Effect of fatigue on force sensation.

Recent experiments have suggested that a sense of effort can be separated from a sense of developed force or tension in muscular contractions. The evidence for this distinction was examined during submaximal fatiguing contractions. Subjects were required to maintain until maximal endurance a constant isometric force with their right, reference arm, and at 15-s intervals they estimated the magnitude of this force with a matching contraction of the contralateral arm. The matching force produced by the unfatigued limb was the measure of force sensation. Both force and the brachial biceps and triceps EMG were recorded from each arm. During the fatiguing contractions the matching force increased linearly as did the biceps EMG of the fatiguing muscle. The rate of increase was dependent on the level of force exerted. A linear relation between the reference arm EMG and the perceived force was observed, which suggested that the over-estimation of force was due to the increase in the excitatory input to the fatiguing muscle. These results provide support for a centrally mediated theory of force perception, and indicate that during fatigue subjects are unable to estimate accurately the force of contraction. Furthermore, they suggest that under those conditions a sense of tension is not distinguishable from a sense of effort.