Effects of motor fatigue on human brain activity, an fMRI study

The main purpose of this study was to investigate effects of motor fatigue on brain activation in humans, using fMRI. First, we assessed brain activation that correlated with muscle activity during brief contractions at different force levels (force modulation). Second, a similar analysis was done for sustained contractions inducing motor fatigue. Third, we studied changes in brain activation due to motor fatigue over time. And fourth, we investigated cross-over effects of fatigue by comparing brain activation before and after the fatiguing condition during simple and high-order motor tasks (reaction time tasks). Several motor areas in the brain showed increased activity with increased muscle activity, both during force modulation and motor fatigue. Interestingly, the cerebellum showed a smaller increase in activation, during compensatory activation due to fatigue, while additional activation was found in the pre-supplementary motor area and in a frontal area. During motor fatigue, there was a decrease in force production, an increase in force variability, and an increase in muscle activity. Brain areas comparable with the aforementioned areas also showed stronger activation over time. After fatigue, reaction time task performance remained the same (compared to before fatigue), while increased activation in orbitofrontal areas was found. Furthermore, there was a reduction in subjects' maximal voluntary contraction force, accompanied by a decrease in activation of the supplementary motor area (SMA). These results suggest that especially the activity in the SMA and frontal areas is affected by motor fatigue.