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Review
, 586 (1), 11-23

Muscle Fatigue: What, Why and How It Influences Muscle Function

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Review

Muscle Fatigue: What, Why and How It Influences Muscle Function

Roger M Enoka et al. J Physiol.

Abstract

Much is known about the physiological impairments that can cause muscle fatigue. It is known that fatigue can be caused by many different mechanisms, ranging from the accumulation of metabolites within muscle fibres to the generation of an inadequate motor command in the motor cortex, and that there is no global mechanism responsible for muscle fatigue. Rather, the mechanisms that cause fatigue are specific to the task being performed. The development of muscle fatigue is typically quantified as a decline in the maximal force or power capacity of muscle, which means that submaximal contractions can be sustained after the onset of muscle fatigue. There is even evidence that the duration of some sustained tasks is not limited by fatigue of the principal muscles. Here we review experimental approaches that focus on identifying the mechanisms that limit task failure rather than those that cause muscle fatigue. Selected comparisons of tasks, groups of individuals and interventions with the task-failure approach can provide insight into the rate-limiting adjustments that constrain muscle function during fatiguing contractions.

Figures

Figure 1
Figure 1. Differences in fatigability between young and old adults
A, average torque exerted by young and old adults during five sets of 30 maximal lengthening contractions with the dorsiflexor muscles. Each data point indicates the mean ± s.e.m. of five successive contractions for 16 subjects in each group. Adapted with permission from Baudry et al. (2007). B, each data point denotes the time to failure for the one young man and one old man who were matched for strength. The task was to sustain an isometric contraction with the elbow flexor muscles at 20% of maximum for as long as possible. The time to failure was longer for the older man of each pair. Adapted from Hunter et al. (2005).
Figure 2
Figure 2. Times to task failure for men and women
A, mean (+s.e.m.) time to task failure for sustained and intermittent isometric contractions performed by strength-matched men and women with the elbow flexor muscles. The target torque was 20% of MVC torque for the sustained contractions and 50% of MVC torque for the intermittent contractions. Reprinted with permission from Hunter et al. (2004b). B, mean (± s.e.m.) time to task failure for sustained isometric contractions performed by men and women with the knee extensor muscles with the blood flow to the limb either occluded or not occluded. The men were stronger than the women in this study. The target was 25% of MVC torque. Data from Clark et al. (2005).
Figure 3
Figure 3. Decreases in MVC torque and voluntary activation during a fatiguing contraction
A, decrease in MVC torque after three sets of maximal shortening contractions (60 deg s−1 over a 75 deg range of motion) and three matched maximal isometric contractions with the knee extensor muscles. The two tasks were performed on separate days and the isometric contractions were sustained to produce the same relative decrease in MVC torque. The data correspond to mean +s.e.m. for the shortening contractions and isometric contractions for the first, second and third sets of shortening contractions and the three isometric contractions. Adapted with permission from Babault et al. (2006). B, corresponding decreases in voluntary activation as assessed with paired stimuli (10 ms apart) during selected maximal contractions. Initial values for voluntary activation were 88.3 ± 3.0% for the session in which the shortening contractions were performed and 89.4 ± 3.1% in the session for the isometric contractions. Data from Babault et al. (2006).
Figure 4
Figure 4. Characteristics of the force and position tasks performed to failure
A, the two tasks were performed with the first interosseus muscle with the hand in the posture shown. The index finger pushed up against a rigid restraint for the force task and supported an inertial load for the position task. B, representative data for the position (top) and force (bottom) tasks as performed by one individual. For each task, the data comprise the EMG for the antagonist (second palmar interosseus) and agonist (first dorsal interosseus) muscles, and either the joint angle (position task) or the abduction force (force task). Adapted with permission from Maluf et al. (2005). C, group data (n = 20) for EMG amplitude for the agonist and antagonist muscles during the force and position tasks. EMG amplitude was calculated for each 1% interval of the time to failure and averaged across subjects. 95% confidence intervals are shown for the first, middle and last data point of each condition. Adapted from Maluf et al. (2005).
Figure 5
Figure 5. Discharge characteristics of single motor units in biceps brachii during the force and position tasks
A, mean ± s.e.m. discharge rate at the beginning, middle and end of the force and position tasks for the 32 motor units. The decrease in mean discharge rate was significantly greater during the position task (13.1–10.6 pps) than during the force task (13.3–12.0 pps). B, the coefficient of variation for interspike interval did not change from the beginning (23.1 ± 7.8%) to the end (22.4 ± 8.6%) of the force task, whereas it increased significantly during the position task (22.4 ± 10.4 to 26.7 ± 9.2%). Data from Mottram et al. (2005).
Figure 6
Figure 6. EMG activity of the elbow flexor muscles during the force and position tasks
A, the two tasks were performed with the elbow joint at a right angle and the upper arm abducted from the trunk by about 0.4 rad (23 deg). B, the average rectified EMG (mean ± s.e.m.) for infraspinatus (circles), supraspinatus (squares) and teres minor (triangles) during the force and position tasks. The target force was 20% MVC force. The EMGs were normalized to the peak values observed during the MVCs and averaged over 30 s epochs at 20% intervals during each task. Adapted with permission from Rudroff et al. (2007a). C, rectified EMG averaged over 30 s epochs at 25% intervals for the duration of the position task and at the end of the force task across the biceps brachii, brachialis and brachioradialis muscles during the two tasks. The target force was 15% MVC force. The EMGs were normalized to the peak values observed during the MVCs. Data from Hunter et al. (2002).

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