Coordination and inhomogeneous activation of human arm muscles during isometric torques

1. In this study we have recorded the activity of motor units of the important muscles acting across the elbow joint during combinations of voluntary isometric torques in flexion/extension direction and supination/pronation direction at different angles of the elbow joint. 2. Most muscles are not activated homogeneously; instead the population of motor units of muscles can be subdivided into several subpopulations. Inhomogeneous activation of the population of motor units in a muscle is a general finding and is not restricted to some multifunctional muscles. 3. Muscles can be activated even if their mechanical action does not contribute directly to the external torque. For example, m. triceps is activated during supination torques and thus compensates for the flexion component of the m. biceps. On the other hand, motor units in muscles are not necessarily activated if their mechanical action contributes to a prescribed torque. For example, there are motor units in the m. biceps that are activated during flexion torques, but not during supination torques. 4. The relative activation of the muscles depends on the elbow angle. Changing the elbow angle affects the mechanical advantage of different muscles differently. In general, muscles with the larger mechanical advantage receive the larger input. 5. We have calculated the relative contributions of some muscles to isometric torques. These contributions depend on the combination of the torques exerted. 6. Existing theoretical models on muscle coordination do not incorporate subpopulations of motor units and therefore need to be amended.