Proprioceptive control of multijoint movement: unimanual circle drawing.

The present experiments addressed whether proprioception is used by the central nervous system (CNS) to control the spatial and temporal characteristics of unimanual circle drawing. Circle drawing is a multijoint movement, in which the muscles crossing the elbow and the shoulder are sequentially activated. The spatial and temporal characteristics of circle drawing depend on the precise coordination of these sequential activation patterns, and proprioception is ideally suited to support this coordination. Blindfolded human subjects produced a counterclockwise circular drawing motion (diameter = 16 cm) with the dominant arm at a repetition rate of 1/s. In some trials, 60-70 Hz vibration was applied to the tendons of the biceps brachii and/or the anterior deltoid. Spatial parameters measured from hand-movement data included the x- and y-axis diameters, circularity, and drift of the hand in the workspace. Vibration of either the biceps brachii or the anterior deltoid caused subjects to draw circles with decreased diameter, with changes in circularity, and with a systematic drift of the hand. These distortions to circle drawing by tendon vibration demonstrate that the CNS uses proprioceptive information to accomplish the spatial characteristics of this motor task. Simultaneous vibration of both muscles produced a drift that exceeded the individual vibration effects, which suggests that the CNS combined proprioceptive information related to elbow and shoulder rotation to control the movement of the hand. The temporal characteristics of circle drawing were quantified from joint angle data. While vibration did not significantly influence the relative phase between elbow and shoulder rotation, the variability of the phase relationship increased significantly, which suggests that proprioception contributes to phase stabilization. During circle drawing, elbow flexion-extension movements were produced with limited activation of the biceps. Nevertheless, biceps vibration distorted the circle metrics, suggesting that a muscle's significance as a sensory transducer is independent of its activity level.