Task-dependent effect of limb immobilization on the fatigability of the elbow flexor muscles in humans.

Because short-term limb immobilization produces selective adaptations in the neuromuscular system that probably interact with the task-dependent expression of muscle fatigue, the purpose of this study was to determine the effects of limb immobilization on the ability of human subjects to sustain isometric contractions at low and moderate submaximal forces. Four weeks of elbow joint immobilization caused a substantial decrease in the daily activity of biceps brachii during immobilization, a significant reduction in the cross-sectional area and volume of the elbow flexor muscles as measured by magnetic resonance imaging, and a decline in the maximum voluntary contraction (MVC) activation and force of the elbow flexor muscles. Immobilization had a task-dependent effect on muscle fatigue with a substantially increased endurance time (reduced fatigability) at a low force (20% MVC) and no statistical effect at a moderate force (65% MVC). Despite atrophy of the elbow flexor muscles due to the immobilization, the twitch force elicited in biceps brachii by electrical stimulation was greater after immobilization. The selective improvement of fatigue resistance for the low-force contraction and the absence of a change in the time course of the twitch suggests that the immobilization-induced adaptations included an improved efficacy of some excitation-contraction processes and underscored the major role of these mechanisms in determining the endurance time for low-force, long-duration contractions.