Anticipatory responses to a self-applied load in normal subjects and hemiparetic patients

A bimanual loading task was studied in eight right-handed normal subjects and nine hemiparetic patients in order to detect anticipatory adjustments and to analyse the reflex and voluntary, responses induced by the perturbation. The left forearm (or the impaired side in patients) was 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 hemiparetic 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-applied situation, but not in the control situation. The anticipatory flexion of hemiparetic patients was slower and longer. The amplitude of the anticipatory flexion at the time of impact and its duration were correlated with the mass of the load and the initial distance between the two hands in both groups. The anticipatory flexion reduced the distance through which the load fell and thus its kinetic energy at impact. The impact induced a brisk extension which was always smaller in the self-applied situation for a given kinetic energy. In normal subjects, the amplitude of the monosynaptic reflex (MSR) following the impact in flexor muscles was usually greater in the self-applied situation, but its gain was either reduced or unchanged. The gain of the functional stretch reflex (FSR) was consistently reduced in the self-applied situation. Depression of the FSR gain occurred in only two hemiparetic patients who had the best recovered motor function. Anticipation always ended up minimizing the perturbation following different motor strategies. The normal subjects fell into two groups. One group adjusted the anticipatory 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 intermediate behavior.