Acoustic myographic activity increases linearly up to maximal voluntary isometric force in the human quadriceps muscle

Sounds produced during voluntary isometric contractions of the quadriceps muscle were recorded by acoustic myography (AMG) in seven healthy adults. With the subject seated, surface AMG and electromyography (EMG) were recorded over rectus femoris (RF) during isometric contractions, at different levels of maximum voluntary force. The AMG and EMG signals were amplified and integrated (IAMG and IEMG). The relationships between force and IAMG (r = 0.98 +/- 0.01, mean +/- 1 SD) and force and IEMG (r = 0.99 +/- 0) were linear in all subjects. The results for EMG confirm previous reports but those for AMG differ from the relationships reported for other muscles. Physiological and technical explanations are proposed for these differences and the necessity for further validation of the AMG technique is stressed.     

Frequency of acoustic myography during isometric contraction of fresh and fatigued muscle and during dynamic contractions

The frequency of the acoustic myographic (AMG) signal was examined during fresh and fatigued isometric contractions of quadriceps and during dynamic contractions of biceps brachii (BB) in healthy subjects. Recordings were obtained from quadriceps over a range of forces between 10% and 100% maximal voluntary contraction prior to, and 15 minutes after, a fatiguing exercise. Recordings from BB were obtained over a range of submaximal forces (0–8.5 kg) during concentric and eccentric contractions. The mean power frequency (MPF) of the AMG signal was analyzed during each of these contractions by fast-Fourier transform (FFT). The MPF was not significantly different (P > 0.05) during fresh and fatigued contractions of quadriceps and increased quadratically with force in both states (r = 0.81, fresh; r = 0.77, fatiqued). During concentric contractions of BB the MPF initially with force, but then decreased at the heavier loads (>5.5 kg). The MPF of eccentric contractions did not significantly (P > 0.05) alter with force. The AMG MPF was within a similar low frequency range for both muscles, during different types of contraction, and was unaltered with fatigue. © 1993 John Wiley & Sons, Inc.     

Quadriceps muscle function and fatigue in women with Addison's disease.

In nine patients with Addison's disease (mean +/- SE: 51 +/- 2 years) receiving conventional steroid treatment, and nine age-matched healthy controls (56 +/- 2 years), we investigated maximum voluntary quadriceps force (MVC) and contractile properties evoked with stimulation and central activation both at rest and during a submaximal intermittent fatigue task. The MVC was similar (-3%), but twitch tension (-27%) and central activation were significantly less (-7%), and tetanic half-relaxation time was approximately 40% slower in the patients. Twitch amplitudes were potentiated by 6% in the patients, but unchanged in the control group. The patients self-terminated a submaximal intermittent fatigue protocol (0.6 duty cycle) at approximately 5 +/- 1 min, whereas the controls stopped when they lost 50% of MVC force ( approximately 10 +/- 1 min). Force loss was similar between groups over the first 5 min of the fatigue task. In the patient group, maximal and submaximal relative integrated electromyogram (IEMG) increased significantly in the first minute of fatigue and remained elevated, whereas the controls exhibited a gradual increase in submaximal IEMG with little change in maximal IEMG. These results indicate that conventionally treated Addison's patients have similar MVC strength, but altered contractile properties and decreased endurance compared with controls.     

Age-related muscle fatigue after a low-force fatiguing contraction is explained by central fatigue

The contribution of central fatigue during and after low- and high-force isometric contractions sustained until failure with age is not established. We compared the time to failure and changes in voluntary activation measured using motor point stimulation of 15 young and 15 old adults for an isometric contraction sustained with the elbow flexor muscles at 20% and 80% of maximal voluntary contraction (MVC) force. Young adults had a briefer time to task failure than old adults for the 20% MVC fatiguing contraction, but a similar duration for the 80% task. Voluntary activation was reduced at the end of the 20% MVC task, but by greater magnitudes for old than young adults. The reduction in MVC torque after the low-force task was associated with the reduction in voluntary activation. After the 80% task, voluntary activation declined to similar levels for the young and old adults. Electromyographic activity levels (% MVC) of the biceps brachii and brachioradialis muscles during the fatiguing contraction were greater for the old than young for the 20% MVC task, but similar with age for the 80% MVC task. Our findings indicate that intensity and duration of contraction can be manipulated in young and old adults to induce varying magnitudes of fatigue within the central nervous system. Aging increases: (1) fatigue within the central nervous system immediately after a low-force fatiguing contraction, and (2) the potential for large neural adaptations during neuromuscular rehabilitation in old adults.     

Mechanisms of fatigue differ after low- and high-force fatiguing contractions in men and women

The magnitude of failure in voluntary drive after fatiguing contractions of different intensities in men and women is not known. The purpose of this study was to compare the time to task failure and voluntary activation of men and women for a sustained isometric contraction performed at a low and high intensity with the elbow flexor muscles. Nine men and nine women sustained an isometric contraction at 20% and 80% of maximal voluntary contraction (MVC) force until task failure during separate sessions. The men had a shorter time to failure than women for the 20% but not the 80% MVC task. Voluntary activation was reduced to similar levels for the men and women at the end of the fatiguing contractions but was reduced less after the 80% MVC task than the 20% MVC contraction. Twitch amplitude was reduced similarly at task failure for both sexes and to similar levels at termination of the 20% and 80% MVC tasks. The rate of change in mean arterial pressure was the main predictor of time to failure for the low-force sustained contraction. These results suggest that women experienced greater muscle perfusion, less peripheral fatigue, and a longer time to task failure than men during the low-force fatiguing contraction. However, the low-force task induced greater central fatigue than the high-force contraction for both men and women. Thus, low-force, long-duration fatiguing contractions can be used in rehabilitation to induce significant fatigue within the central nervous system and potentially greater neural adaptations in men and women.     

Sex differences in time to task failure and blood flow for an intermittent isometric fatiguing contraction

The purpose of this study was to compare the time to task failure, postcontraction hyperemia, and vascular conductance of young men and women for a submaximal intermittent fatiguing contraction performed with the handgrip muscles. Twenty men and 20 women (mean ± SD: 22 ± 4 years) performed an isometric contraction at 50% of maximal voluntary contraction (MVC) (6‐s contraction, 4‐s rest) until task failure. Forearm venous occlusion plethysmography was used to estimate the peak blood flow (after 10‐min occlusion) and blood flow at rest after 6‐s submaximal contractions of varying intensities, and during an intermittent fatiguing contraction at 1‐min intervals and task failure. The time to task failure was longer for the women compared with the men (408 ± 205 s vs. 297 ± 57 s, P < 0.05). Postcontraction hyperemia and vascular conductance were greater for men than for women after nonfatiguing 6‐s submaximal contractions performed at 20%, 40%, 50%, 60%, and 80% of MVC force (P < 0.05). In contrast, hyperemia and vascular conductance were similar for both genders when measured at 50 s into the fatiguing contraction, at each minute thereafter, and at task failure. Regression analysis indicated that the rate of electromyographic activity and perceived exertion were the significant predictors of the time to task failure. The longer time to task failure for women compared with men for an intermittent fatiguing contraction with handgrip muscles was not explained by postcontraction hyperemia or vascular conductance with fatigue. © 2008 Wiley Periodicals, Inc. Muscle Nerve 39: 42–53, 2009     

Muscle fatigue measured with evoked muscle vibrations.

Skeletal muscle vibrates laterally during voluntary and evoked muscle contractions. We hypothesized that the vibration amplitude from evoked muscle twitches is directly related to evoked twitch force from fatiguing muscle. To test the hypothesis, vibrations produced by evoked muscle twitches were recorded during short (5-second) rest periods as the muscle was intermittently exercised with voluntary contractions. Trials were performed at 30%, 50%, and 70% of maximal voluntary contraction. Evoked muscle twitches eliminated the problems of motivation and tremor that complicate sound and vibration measurements during voluntary contractions. Results from the first dorsal interosseus hand muscle in 11 normal adult volunteers revealed that the vibration amplitude is highly correlated (r2 = 0.93, at 70% MVC, r2 = 0.97, at 50% MVC; r2 = 0.85, at 30% MVC) with force. Both potentiation and reduction of force with exercise were accompanied by parallel changes in vibration amplitude, as measured with an accelerometer. Compound muscle action potentials did not increase with exercise-induced twitch potentiation, and did not correlate as highly with force during fatigue.     

Electromyographic and mechanical characteristics of human skeletal muscle during fatigue under voluntary and reflex conditions

Effects of fatigue produced by a maintained 50% isometric loading on electromyographic (EMG) and mechanical characteristics of voluntary and reflex contractions of human skeletal muscles were studied in 14 males. A continuous isometric loading of the knee extensors caused significant (P less than 0.001) increase in integrated EMG (IEMG) and decrease (P less than 0.001) in mean power frequency (MPF) of the EMG spectrum. The decrease in MPF was related (P less than 0.05) to percentage fast twitch (FT) fibre area of the vastus lateralis muscle. In reflexly induced contractions no changes were observed during fatigue in reflex latency (LAT) but electromechanical delay (EMD) increased significantly (P less than 0.01). The decrease in voluntary MPF and increase in reflex EMD were interrelated (P less than 0.05). Fatigue loading increased (P less than 0.05) the peak-to-peak amplitude of EMG of the reflex contraction but decreased (P less than 0.01) the corresponding force amplitude. The increase (P less than 0.01) in this reflex EMG/force ratio was related (P less than 0.05) to the corresponding increase observed in IEMG/force ratio of the voluntary contraction. The present findings support the differential fatiguing properties of fast twitch and slow twitch muscle fibres. In addition the fatigue effects on reflexly induced contractions show that fatigue took place primarily in the contractile processes and that muscle spindle sensitivity was increased during fatigue loading.     

Cumulative effects of intermittent maximal contractions on voluntary activation deficits

The purpose of this research was to provide more definitive support for the hypothesis that prolonged muscle activation at high intensities increases voluntary activation deficits. Interpolated twitch responses were evoked during maximal and sub-maximal voluntary contractions of the soleus muscle in 10 college-aged students. Maximal voluntary contractions (MVC), maximal muscle twitches, and interpolated twitch responses were measured before, during, and after fatiguing isometric exercise, five bouts of 20 intermittent MVCs. The relationship between voluntary activation and force was studied by evoking interpolated twitches during sub-maximal voluntary contractions on Day 1 and pre-post fatigue on Day 2. Intraclass reliability coefficients for the MVC, maximal muscle twitch, and interpolated twitch responses were adequate across trials and days (R > or = .80). MVC force and maximal twitch force decreased after the fatiguing exercise bouts by 28% and 32%, respectively (p < .05). Voluntary activation of the fatigue-resistant soleus muscle decreased by 10% after the first five min of maximal exercise with a subsequent decrease of 9% occurring after 25 min of maximal exercise (p < .05). At the end of the experimental session, approximately 30 min after the end of the fatiguing exercise, decreases in 100% MVC force, maximal muscle twitch force, and voluntary activation were still evident: 22%, 23%, and 11%, respectively (p < .05). Post-fatigue, there were also changes in neural strategies for voluntary activation of the soleus muscle at the higher sub-maximal efforts, > or = 70% MVC target levels (p < .05). These data demonstrate the cumulative effects of prolonged exercise on voluntary activation.     

Relationship between electrical and vibratory output of muscle during voluntary contraction and fatigue

Measurements were done on the biceps muscles of 6 healthy volunteers to record simultaneously the surface electromyogram (EMG) and vibromyogram (VMG) by means of a piezoelectric device (accelerometer). The VMG is generated by mechanical waves due to the contraction mechanism and often measured as sound. The frequency spectrum and integrated value (IEMG and IVMG) of both signals were calculated. Both IEMG and IVMG showed a clear linear correlation with force, although at high forces, the variability of the VMG became rather high. Two series of experiments were performed to study the EMG and VMG changes in relation to changes induced by fatigue: (1) during constant force at 50% of the maximal voluntary contraction (MVC) and recovery; (2) for one 1 minute during declining force at MVC. The main finding was that the IVMG was related to the absolute force, irrespective of the fatigue state of the muscle. In contrast, the IEMG showed the well-known changes during fatigue, such as an increase during endurance. The spectral changes of the two measurements also showed a divergence. The spectra of the EMG shifted to lower frequencies in both fatigue protocols. In contrast, the spectra of the vibratory signal did not shift, except for several measurements at MVC. However, the shape did change to a somewhat flatter spectrum with less pronounced peaks. Possible explanations for this different behavior are discussed. It is concluded that the vibratory energy generated by the contraction mechanism is linearly related to force. Changes induced by fatigue do not alter this relationship: this contrasts with the behavior of the EMG.(ABSTRACT TRUNCATED AT 250 WORDS)     

CUMULATIVE EFFECTS OF INTERMITTENT MAXIMAL CONTRACTIONS ON VOLUNTARY ACTIVATION DEFICITS

The purpose of this research was to provide more definitive support for the hypothesis that prolonged muscle activation at high intensities increases voluntary activation deficits. Interpolated twitch responses were evoked during maximal and sub-maximal voluntary contractions of the soleus muscle in 10 college-aged students. Maximal voluntary contractions (MVC), maximal muscle twitches, and interpolated twitch responses were measured before, during, and after fatiguing isometric exercise, five bouts of 20 intermittent MVCs. The relationship between voluntary activation and force was studied by evoking interpolated twitches during sub-maximal voluntary contractions on Day 1 and pre-post fatigue on Day 2. Intraclass reliability coefficients for the MVC, maximal muscle twitch, and interpolated twitch responses were adequate across trials and days (R ≥ .80). MVC force and maximal twitch force decreased after the fatiguing exercise bouts by 28% and 32%, respectively (p < .05). Voluntary activation of the fatigue-resistant soleus muscle decreased by 10% after the first five min of maximal exercise with a subsequent decrease of 9% occurring after 25 min of maximal exercise (p < .05). At the end of the experimental session, approximately 30 min after the end of the fatiguing exercise, decreases in 100% MVC force, maximal muscle twitch force, and voluntary activation were still evident: 22%, 23%, and 11%, respectively (p < .05). Post-fatigue, there were also changes in neural strategies for voluntary activation of the soleus muscle at the higher sub-maximal efforts, ≥70% MVC target levels (p < .05). These data demonstrate the cumulative effects of prolonged exercise on voluntary activation.     

Co-activation of ipsi- and contralateral muscle groups during contraction of ankle dorsiflexors.

Seventeen adult, healthy subjects, age 38.4 +/- 0.24 years (mean +/- SEM) 7 of which were females, were studied. Each subject was seated on a specially designed chair with trunk and legs fixed and the foot strapped to a rigid plate that was attached to a load cell. The position of the strap was adjusted so as to lie across the foot at the level of the metatarsal bones. The knee and ankle joints were adjusted to 90 degrees. To record EMG activity, pairs of surface electrodes were placed over the belly of both the right and left tibialis anterior, quadriceps, hamstring and contralateral triceps surae muscles. Two experimental paradigms were used, A and B. In A the subject was asked to sustain maximum voluntary contraction (MVC) of the ankle dorsiflexors until the force decreased to 50% of the initial value; in B the subject was asked to carry out contractions of the ankle dorsiflexors for 6 seconds followed by 4 sec relaxation periods. The initial contraction was 20% of MVC followed by 40, 60, 80 and 100% of MVC which represented one cycle. The subject was asked to repeat this cycle 10 times. Voluntary contraction of ankle dorsiflexors was regularly accompanied by activation of other muscles, usually first in the same leg, later in the contralateral leg during MVC of ankle dorsiflexors. When intermittent contractions with step wise increments of force developed by the ankle dorsiflexors were carried out, co-activation of ipsilateral and contralateral muscle groups occurred before the force of the contracting muscles decreased.(ABSTRACT TRUNCATED AT 250 WORDS)     

Load-dependence of fatigue related changes in tremor around 10 Hz.

OBJECTIVES: The aim of the study was to investigate the effects of different loads on tremor around 10 Hz during fatiguing contractions. METHODS: Eighteen healthy volunteers performed sustained isometric knee extensions at 30%, 50% and 70% maximum voluntary contraction (MVC). During the fatiguing contractions, mechanical recordings were made with a high-resolution force sensor. Tremor-power was calculated for the 6-20 Hz frequency window as a function of time normalized to endurance time. RESULTS: Initial tremor power was different between the high and low load tasks. Changes of tremor with contraction time differed between the three tasks, in that tremor of the 30% MVC contraction showed the least decrease throughout the sustained contraction, whilst that of the 50% and 70% MVC showed progressively higher decreases. At failure, all 3 contractions merged to the same tremor level. CONCLUSION: Load-dependent, fatigue-related 6-20 Hz tremor changes during sustained submaximum voluntary contractions seem mainly the consequence of recruitment of new units and fatigue-related properties of the high threshold motor units of muscles.     

A comparison of electromyographic and mechanical fatigue properties in motor units of the cat hindlimb

Single motor units were isolated in medial gastrocnemius (MG) or soleus (Sol) muscles of the cat. Single shocks delivered to the motor axon elicited EMG waveforms which were recorded in the muscle. The amplitude of each individual EMG waveform as well as the area under the full-wave rectified waveform were measured. Mechanical properties of the motor units were then measured so that each unit could be classified as FF, FR or S according to the criteria of Burke. Finally, each unit was stimulated continuously at 80 pulses per second (pps), and changes in electrical and mechanical responses were recorded and compared. IEMG was positively correlated with maximum tetanic tension of motor units in MG. The relationship could be fitted with a parabola showing that 'large' motor units produce relatively more electrical activity than do 'small' motor units. Two types of electrical changes were seen during continuous stimulation of motor units at 80 pps: a smooth continuous decline in IEMG was attributed to electrical changes occurring at a site or sites distal to the neuromuscular junction; and random, abrupt changes in unit-EMG waveform and hence IEMG amplitude, were attributed to failure of the action potential to propagate past axonal branch points, resulting in the intermittent failure of groups of muscle fibers to respond to stimuli. The rates of decline of electrical and mechanical activity were compared for motor units. It was found that in fast fatiguing motor units, tetanic tension declined more rapidly than did IEMG, while in slowly fatiguing motor units, IEMG declined more rapidly than did force. We conclude that fatigue of motor units induced by continuous 80 pps stimulation can occur at different sites in the motor unit, and that the site of fatigue depends on motor unit properties or motor unit type.     

Mechanisms of force failure during repetitive maximal efforts in a human upper airway muscle

The upper airway respiratory muscles play an important role in the regulation of airway resistance, but surprisingly little is known about their contractile properties and endurance performance. We developed a technique that allows measurement of force and the electromyogram (EMG) of human nasal dilator muscles (NDMs). Endurance performance was quantified by measuring NDM "flaring" force and EMG activity as healthy human subjects performed 10 s maximal voluntary contractions (MVCs), separated by 10 s rest, until the area under the force curve fell to 50% MVC (the time limit of the fatigue task, Tlim), which was reached in 34.2 +/- 3.1 contractions (685.0 +/- 62.3 s). EMG activity was unchanged except at Tlim, where it averaged 78.7 +/- 3.6% of pretest activity (P < 0.01). M-wave amplitude did not change, suggesting that neuromuscular propagation was not impaired. MVC force increased to 80% of the pretest level within 10 min of recovery but twitch force failed to recover, suggesting low-frequency fatigue. The data suggest that a failure of the nervous system to excite muscle could explain at most only a small fraction of the NDM force loss during an intermittent fatigue task, and then only at Tlim. Thus, the majority of the force failure during this task is due to impairment of mechanisms that reside within the muscle fibers.     

The relationship of motor unit size, firing rate and force.

OBJECTIVE: Using a clinical electromyographic (EMG) protocol, motor units were sampled from the quadriceps femoris during isometric contractions at fixed force levels to examine how average motor unit size and firing rate relate to force generation. METHODS: Mean firing rates (mFRs) and sizes (mean surface-detected motor unit action potential (mS-MUAP) area) of samples of active motor units were assessed at various force levels in 79 subjects. RESULTS: MS-MUAP size increased linearly with increased force generation, while mFR remained relatively constant up to 30% of a maximal force and increased appreciably only at higher force levels. A relationship was found between muscle force and mS-MUAP area (r2 = 0.67), mFR (r2 = 0.38), and the product of mS-MUAP area and mFR (mS-MUAP x mFR) (r2 = 0.70). CONCLUSIONS: The results support the hypothesis that motor units are recruited in an orderly manner during forceful contractions, and that in large muscles only at higher levels of contraction ( > 30% MVC) do mFRs increase appreciably. MS-MUAP and mFR can be assessed using clinical EMG techniques and they may provide a physiological basis for analyzing the role of motor units during muscle force generation.     

Assessment of muscle fatigue during exercise in chronic obstructive pulmonary disease

Contractile fatigue is associated with exercise intolerance in patients with chronic obstructive pulmonary disease (COPD). Contractile fatigue may be assessed by quantifying the decline in strength after a fatiguing protocol but this may pose practical problems. The purpose of this study was to investigate the relationship between the decline in quadriceps strength, quadriceps electrical activity, perception of leg fatigue, and arterial lactate level in patients with COPD during constant work-rate cycling exercise. The decline in quadriceps strength was significantly associated with the decrease in electromyographic median frequency (r = 0.606), leg fatigue perception (r = 0.453), and arterial lactate level (r = 0.384). Using the receiver-operating-characteristic curve, it was found that a 4% decline in electromyographic median frequency had a 94% sensitivity and a 75% specificity to predict contractile fatigue. We conclude that contractile fatigue commonly occurs during cycling exercise in COPD. The electromyographic median frequency appears to be a valuable indirect marker to predict contractile leg fatigue.     

Comparison of potentiated and unpotentiated twitches as an index of muscle fatigue

Recent data suggest that the potentiated twitch is a more sensitive index of contractile fatigue than is the unpotentiated twitch. We hypothesized that after a potentially fatiguing load, the fall in twitch amplitude of the potentiated twitch would be significantly greater than that of an unpotentiated twitch. We compared the response of the potentiated and unpotentiated twitches to a series of potentially fatiguing loads using magnetic stimulation of the femoral nerve in 10 healthy subjects. The baseline unpotentiated quadriceps twitch force (TwQu), potentiated quadriceps twitch force (TwQp), and maximal voluntary contraction (MVC) were 129 plus minus 6 N, 198 plus minus 6 N, and 622 plus minus 25 N, respectively. During a fatigue protocol that was designed to induce a spectrum of fatigue from mild to marked, the percent fall in quadriceps twitch force was significantly greater for the potentiated method than for the unpotentiated method at all levels of fatigue (P <.005). The within-subject within-day coefficient of variation was 7.5 plus minus 0.5% for TwQu and 5.6 plus minus 0.9% for TwQp. Thus, TwQp is reproducible and is superior to TwQu for detecting early muscle fatigue.     

Activation linearity and parallelism of the superficial quadriceps across the isometric intensity spectrum

The purpose of this study was to assess neuromuscular activation of the three superficial portions of the quadriceps femoris muscles during linearly increasing isometric contraction intensities. Thirty healthy volunteers were assessed for isometric electromyographic (EMG) activity of the vastus medialis (VM), vastus lateralis (VL), and rectus femoris (RF) muscles with the knee at 60 degrees of flexion. For 5 s, subjects performed isometric contractions equivalent to 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, and 90% of the average of three maximal voluntary contractions (MVC), in random order. Full-wave rectified and integrated EMG signals over the middle 3 s of each contraction were expressed as a percentage of the activity recorded during the three averaged MVCs. One sample t-tests and 95% confidence intervals were calculated at each relative torque level. A two-factor analysis of variance (muscle by intensity) with repeated measures was performed to evaluate parallel activation across the intensity levels. Activation linearity was assessed via regression analysis for each muscle. VM activation was shown to be significantly lower than expected at 20-70% MVC. VL and RF activations were significantly higher than expected at 10% MVC, and RF EMG was less than expected at 40-70% MVC. EMG of VM was shown to increase significantly more than VL and RF from 80% to 90% MVC. Significant linear and quadratic relations were also demonstrated for all three muscles. Parallel activation of the superficial quadriceps muscles occurred from low to moderate intensities, whereas convergence was noted at near maximal intensities.     

Fatigue induced by intermittent maximal voluntary contractions is associated with significant losses in muscle output but limited reductions in functional MRI-measured brain activation level

The main purpose of this study was to characterize brain activation patterns during a fatigue task involving repetitive maximal voluntary contractions (MVC) of finger flexor muscles. Fourteen young, healthy human participants performed approximately 100 handgrip MVCs (each 2-s contraction was followed by a 1-s rest) while their brain was imaged by functional MRI (fMRI). The handgrip force and electromyograms (EMG) of the finger flexors declined progressively to about 40% of the initial values at the end of the fatigue task, suggesting that significant muscle fatigue had occurred. In contrast, the level of the fMRI signal in the primary (sensorimotor), secondary (supplementary motor), and association (prefrontal and cingulate) motor-function cortices did not change significantly throughout the fatigue task (although the signal of the primary sensorimotor cortex showed a clear trend of decline). The fMRI data from the task of intermittent handgrip MVCs differed dramatically from those obtained in a 2-min sustained handgrip MVC published in a recent report, in which the overall fMRI-measured brain activation level was substantially lower and followed an increase-then-decrease pattern compared to the linear decreases in force and EMG. These results support the notion that the motor cortical centers control the tasks of repetitive and continuous muscle contractions differently and that there is a decoupling in the signal changes of the brain and muscles during muscle fatigue processes induced by maximal voluntary contractions.