The dominant mechanisms limiting time to failure of sustained isometric contractions with the elbow flexor muscles appear to change at target forces between 30 and 45% MVC, with load compliance being a significant factor at lower forces only. There was no difference in the estimates of external mechanical work at any target force.
The briefer times to failure for the position task at the lower forces were accompanied by greater rates of increase in elbow flexor muscle activity, mean arterial pressure, heart rate, and rating of perceived exertion. The time to task failure was longer for the force tasks (576 ± 80 and 325 ± 70 s) than for the position tasks (299 ± 77 and 168 ± 35 s) at target forces of 20 and 30% ( P 0.19). The maximal voluntary contraction (MVC) force was similar ( P = 0.95) before the subjects performed the force and position tasks at each of the four target forces: 20, 30, 45, and 60% of MVC force. Twenty-one healthy adults (23 ± 6 yr 11 men) participated in the study. Each task was sustained for as long as possible. Subjects pulled against a rigid restraint during the force task and maintained a constant elbow angle, while supporting an equivalent inertial load during the position task. The primary purpose of this study was to determine the influence of load compliance on time to failure during sustained isometric contractions performed with the elbow flexor muscles at four submaximal target forces.
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