We plan to study two different aspects of the disturbance in movement associated with spastic hemiparesis. Our subject group will consist largely of stroke and head trauma victims. First, stretch reflex dynamics and alterations in mechanical properties of elbow muscles and related soft-tissues will be estimated from the EMG and torqued responses elicited by a variety of perturbations, including pseudorandom binary pulse (PRBS) and sinusoidal perturbations of elbow joint angle. We anticipate that these measurements will reveal no systematic modification of reflex dynamics in spastic limb (as compared with the contralateral side) and that increases in measured torque will result from alterations in viscoelastic properties of muscle and of other elbow soft tissues. Second, the nature and origins of the disturbance in muscle synergy that accompanies spastic hemiparesis will be evaluated, using simultaneous intramuscular EMG recordings from most of the significant elbow muscles during precisely regulated isometric contractions of elbow musculature. Synergic relations will be characterized by plots of EMG in one muscle against EMG in a synergist, for a constant torque direction, over increasing torque magnitudes. Major disruptions of synergic relations which are insensitive to voluntary control or to alterations in the limb mechanical load will indicate that new sources of motoneuron innervation have developed. This finding may, in turn, indicate the deelopment of new connections to motoneurons or simply the preferential use of preexisting but less appropriate pathways. Understanding the characteristics and origins of disturbances in synergy is of great potential significance in understanding the origin of motor dysfunction in hemiparesis, and it will also help define potential mechanisms of functional recovery.