| This work examined how humans react to unexpected mechanical perturbations during reaching movement. Important aspects of this behavior were examined and a mathematical model of a limited portion of the response to perturbations was developed. In a study of planar point-to-point reaching motion, small force pulses were unexpectedly applied during one of various stages of a repeated movement (early, mid, and late) to observe whether there were permanent changes in endpoint without vision of the limb. The statistical analysis of endpoint shift vs. unperturbed motion only used movements in which there was insufficient time to react voluntarily to the perturbation (as measured in a supplementary experiment). The results of the study showed the lack of stability of the endpoint in the face of transient perturbations during movement, a finding inconsistent with the "Equilibrium Point Hypothesis" (EPH), a well known theory of motor control.;In a second study involving point-to-point extensions of the elbow, pulses were again applied during various stages of movement. Since we expect reflexes will not have sufficient time to influence elbow trajectory within the 50 ms of perturbation onset, to which a K-B-I (stiffhess-damping-inertia) model was fit, intrinsic rather than effective muscle and limb properties were measured. Reflex activity was monitored using EMG recording. Although co-contraction increased stiffness greatly, stiffness was not significant either in the moving or the relaxed limb, although the damping coefficient was always significant and increased slightly at the end of movement. The presence of significant damping allows the programming of agonist, then antagonist bursts of activation which do not have to be precisely matched to achieve movement termination.;In conclusion, for novel dynamics or unexpected loads, resistance is initially provided by limb properties such as viscosity and inertia. Then the stretch reflex intervenes, although limited in gain by its automatic nature, followed by non-automatic supraspinal responses, which are able to co-contract the limb. This altered muscle activity, which achieves the task, could then be used to aid in motor learning. |
