DIRECTIONAL CONTROL OF PLANAR HUMAN ARM MOVEMENT

We examined the patterns of joint kinematics and torques in two kinds of sagittal plane reaching movements. One consisted of movements from a fixed initial position with the arm partially outstretched, to diffe rent targets, equidistant from the initial position and located accord ing to the hours of a clock. The other series added movements from dif ferent initial positions and directions and >40-80 cm distances. Dynam ic muscle torque was calculated by inverse dynamic equations with the gravitational components removed. In making movements in almost every direction, the dynamic components of the muscle torques at both the el bow and shoulder were related almost linearly to each other. Both were similarly shaped, biphasic, almost synchronous and symmetrical pulses . These findings are consistent with our previously reported observati ons, which we termed a linear synergy. The relative scaling of the two joint torques changes continuously and regularly with movement direct ion. This was confirmed by calculating a vector defined by the dynamic components of the shoulder and elbow torques. The Vector rotates smoo thly about an ellipse in intrinsic, joint torque space as the directio n of hand motion rotates about a circle in extrinsic Cartesian space. This confirms a second implication of linear synergy that the scaling constant between the linearly related joint torques is directionally d ependent. Multiple linear regression showed that the torque at each jo int scales as a simple linear function of the angular displacement at both joints, in spite of the complex nonlinear dynamics of multijoint movement. The coefficients of this function are independent of the ini tial arm position and movement distance and are the same for all subje cts. This is an unanticipated finding. We discuss these observations i n terms of the hypothesis that voluntary, multiple degrees of freedom, rapid reaching movements may use rule-based, feed-forward control of dynamic joint torque. Rule-based control of joint torque with separate dynamic and static controllers is an alternative to models such as th ose based on the equilibrium point hypotheses that rely on a positiona lly based controller to produce both dynamic and static torque compone nts. It is also an alternative to feed-forward models that directly so lve the problems of inverse dynamics. Our experimental findings are no t necessarily incompatible with any of the alternative models, but the y describe new, additional findings for which we need to account. The rules are chosen by the nervous system according to features of the ki nematic task to couple muscle contraction at the shoulder and elbow in a linear synergy. Speed and load control preserves the relative magni tudes of the dynamic torques while directional control is accomplished by modulating them in a differential manner. This control system oper ates in parallel with a positional control system that solves the prob lems of postural stability.