Inhomogeneous activation of motoneurone pools as revealed by co-contraction of antagonistic human arm muscles

There is now considerable evidence that motoneurone pools in several human arm muscles are activated inhomogeneously during isometric and reflex-induced contractions in different directions (van Zuylen et al. 1988; Gielen et al. 1988; ter Haar Romeny et al. 1984). In this paper we investigate the activation of antagonist muscles (m. brachialis, m. brachioradialis, m. biceps and m. triceps) during co-contraction of the upper arm muscles. The results show that there is a marked difference between the distribution of the activities of synergistic flexor muscles, or even within these muscles, in co-contraction tasks and in flexion tasks. This discrepancy may be attributed to the existence of inhibitory mechanisms between motoneurone pools of antagonist muscles. These mechanisms can also account for different types of recruitment behaviour of motor unit populations in a single muscle.