Modulation of the stretch reflex during volitional sinusoidal tracking in Parkinson's disease. Sinusoidal visually-guided wrist tracking, in normal and parkinsonian subjects, was perturbed by torque transients every 90 degrees throughout the movement. Long-latency stretch reflex and volitional EMG amplitude modulations were assessed as functions of the tracking phase. Reflex modulation during tracking, both in wrist flexor and extensor muscles, was found to differ significantly between parkinsonian and normal subjects. In the parkinsonian group, the abnormality consisted of an increased reflex activity during tracking phases in which the muscle was lengthening. At these phases the reflex generated torque is opposite in direction to the volitionally generated torque and the tracking movement. No differences in the unperturbed volitional EMG modulation were observed between groups for this error constrained tracking paradigm. Significant correlations were found between ratings of bradykinesia and the amount of abnormal reflex modulation in the wrist flexor. These data suggest that a component of bradykinesia results from a defective coordination of supraspinal reflex and volitional control systems. 