Abstract
The motor system adapts to novel dynamic environments by forming internal models that predict the muscle forces needed to move skillfully. The goal of this study was to determine how muscle fatigue affects internal model formation during arm movement and whether an internal model acquired while fatigued could be recalled accurately after rest. Twelve subjects adapted to a viscous force field applied by a lightweight robot as they reached to a target. They then reached while being resisted by elastic bands until they could no longer touch the target. This protocol reduced the strength of the muscles used to resist the force field by approximately 20%. The bands were removed, and subjects adapted again to the viscous force field. Their adaptive ability, quantified by the amount and time constant of adaptation, was not significantly impaired following fatigue. The subjects then rested, recovering approximately 70% of their lost force-generation ability. When they reached in the force field again, their prediction of the force field strength was different than in a nonfatigued state. This alteration was consistent with the use of a higher level of effort than normally used to counteract the force field. These results suggest that recovery from fatigue can affect recall of an internal model, even when the fatigue did not substantially affect the motor system's ability to form the model. Recovery from fatigue apparently affects recall because the motor system represents internal models as a mapping between effort and movement and relies on practice to recalibrate this mapping.
Highlights
Title Effect of muscle fatigue on internal model formation and retention during reaching with the arm
A key strategy used by the nervous system to adapt to altered dynamic environments is to form an internal model, a feedforward neural mapping between limb state and muscle activations, which allows the nervous system to predict the muscle activations needed to achieve a desired movement in environments characterized by patterns of forces applied to the limbs [9, 39, 56]
Post hoc analysis revealed a significant linear contrast of direct effect size in exposure 4 (E4) as a function of exercise protocol and significant differences between aftereffect size for the fatigue left (FL) and no fatigue (NF), and fatigue right (FR) and NF exercise protocols (P Ͻ 0.005, Tukey’s test). These results demonstrate that the motor system is able to form an internal model of a novel dynamic environment even when the muscles used to compensate for the environment are substantially fatigued
Summary
Title Effect of muscle fatigue on internal model formation and retention during reaching with the arm. Once an internal model is learned, it can be recalled following a period when it is not accessed [3, 7, 55] This process, called “retention,” is partially characterized by a reduction in the size of the direct effect when a person revisits a force field. Understanding the effect of fatigue on the use of internal models is important for understanding and optimizing exercise, sports training, and rehabilitation, in which it is common for a person to practice a particular motor task while fatigued, but seek to perform the task later in a rested state. The goal of this study was, to test how fatigue affects the ability of healthy adults to acquire and recall an
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