Stability of inter-joint coordination during circle drawing: effects of shoulder-joint articular properties

The present study addressed the effect of articular conformity of the shoulder joint on the stability of inter-joint coordination during circular drawing movements. Twelve right-handed participants performed clockwise and counter-clockwise circular drawing movements at nine locations in the mid-sagittal plane. The task was paced acoustically at 1.0, 1.5 and 2.0 Hz and performed without visual control. Displacements of seven infrared light emitting diodes that were fixated at relevant joints were sampled at 100 Hz by means of a 3D-motion tracking system (Optotrak 3020). From these data, shoulder, elbow and wrist angular excursions were derived as well as the continuous relative phase of the proximal and distal joint pairs of the arm. The results confirmed earlier observations that the shoulder and elbow are more strongly coupled than the elbow and wrist in sagittal-plane movements. However, a typical characteristic of the architecture of the shoulder joint, that is, its built-in mechanical "joint play", was shown to induce a position-dependent variation in inter-joint coordination stability. We conclude that besides polyarticular-muscle induced synergies and inertial coupling, articular conformity of the shoulder joint constitutes an additional determinant of inter-joint coordination stability that, to date, has been neglected.