Instantaneous interjoint rescaling and adaptation to balance perturbation under muscular fatigue.

Adaptation of automatic postural responses (APR) to balance perturbations might be thought to be impaired by muscle fatigue, given the associated proprioceptive and effector deficits. In this investigation, we aimed to evaluate the effect of muscular fatigue on APR adaptation over repetitive balance perturbations through support base backward translations. APR adaptation was evaluated in three epochs: (a) pre-fatigue; (b) post-fatigue, immediately following fatigue of the plantiflexor muscles through isometric contractions and (c) post-recovery, 30 min after the end of fatiguing activity. Results showed the following: (a) Decreasing amplitudes of joints' maximum excursion over repetitive perturbations in the three fatigue-related epochs. (b) Modulation of joints' excursion was observed in the first trial in the post-fatigue epoch. (c) In the post-fatigue epoch, we found interjoint rescaling, with greater amplitude of hip rotation associated with reduced amplitude of ankles' rotation. (d) Amplitudes of ankles' rotation were similar between the post-fatigue and post-recovery epochs. These findings lead to the conclusions that adaptation of automatic postural responses over repetitive trials was effective under focal muscular fatigue; modulation of the postural response took place in the first perturbation under fatigue, and generalization of response characteristics from post-fatigue to post-recovery suggests that feedforward processes in APRs generation are affected by the recent history of postural responses to stance perturbations.