The effects of neuromuscular fatigue on task performance during repetitive goal-directed movements.

Proper movement timing is essential to the successful execution of many motor tasks and may be adversely affected by muscle fatigue. This study quantified how muscle fatigue affected task performance during a repetitive upper extremity task. A total of 14 healthy young adults pushed a low load back and forth along a low-friction horizontal track in time with a metronome until volitional exhaustion. Kinematic, force, and electromyography (EMG) data were measured continuously throughout the task. The first and last 3.5 min were analyzed to represent "early" and "late" fatigue. Means and standard deviations of movement distance, speed, and timing errors were computed. We also decomposed variations in movement distance and speed into deviations that directly affected achieving the task goal and those that did not, by identifying the goal equivalent manifold (GEM) of all valid solutions to this task. Detrended fluctuation analysis was used to quantify the temporal persistence in each time series. Principle components analysis provided a direct measure of alignment with the GEM. Median power frequencies of the EMG significantly decreased in six of the nine muscles tested indicating that subjects did fatigue. However, there were no differences in the means or variability of movement distance, speed, or timing errors. Thus, subjects maintained overall performance despite fatigue. Subjects applied slightly higher peak handle forces when they were fatigued (P = 0.032). Muscle fatigue caused significant reductions in the temporal persistence of movement speed (P = 0.037) and timing errors (P = 0.046), indicating that subjects corrected errors more quickly when fatigued. Mean deviations and variability perpendicular to the GEM were much smaller than variability along the GEM (P < 0.001). Deviations perpendicular to the GEM were also corrected much more rapidly than those along the GEM (P < 0.001). Subjects aligned themselves very closely (<+/-7 degrees ), but not exactly (P < 0.001), with the GEM. These measures were not significantly affected by muscle fatigue. Overall, these results indicated that subjects altered their biomechanical movement patterns in response to muscle fatigue, but did so in a way that specifically preserved the goal relevant features of task performance.