Effect of fatigue on the precision of a whole-body pointing task.

We addressed the issue of the possible degradation of the aiming precision of a whole-body pointing task, when movement coordination is deranged by selective fatigue of the postural task component. The protocol involved continuous repetition (0.1 Hz frequency) of rapid whole-body pointing trials toward a target located beyond arm length, starting from stance and requiring knee flexion. Six healthy human subjects repeated the trials until exhaustion. Such repetition led to electromyography signs of fatigue in rectus femoris (active in body lowering and raising), but not in deltoid (prime mover for arm reaching component). Rectus femoris fatigue affected the equilibrium control strategy, since the anteroposterior displacement of the center of foot pressure was reduced during the fatigued compared with the initial trials. Conversely, the precision of the aiming movement was unaffected by the rectus femoris fatigue in spite of changes in finger trajectory. Trunk inclination at the end of whole-body pointing task and hip and shoulder marker trajectories were unaffected by rectus femoris fatigue. Control experiments were made, whereby fatiguing repetitions of the postural component of the task were performed without finger pointing, except in the first and last five complete whole-body pointing trials. The results were not different from those of the main protocol, except for a transient change in finger trajectory in the very first trial after fatigue. The CNS takes into account the state of postural muscles' fatigue and the concurrently ensuing equilibrium constraints in order to appropriately modify whole-body pointing strategy and keep pointing precision at the target.