Muscle fatigue is known to be associated with a deteriorated muscle coordination and impaired movement performance in variety of voluntary movements. The aim of this study was to investigate the generally underexplored effect of muscle fatigue on both the coordination between grip force (GF; the force component perpendicular to the hand-object contact area that provides friction) and load force (LF; the parallel force component that can move the object or support the body) as well as movement performance in manipulation tasks. Fifteen participants performed a variety of static and dynamic manipulations both with and without a preceding procedure designed to fatigue the arm and hand muscles. The tasks involved exertion of ramp-and-hold and oscillation patterns of LF against an externally fixed instrumented device, and a simple lift of a freely moving device. The results revealed a fatigue-associated decrease in GF scaling (i.e., the magnitude of GF relative to LF) and GF-LF coupling (correlation between GF and LF), while the task performance regarding the accuracy of exertion of the prescribed LF profiles remained unaffected. We conclude that muscle fatigue both partly decouples GF from LF and reduces the overall GF magnitude, which could potentially explain why hand-held objects are more likely to drop when manipulated with fatigued muscles. However, the unaffected task performance could be explained either by the relatively low level of muscle forces required by the tested tasks, the moderate level of the fatigue imposed, or both.
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