ROLE OF INTRINSIC MUSCLE PROPERTIES IN PRODUCING SMOOTH MOVEMENTS

Human upper limb movement trajectories have been shown to be quite smo oth, in that time derivatives of end point position (r), including d(3 )r/dt(3) (i.e., jerk), appear to be minimized during rapid voluntary r eaching tasks. Studies have suggested that these movements are impleme nted by an optimal neural controller which seeks to minimize a cost fu nction, such as average jerk cost, over the course of these motions, W hile this hypothetical control strategy is widely supported, there are substantial difficulties associated with implementing such a controll er, including ambiguities inherent in transformations from Cartesian t o joint coordinates, and the lack of appropriate transducers to provid e information about higher derivatives of limb motion to the nervous s ystem. Given these limitations, we evaluate the possibility that smoot hing of movement might be induced primarily by the intrinsic mechanica l properties of muscle by recording the trajectories of inertially loa ded muscle with the excitatory input held constant, These trajectories are compared with those predicted by a minimum-jerk optimization mode l, and by a Hill-based muscle model. Our results indicate that traject ories produced by inertially loaded muscle alone are smooth (in the mi nimum-jerk sense), and that muscle properties may suffice to account f or much of the observed smoothing of voluntary motion, obviating the n eed for an optimizing neural strategy.