Sustaining a constant effort by the tongue and hand: effects of acute fatigue.

A constant-effort task was used previously as a potential assessment technique for fatigue. Participants sustained submaximal target effort levels with the tongue and hand against soft air-filled bulbs. For 80% of all trials, pressure decreased exponentially to a positive asymptote. In addition, pressure decreased faster when the muscles were fatigued than when they were rested. This study attempted to replicate the previous findings with new participants and to extend the findings to include surface electromyographic (EMG) data. Pressure and surface EMG signals were collected simultaneously while 10 neurologically normal young adults performed the constant-effort task at 50% of maximum pressure with the tongue and the hand. Eighty-one percent of the pressure data were modeled by a negative exponential equation with a nonzero asymptote. Seventy-three percent of the corresponding EMG data also fit this mathematical model. The pressure signals decayed more slowly than the corresponding EMG signals, particularly for the hand. After participants fatigued the tongue and hand with repeated brief maximal voluntary contractions, the time constants were reduced (rate of decay increased) for the tongue but not the hand. These results corroborate the previous finding that the time constant, determined from an exponential curve-fitting procedure, is a replicable measure. Furthermore, the reduction in the time constant after inducing acute fatigue in the tongue was replicated, although this same relationship was not replicated for the hand. The EMG data suggest that decreases in neuromuscular drive, including increased early adaptation, motor unit derecruitment, and motor unit desynchronization, contributed to the decrease in pressure during the constant-effort task, especially after acute fatigue was induced. These observations support the hypothesis that the task reflects, at least in part, central fatigue processes.