Horizontal-plane arm movements with direction reversals performed by normal individuals and individuals with Down syndrome

We examined the systematic variation in shoulder and elbow torque, as well as movement kinematics, for horizontal-plane arm movements with direction r eversals performed by normal individuals and individuals with Down syndrome . Eight neurologically normal individuals and eight individuals with Down s yndrome performed horizontal, planar reversal movements to four different t arget locations. The four locations of the targets were chosen such that th ere is a systematic increase in elbow interaction torque for each of the fo ur different target locations. This systematic increase in interaction torq ue has previously been shown to lead to progressively larger movement rever sal errors, and trajectories that do not show a sharp reversal of direction , for movements to and from the target in patients who have proprioceptive abnormalities. We computed joint torques at the elbow and shoulder and foun d a high correlation between elbow and shoulder torque for the neurological ly normal subjects. The ratio of joint torques varied systematically with t arget location. These findings extend previously reported findings of a lin ear synergy between shoulder and elbow joints for a variety of point-to-poi nt movements. There was also a correlation between elbow and shoulder torqu e in individuals with Down syndrome, but the magnitude of the correlation w as less. The ratio of joint torques changed systematically with target dire ction in individuals with Down syndrome but was slightly different from the ratio observed for neurologically normal individuals. The difference in th e ratio was caused by the generation of proportionately more elbow torque t han shoulder torque. The fingertip path of individuals with Down syndrome s howed a sharp reversal in moving toward and then away from the target. In t his respect, they were similar to neurologically normal individuals but dis similar to individuals with proprioceptive deficits. Finally, we observed t hat individuals with Down syndrome spend proportionately more time in the v icinity of the target than normal individuals. Collectively these results s how that there is a systematic relationship between joint torques at the el bow and shoulder. This relationship is present for reversal movements and i s also present in individuals with Down syndrome.