Motor fatigue is an exercise-induced reduction in the force-generating capacity. The underlying mechanisms can be separated into factors residing in the periphery or in the central nervous system. We designed an experiment in which we investigated central processes underlying motor fatigue by means of magnetic resonance imaging in combination with the twitch interpolation technique. Subjects performed a sustained maximal abduction (2 min) with the right index finger. Brain activation was recorded with an MR scanner, together with index finger abduction force, EMG of several hand muscles and interpolated twitches. Mean activity per volume was calculated for the primary motor cortex and the secondary motor areas (supplementary motor, premotor, and cingulate areas) as well as mean force and mean rectified EMG amplitude. Results showed a progressive decline in maximal index finger abduction force and EMG of the target muscles combined with an increase in brain activity in the contralateral primary motor cortex and secondary motor areas. Analysis of the twitches superimposed on the sustained contraction revealed that during the contraction the voluntary drive decreased significantly. In conclusion, our data showed that despite an increase in brain activity the voluntary activation decreased. This suggests that, although the CNS increased its input to the relevant motor areas, this increase was insufficient to overcome fatigue-related changes in the voluntary drive.