Muscle load can affect performance of concurrent cognitive task. This effect is often explained by limited resources in the voluntary attention system. To examine whether earlier stages of cognitive information processing might be affected, we recorded the Mismatch Negativity component (MMN) of the auditory event-related brain potential before, during and after sustained handgrip at 7% and 30% of maximal voluntary contraction. MMN is an index of automatic detection of a deviating auditory event. MMN was not affected by force level. However, its amplitude at fronto-central sites decreased during the fatiguing 30% contraction, while it tended to increase during the light 7% work. Thus, muscle fatigue may affect auditory information processing at preattentive and preconscious stages, which could modify cognitive performance