Coactivation of biceps and triceps. The biceps’ EMG is on top, while the triceps’ EMG is on the bottom.
Muscle coactivation is a phenomenon in which a muscle is activated coordinately with another muscle. The EMG shown demonstrates the antagonistic muscle activity in the biceps and triceps of a relaxed and seated subject, with the elbow bent at 90 degrees increase in the activity of the triceps. Likewise, there is a minor increase of activity in the biceps trace when the triceps is activated. Coactivation of the muscles stabilizes a joint, although the mechanism is not well understood. Muscular coactivation is thought to be required for fine movements.