| The mechanisms contributing to disturbances of motor function in hemiparetic patients remain poorly understood. Clinically, hemiparesis is accompanied by abnormalities of muscle tone and disturbances of coordination, which are particularly debilitating and difficult to characterize. This dissertation examined disturbances of upper limb motor function in hemiparetic patients under both dynamic and static conditions with an emphasis on addressing the mechanisms underlying incoordination of voluntary movement.; The dynamic studies focused on spatial disturbances of rapid point-to-point movements performed in the horizontal plane, and their dependence on target direction and support condition. Movements of the passively supported paretic limb were found to be misdirected systematically in association with an excessive rotation of the elbow relative to the shoulder. An inverse dynamics analysis revealed an abnormal spatial tuning of the elbow torque used to initiate movement, which could not be attributed to previously identified mechanisms. It is hypothesized that the spatial abnormalities reflect a deterioration in the internal (cerebral) representation of limb dynamics, either as a primary consequence of brain injury, or secondary to altered usage.; In the actively supported condition, the ability to perform movements with the paretic limb deteriorated, particularly for targets requiring an initial elbow extension torque. This finding is hypothesized to reflect an abnormal muscular coupling, or "synergy", involving the shoulder abductors and elbow flexors.; The aims of the static studies were to examined abnormalities in the torque synergies exhibited by the paretic limb during the generation of maximum voluntary torques at the shoulder and elbow and to characterize the residual strength of torque generation and its task dependence. It was found that the torque synergies exhibited by the paretic limb differed from those observed for the ipsilesional limb and control group and largely paralleled the classical clinical descriptions of the flexor and extensor movement synergies. Contralesional weakness was strongly task dependent, in a manner consistent with a limited ability to generate torques in combinations deviating from the identified abnormal synergies.; Results of the movement and isometric studies are synthesized to provide additional insights into potential mechanisms, and the implications of these studies for therapy are addressed. |
