Perturbation of a skilled action 1. The responses of neurologically normal and cerebral palsied individuals

Subjects were trained to execute rhythmical forward and backward arm swings, with consistent timing and displacement characteristics. A DC servomotor was used to introduce brief perturbations, opposing elbow extension. Pilot studies indicated that: (1) Weak disturbances tended to foreshorten the voluntary action, while larger perturbations (strong enough to reverse the ongoing action) caused positional overshooting. (2) When prolonged ( > 500 msec) perturbations were investigated, there was no evidence that EMG responses within the first 100 msec provided much force to counter the disturbance. (3) Subjects were unable to learn to curb the steep rises in velocity observed when strong perturbations were released, despite extensive feedback experience. The main study confirmed that electromyographic and movement parameters reflect the magnitude of the perturbation. The actions of cerebral palsied subjects were more disrupted by perturbations, abnormal antagonist activity and clonus were recorded. Models of compensation are discussed.