Adaptation to fatigue of long duration in human wrist movements.

Subjects made fast, accurate, consistent wrist flexions under normal conditions and under conditions of low-frequency fatigue. Movements made 1 h after fatiguing exercise were indistinguishable from those made before exercise, even though twitch tensions were only approximately 60% of their fresh values. Electromyograms (EMGs) recorded from the fatigued muscles were, however, different from those recorded before exercise. EMGs during unfatigued movements showed multiple bursts typical for rapid movements. In the presence of low-frequency fatigue, the duration of the first burst was longer than that under normal conditions, and its onset occurred earlier relative to the initiation of movement. The area of the second agonist burst and, in some cases, the antagonist burst, was increased, although changes in their timings were unclear. We conclude that subjects adapted to low-frequency fatigue by changing the neural patterns controlling their muscles and present a simple model of excitation-contraction coupling that demonstrates how the observed changes in excitation can produce the same kinematics.