Neural mechanisms contributing to muscle fatigue in humans
The mechanisms that contribute to the decline in muscle force during a voluntary contraction depend on the details of the task. One task variable that influences the prevailing mechanisms is the type of load supported by the limb. For example, the time to task failure is briefer when light loads are supported (position task) compared with exerting an equivalent force against a rigid restraint (force task). This difference in the time to failure occurs despite a similar net muscle torque during the two tasks. Although the rate of increase in the muscle activity (electromyogram, EMG) for the elbow flexor muscles was similar for the two tasks, measures of central neural activity, including mean arterial pressure, heart rate, ratings of perceived exertion, and fluctuations in motor output (force and acceleration), all increased more rapidly during the position task. This dissertation examined the contribution of descending drive and afferent feedback to the difference in time to failure for the two tasks. This was accomplished by examining the effects of load type, visual feedback, and sensory feedback on the ability of young adults to maintain a submaximal contraction to failure. The first study compared the discharge characteristics of the same motor unit in biceps brachii during the performance of both tasks. The findings indicated more marked changes in motor unit activity during the position task than the force task, including a greater reduction in discharge rate, and a more pronounced increase in discharge rate variability. The second study examined the contribution of the frequency modulation of motor unit discharge to the fluctuations in the motor output (force or acceleration) during the force and position tasks. Different frequencies from the power spectra of the motor unit discharge contributed to the fluctuations in motor output for the force and position tasks. The third study examined the influence of varying the gain of the feedback signal on time to failure during the position task. The results indicated that despite the same mechanical load, increasing the gain of the feedback signal reduced the time to failure during the position task. The fourth study examined the influence of vibration on time to failure during the position task, and revealed that applying vibration to the common tendon of biceps brachii while performing the position task also reduced the time to task failure. Taken together, these results indicate that the time to task failure is influenced by changes in the activity of the motor neuron pool caused by load type, gain of the feedback signal, and vibration of the muscle.