Force level modulates human cortical oscillatory activities.

We studied the relationship between cortical and muscular oscillatory activities and muscular force level during a tonic contraction task using electroencephalography (EEG). Within the weak to moderate force level, the normalized power in the alpha band in the contralateral sensorimotor areas was inversely linearly correlated with the force. Cortical-muscular coherence was observed in the beta band and its magnitude was not affected by the force. In contrast, during strong contractions, EEG power in the gamma band increased and was partly correlated with the Piper rhythm in the electromyogram. Our results show that the multiple oscillatory activities in the cortex are correlated with the force level in different ways. Cortical gamma band oscillation may reflect both focused attention and the efferent drive to the muscle during very strong tonic contraction.