Bilateral voluntary contractions involve functional changes in both primary motor cortices. We investigated whether a voluntary contraction controlled by one hemisphere can influence oscillatory processes contralaterally. Corticomuscular coherence was calculated between EEG recorded over the motor cortex hand representation and electromyogram from the first dorsal interosseous muscle when the nondominant hand performed a precision grip task. The dominant arm remained at rest or performed a finger abduction or an elbow flexion task at 10, 40, and 70% of maximal isometric voluntary contraction (MVC). Mean coherence in the 15- to 30-Hz range in the hand performing a precision grip increased during 40% (by 72%) and 70% (by 73%) but not during 10% of MVC in the finger abduction task. Similarly, in the elbow flexion task, mean coherence increased during 40% (by 40%) and 70% (by 48%) but not during 10% of MVC. No differences were observed between the increments in coherence between the finger abduction and elbow flexion tasks at a given force level. We speculate that these results reflect the increased complexity of controlling a fine motor task with one hand while performing a strong contraction with the contralateral hand and suggest that increased oscillatory corticomuscular coupling may contribute to successful task performance.