Coordination of mono- and bi-articular muscles in multi-degree of freedom elbow movements.

We investigated the coordination of mono- and bi-articular muscles during movements involving one or more degrees of freedom at the elbow. Subjects performed elbow flexion (or extension) alone, forearm pronation (or supination) alone, and combinations of the two. In bi-articular muscles such as biceps brachii and pronator teres, the amplitude of agonist electromyographic (EMG) activity was dependent on motion in the two degrees of freedom. Agonist burst amplitudes for combined movements were approximately the sum of the agonist burst amplitudes for movements in the individual degrees of freedom. Activity levels in individual degrees of freedom were, in turn, greater than activity levels observed when a muscle acted as agonist in one degree of freedom and antagonist in the other. Other muscles such as triceps, brachialis, and pronator quadratus acted primarily during motion in a single degree of freedom. The relative magnitude and the timing of activity between sets of muscles also changed with motion in a second degree of freedom. These patterns are comparable with those reported previously in isometric studies.