Muscle fatigue reduces the force that a muscle can produce and causes sensations of weakness and discomfort. People can adapt to muscle fatigue by adopting new movement patterns and control strategies. These motor adaptations can affect performance and sometimes predispose people to injuries. However, the effects of fatigue are situation-dependent. This research examined how the effects of muscle fatigue are moderated by which muscles are fatigued, the fatigue state, and the sensation of pain. It was expected that these moderating factors would affect the motor adaptations that people use when fatigued, the rate of muscle fatigue, and the risk of error and injury.

Since different joints make different contributions to the completion of movement tasks, fatigue that is localized in different muscle groups is expected to affect movement differently. Aim 1 compared changes in whole-body movement coordination following fatigue of proximal and distal muscle groups in the upper extremity. Subjects maintained task performance after both proximal and distal fatigue. Proximal fatigue led to widespread movement changes across several joints, but distal fatigue primarily caused changes at the distal joints. The observed changes after proximal fatigue may increase the risk of back pain and injury, while changes after distal fatigue may predispose people to errors in manipulation.

Muscle fatigue is a complex, multifactorial process, and people may adapt to fatigue by changing the movement patterns of many joints. It is difficult to quantify the effects of muscle fatigue on multi-joint coordination. In Aim 2, I used principal components analyses to determine how multi-joint coordination changes during and after muscle fatigue. During fatigue, inter-joint coordination decreased, and subjects utilized a stiffening strategy that may have reduced the complexity of movement. However, these changes began to reverse after cessation of a fatiguing task. The observed changes suggest that people learn novel coordinative patterns as they adapt to muscle fatigue.

Adapting to muscle fatigue is a cognitively demanding process. In working environments, biological stimuli such as pain may compete for limited cognitive resources during movement tasks. Aim 3 used a goal equivalent manifold (GEM) approach to determine how experimental pain influences the ability to adapt to muscle fatigue. Ischemic muscle pain in the contralateral arm caused people to reduce movement control, but this did not lead to significantly faster fatigue rates. However, order of the painful, and non-painful experimental sessions had a significant effect on fatigue rate. Furthermore, people who exhibit catastrophic thinking used different movement strategies than those who did not exhibit catastrophic thinking.

Together these results demonstrate that the fatigue state of the muscles and the presence of a noxious stimulus moderate the way that people adapt to muscle fatigue. People can adapt to muscle fatigue by modifying their movement patterns, but motor adaptation does not necessarily lead to optimal movement strategies. While motor adaptation did not affect the task outcome in these or previous fatigue studies, the lack of performance deficits is likely attributable to the selection of simple experimental tasks in highly controlled environments. The moderating factors examined here are likely to affect complex fatiguing situations encountered in real world environments where the observed reduction in movement coordination and control could negatively impact task execution and lead to inefficient movement and injury. The current results emphasize the need to better understand how fatigue affects movement in realistic working environments.