PERTURBATIONS OF FAST GOAL-DIRECTED ARM MOVEMENTS - DIFFERENT BEHAVIOR OF EARLY AND LATE EMG RESPONSES

Subjects made fast goal-directed elbow flexion movements against an in ertial load. Target distance was 8 or 16 cm, randomly chosen. In 20% o f the trials, the inertial load (the mass) was increased or decreased without the subject's knowledge. The change of mass elicited appreciab le electromyographic (EMG) responses in elbow muscles as well as in sh oulder muscles. The latency of these responses was 25-35 ms, relative to the point at which the velocity of the hand differed by more than 0 .2 m/s from the velocity in the control trials. Whether these response s could be altered during fast movements was tested by giving instruct ions to the subject or by changing the mass during the movement instea d of just after the start. The first 25 ms of the response were found to depend only on the velocity change induced by the perturbation, not on the instruction to the subject, regardless of the phase of the mov ement. The part of the response with a longer latency (60 ms) could ha ve a sign opposite to that of the early response, anticipating future effects of the mass change. Instructions to the subject to stop the mo vement if the mass was unexpectedly large had a slight effect on this late response but no effect on the early response. The sequence of an early stereotyped response and a flexible late response resembles the sequence of responses to muscle stretch, which makes it likely that th e same neural circuits are used. The responses seem more adequate for correcting for changes in inertia than for correcting for position err ors. Probably, the stretch reflex is an epiphenomenon of a mechanism t hat is tuned for dealing with changes in inertia.