Net excitation of the motor unit pool varies with load type during fatiguing contractions.

To identify the underlying physiological mechanisms for the difference in the time to failure for two types of fatiguing contractions, 20 subjects performed force and position tasks with the elbow flexor muscles at a comparable net muscle torque for a similar duration. Prior to terminating each task, blood flow was occluded to estimate the relative amount of feedback transmitted by small-diameter afferents to the spinal cord. Mean arterial pressure at the conclusion of the fatiguing contraction increased similarly for the two tasks (force: 119% +/- 14%; position: 114% +/- 15%). However, the final values for the electromyographic activity for the elbow flexor muscles (26% +/- 14% and 21% +/- 11%, respectively; P < 0.05), and the increase in the fluctuations in acceleration and force (225% +/- 152% and 154% +/- 53%, respectively; P < 0.05) in the sagittal plane, were significantly greater during the position task compared with the force task. These results suggest a different balance in the excitatory and inhibitory inputs to the spinal motor neurons for the two tasks, which has implications for the design of work tasks and exercise prescription in rehabilitation.