Rhythmic and discrete elements in multi-joint coordination.
Everyday actions invariably consist of a combination of discrete and rhythmic elements within or across joints. The study investigated constraints arising from the co-occurrence of the two actions in a two-joint task and how endpoint trajectories are shaped due to these action elements at the joint level. The task consisted of an elbow oscillation in the plane that was to be merged with a fast discrete adduction or abduction in the shoulder triggered by an auditory signal. The task was performed with and without explicit instruction about the joint involvement. Two hypotheses were tested: (1) kinematic constraints for the coupling of discrete and rhythmic elements arise at the neuro-muscular level, such that EMG bursts of the discrete and rhythmic movement have a tendency to synchronize. This effect was documented previously in a comparable single-joint task. (2) The merging of the two elements is constrained by intersegmental torques such that initiation and performance of the discrete movement utilizes interaction torques. This hypothesis rests on the assumption that the CNS has an internal model of the limb dynamics and exploits passive torques. Key results support hypothesis 1: (i) the discrete action's initiation at the shoulder was constrained to a preferred phase of the ongoing elbow oscillation. (ii) The rhythmic elbow movement showed a systematic phase advance during the discrete shoulder shift, similar to those reported for the single-joint variant of the task. Reaction times of the discrete movement were longer and peak velocities slower than reported for isolated discrete movements, due to the simultaneous presence of the oscillation. (iii) Interaction torques acting from the elbow onto the shoulder joint were not selectively exploited for the acceleration of the discrete shoulder movement. Indirectly however, hypothesis 2 also found support: torques at the elbow generated compensatory muscle activity in the shoulder that stabilized the stationary joint. It was this rhythmic activity that posed the direct constraints on the initiation of the discrete movement.