Non-Linear aspects of joint coordination in blackboard writing

Abstract

The present study focuses on the inter-joint coordination of the shoulder, elbow and wrist joints in circular hand displacements that were performed in the sagittal and fronto-parallel plane. In the latter plane, the experimental task was comparable to blackboard writing. Twelve right-handed participants drew in each plane clockwise and counter-clockwise circles at nine positions. The task, audibly paced at 1, 1.5 and 2 Hz, was performed without visual control. The displacements of seven IREDs that were fixated on the relevant joints were sampled at a rate of 100 Hz by means of a 3D-motion tracking system (Optotrak 3020). Shoulder, elbow and wrist angular excursions were derived as well as three coordination measures, viz. correlations between joint velocities and means and standard deviations of the relative phase between neighbouring joints. The results confirm the expectation that in sagittal-plane movements the shoulder and elbow are more strongly coupled than the elbow and wrist. However, effects due to typical characteristics of joint architecture, like built-in mechanical "joint-play" and closed packed positions, also proved to differentially affect the stability of the joint coordination. These factors induced a plane and position-dependent proximal-to-distal shift of joint coupling. It is concluded that besides polyarticular muscles and inertial coupling, functional properties of joint surfaces form an important determinant of joint coordination which fact, to date, has been neglecte