The stability of synergy in agonists during the execution of a simple voluntary movement.

The characteristics of muscular synergy between the main elbow flexors (biceps brachii, brachialis, brachoradialis were considered. The activities of these different muscles were recorded simultaneously with surface and wire electrodes. A quantitative analysis of the activity of each of these muscles and of their excitation levels was carried out during movements performed at various velocities and against different inertias. It was shown that: (1) the onset as well as the cessation of activity in the different muscles occur practically simultaneously and independently of the velocity and inertia of the movement; (2) the well-known linear relation between the integrated EMG of biceps brachii and the work can be extended to the other main flexors. This implies that the relation between the activities of the main flexors remains constant whatever the velocity and inertia may be. These results confirm the notion of 'Flexor Equivalent'. They also demonstrate a stability of the synergy between agonist muscles which must be distinguished particularly from the synergy between agonists and antagonists.