ANTICIPATORY RESPONSES TO A SELF-APPLIED LOAD IN NORMAL SUBJECTS AND HEMIPARETIC PATIENTS

A bimanual loading task was studied in eight right-handed normal subje cts and nine hemiparetic patients in order to detect anticipatory adju stments and to analyse the reflex and voluntary responses induced by t he perturbation. The left forearm (or the impaired side in patients) w as flexed at approximately 90 degrees and free to rotate in a vertical sagittal plane. It was held to resist a load (2-3 kg) dropped either by an experimenter (control situation), or by the subject himself with no visual control (self-applied situation). The load was dropped from the right hand by normal subjects and from the unimpaired hand by hem iparetic patients. The initial distance the load fell was 0.05-0.35 m. The elbow movements of the limb receiving the load were recorded with a linear accelerometer at the wrist, a potentiometer at the elbow and via the EMG signals from flexor muscles. Normal subjects always made an anticipatory flexion movement prior to the impact in the self-appli ed situation, but not in the control situation. The anticipatory flexi on of hemiparetic patients was slower and longer. The amplitude of the anticipatory flexion at the time of impact and its duration were corr elated with the mass of the load and the initial distance between the two hands in both groups. The anticipatory flexion reduced the distanc e through wich the load fell and thus its kinetic energy at impact. Th e impact induced a brisk extension which was always smaller in the sel f-applied situation for a given kinetic energy. In normal subjects, th e amplitude of the monosynaptic reflex (MSR) following the impact in f lexor muscles was usually greater in the self-applied situation, but i ts gain was either reduced or unchanged. The gain of the functional st retch reflex (FSR) was consistently reduced in the self-applied situat ion. Depression of the FSR gain occurred in only two hemiparetic patie nts who had the best recovered motor function. Anticipation always end ed up minimizing the perturbation following different motor strategies . The normal subjects fell into two groups, One group adjusted the ant icipatory forearm flexion to correct the extension disturbance as fast as possible: the second group relied on an accurate adjustment of the final position. Hemiparetic patients showed idiosyncratic intermediat e behavior.