor unit discharge rates and H-reflex amplitude have been observed in separate experiments during fatiguing submaximal contractions in humans. The purpose of this experiment was to investigate motor unit discharge rate, H-reflex amplitude, and twitch contractile properties concurrently during a fatiguing submaximal isometric contraction of the ankle plantarflexors. Eleven healthy subjects performed fatiguing contractions of low force (25% maximal voluntary contraction (MVC)) or high force (42–66% MVC). Hoffmann (H)-reflexes, muscle compound action potentials (M-waves), twitch contractile properties, and motor unit discharges were recorded from the soleus muscle. In the low-force fatigue task, motor unit firing rate increased gradually over time, whereas the resting H-reflex was significantly depressed at 15% of endurance time and remained quasiconstant for the rest of the task. This suggests that the processes mediating the resting H-reflex depression are relatively independent of those modulating the motor unit firing rate during a low-force fatigue task. In the high-force fatigue task, a decline in the average motor unit discharge rate was accompanied by a decrease in the resting H-reflex amplitude and a prolongation of the twitch half-relaxation time (HRT) at the completion of the fatigue task. Overall, motor unit firing rate was modulated in parallel with changes in the twitch HRT, consistent with the muscle wisdom hypothesis.     

The origins of neuromuscular fatigue post-stroke

Fatigue post-stroke is a disabling and persistent symptom affecting many stroke survivors. Despite its high prevalence, the pathophysiology underlying this phenomenon remains obscure. The aim of the present study was to investigate the origins of neuromuscular fatigue post-stroke. Ten chronic stroke survivors and 10 controls sustained an isometric contraction at 30% of maximal voluntary contraction (MVC) with the ankle dorsiflexors. Motor evoked potential (MEP), cortical silent period (SP), voluntary activation, M wave and contractile properties were evaluated before, during and after fatigue among the paretic, non-paretic and control limbs. The pattern of response to fatigue in the non-paretic and control limbs was comparable; therefore, results are presented between the paretic and non-paretic limbs. Before fatigue, reduced MVC peak torque and MEP amplitude were observed on the paretic side in comparison with the non-paretic side. During fatigue, the cortical SP duration increased significantly in both limbs, whereas the MEP amplitude significantly increased only in the non-paretic limb. After fatigue, MVC peak torque decreased significantly in both limbs. Significant reductions in M wave and twitch peak torque were observed in both limbs, pointing to the development of peripheral fatigue. However, central fatigue, evident by a significant reduction in voluntary activation, was greater in the paretic than in the non-paretic limb. After stroke, an inability to increase central excitability in response to an increased cortical inhibition associated with the fatiguing contraction may contribute to central fatigue observed in the paretic limb, which may also be linked to increased self-reported fatigue during activities of daily living. These findings advance our understanding of the neuromuscular basis of fatigue post-stroke.     

H-reflex and reciprocal Ia inhibition after fatiguing isometric voluntary contraction in soleus muscle

This study investigated the H-reflex and reciprocal Ia inhibition during fatigue in the human soleus muscle. Ten healthy subjects participated in this study, and performed intermittent isometric voluntary contraction of the ankle plantarflexion at 50% MVC as the fatiguing task. Reciprocal Ia inhibition was evaluated by the degree of H-reflex amplitude depression in the soleus muscle by the test stimulus following conditioning stimulus to the common peroneal nerve. The difference in H-reflex amplitude between before and after fatiguing task was also checked. There was no significant difference in the degree of H-reflex amplitude depression, although the H-reflex amplitude significantly decreased after the fatiguing task (p < 0.01). From the results of this study, it was considered that the decrease in H-reflex amplitude was caused by descending inhibitory input from the supraspinal to alpha-motoneuron, and the excitability of the Ia inhibitory interneuron was not involved. It was suggested that the function of reciprocal Ia inhibition was difficult to modulate during fatigue caused by isometric voluntary contraction in this study.     

Endurance time characteristics of human ankle dorsiflexors and plantarflexors

The endurance time provides a convenient means to assess muscle fatigue resistance. The purpose of the experiments was to study endurance time characteristics of human ankle dorsiflexion and plantarflexion as a function of level of contraction. Nine subjects (four men, five women) were examined. The experimental protocol consisted of determining each individual maximum voluntary contraction (MVC) before each fatiguing experiment and undertaking only one fatiguing isometric contraction (dorsiflexion or plantarflexion) per day. Each subject produced at least six plantarflexion and six dorsiflexion contractions chosen from 15% to 90% MVC. An exponential model was fit to data for each individual and was then fit to the pooled data. The variance accounted for was over 99% for both dorsiflexion and plantarflexion mean values. As expected, endurance time declined as the contraction level increased. Plantarflexing responses presented a higher variation from subject to subject than for dorsiflexing. Inter-subject variability primarily seemed to involve a change in the shape of the endurance curve rather than a shift of the curve. When the data were pooled, the exponential curve had parameters similar to the average of the individual fits. Human ankle dorsiflexion and plantarflexion endurance times as a function of level of contraction were found to be similar under the present experimental conditions.     

Neuromuscular fatigue induced by whole-body vibration exercise

The aim of this study was to examine the magnitude and the origin of neuromuscular fatigue induced by half-squat static whole-body vibration (WBV) exercise, and to compare it to a non-WBV condition. Nine healthy volunteers completed two fatiguing protocols (WBV and non-WBV, randomly presented) consisting of five 1-min bouts of static half-squat exercise with a load corresponding to 50 % of their individual body mass. Neuromuscular fatigue of knee and ankle muscles was investigated before and immediately after each fatiguing protocol. The main outcomes were maximal voluntary contraction (MVC) torque, voluntary activation, and doublet peak torque. Knee extensor MVC torque decreased significantly (P < 0.01) and to the same extent after WBV (?23 %) and non-WBV (?25 %), while knee flexor, plantar flexor, and dorsiflexor MVC torque was not affected by the treatments. Voluntary activation of knee extensor and plantar flexor muscles was unaffected by the two fatiguing protocols. Doublet peak torque decreased significantly and to a similar extent following WBV and non-WBV exercise, for both knee extensors (?25 %; P < 0.01) and plantar flexors (?7 %; P < 0.05). WBV exercise with additional load did not accentuate fatigue and did not change its causative factors compared to non-WBV half-squat resistive exercise in recreationally active subjects.     

Perceived exertion is elevated in old age during an isometric fatigue task

The purposes of this study were to compare the ratings of perceived exertion (RPE) of young [25 (1) years] and old men [84 (1) years] during intermittent (3 s on, 2 s off) voluntary isometric contractions of the elbow flexors at 60% of each subject's maximal voluntary contraction (MVC) force, and to determine whether potential differences in RPE were accompanied by altered neuromuscular parameters during fatigue. Subjects performed the fatigue protocol on two different visits. All subjects in both groups reported a maximal (10) perceived exertion using a modified Borg-scale at fatigue, but the old men reported a greater perceived exertion compared to the young men at an initial stage of the protocol during the two visits (P<0.05). The amount of prefatigue MVC force remaining at fatigue was not different between groups (~57%), and voluntary activation, as assessed with twitch interpolation, was not different from the prefatigue value for either group throughout the fatigue protocol (>95%), and did not show any age-related difference. Moreover, there was no significant difference in 60% target electromyography between groups at any time point during the fatigue protocol, and there was no significant effect of age on the decline in 50-Hz tetanic force. Thus, the old men were able to exert themselves to the same relative degree of muscle fatigue as the young men, and despite an elevated perceived exertion compared to the young men during initial stages of the protocol, they did not terminate the protocol prematurely, or demonstrate an inability to sustain a high level of voluntary activation. Electronic Publication     

The influence of torque and velocity on erector spinae muscle fatigue and its relationship to changes of electromyogram spectrum density

The influence of contraction force and velocity during isokinetic contractions on the development of fatigue in the erector spinae muscle was studied. Seven male subjects performed a series of 250 contractions at 25% and 50% of their isometric maximal voluntary contraction (MVC) at 40 and 80°·s^–1. Fatigue defined as a decrease of the contractile capacity of the muscles was studied by means of a 15-s maximal test-contraction following the exercise. Both the initial force and the force decrement during the test-contraction were studied. Surface electromyogram (EMG) signals of the main tracts of the erector spinae muscle were recorded. The frequency content was studied by calculating the zero-crossing rate for the signals obtained during dynamic contractions and by means of fast Fourier transformation for the test contraction. After the 50% MVC dynamic contractions the initial force during the postexercise test-contraction was significantly lower than after the 25% MVC contractions. No significant influence of contraction velocity on fatigue development was found. The force decrement during the test-contraction did not depend on the experimental conditions. The EMG amplitude indicated that the subjects were better able to relax their muscles during the counter movement (flexion) at high forces and high velocities compared to the other experimental conditions. The frequency content of the EMG signals during the dynamic contractions and the postexercise test-contraction showed only very weak relationships with fatigue. Therefore, spectrum EMG parameters as determined in the present study do not seem suitable as indicators of muscle fatigue as a consequence of dynamic contractions of trunk extensor muscles.     

Comparison of brain activation after sustained non-fatiguing and fatiguing muscle contraction: a positron emission tomography study

The concept of fatigue refers to a class of acute effects that can impair motor performance, and not to a single mechanism. A great deal is known about the peripheral mechanisms underlying the process of fatigue, but our knowledge of the roles of the central structures in that process is still very limited. During fatigue, it has been shown that peripheral apparatus is capable of generating adequate force while central structures become insufficient/sub-optimal in driving them. This is known as central fatigue, and it can vary between muscles and different tasks. Fatigue induced by submaximal isometric contraction may have a greater central component than fatigue induced by prolonged maximal efforts. We studied the changes in regional cerebral blood flow (rCBF) of brain structures after sustained isometric muscle contractions of different submaximal force levels and of different durations, and compared them with the conditions observed when the sustained muscle contraction becomes fatiguing. Changes in cortical activity, as indicated by changes in rCBF, were measured using positron emission tomography (PET). Twelve subjects were studied under four conditions: (1) rest condition; (2) contraction of the m. biceps brachii at 30% of MVC, sustained for 60 s; (3) contraction at 30% of MVC, sustained for 120 s, and; (4) contraction at 50% of MVC, sustained for 120 s. The level of rCBF in the activated cortical areas gradually increased with the level and duration of muscle contraction. The fatiguing condition was associated with predominantly contralateral activation of the primary motor (MI) and the primary and secondary somatosensory areas (SI and SII), the somatosensory association area (SAA), and the temporal areas AA and AI. The supplementary motor area (SMA) and the cingula were activated bilaterally. The results show increased cortical activation, confirming that increased effort aimed at maintaining force in muscle fatigue is associated with increased activation of cortical neurons. At the same time, the activation spread to several cortical areas and probably reflects changes in both excitatory and inhibitory cortical circuits. It is suggested that further studies aimed at controlling afferent input from the muscle during fatigue may allow a more precise examination of the roles of each particular region involved in the processing of muscle fatigue.     

Stimulated and voluntary fatiguing contractions of quadriceps femoris similarly disturb postural control in the bipedal stance

The aim of this work was to compare the effects of fatigue of the quadriceps femoris after fatiguing voluntary contractions (VOL) and fatiguing neuromuscular electrical stimulation (ES) on bipedal postural control. Nineteen active male subjects (22.2 ± 1.7 years) completed these two fatiguing exercises. Isometric maximal voluntary contraction (MVC) and postural control were recorded using an ergometer and a force platform that registered the center of foot pressure (COP). We analyzed the COP surface, the mean COP velocity and the spectral power density given by the wavelet transform. Recordings were performed before (PRE condition) and after the completion of each fatiguing task (immediately POST condition, after a 5 min recovery POST 5 condition). In POST condition, the ES exercise affected MVC more than the VOL exercise. However, bipedal postural control was similarly deteriorated for both exercises. In POST 5 condition, for both fatiguing exercises, muscle strength and postural control did not recover their initial level. These results suggest that the postural control disturbance could not be distinguished for the two fatiguing exercises in the bipedal stance. In addition, the recovery speeds of postural control and muscle strength abilities did not differ for the ES exercise and the VOL exercise.     

Habitual level of physical activity and muscle fatigue of the elbow flexor muscles in older men

Reduced muscle performance, related to the loss of muscle mass and strength, is a common and natural part of ageing. Nevertheless, it is generally believed that regular participation in activities of moderate intensity may slow down these age-related changes. This study investigated the relationship between the habitual level of physical activity (PA), assessed by the modified Baecke Questionnaire, and the mechanical and fatigue characteristics of the right elbow flexor muscles, m. biceps brachii and m. brachioradialis, in men over the age of 55 years. Muscle fatigue was quantified both by measuring the maximal voluntary contraction (MVC) torque before and after a sustained isometric contraction at 25% MVC until exhaustion, and also by the temporal changes observed in the surface electromyographic (EMG) signal recorded during the fatigue task. Results showed a decreased MVC torque at the end of the fatiguing contraction. After 20 min of recovery, the MVC force was still significantly lower than the pre-fatigue value, except for the most active subjects. Typical myoelectrical indications of fatigue were also observed: a shift in the frequency spectrum of the signal towards lower frequencies accompanied by an increase in the EMG amplitude. We concluded from this study that the level of PA was related to the absolute isometric MVC values and the measurement of neuromuscular efficiency after 20 min of recovery, but did not influence the indications of muscle fatigue during an isometric fatigue task. Electronic Publication