THE ROLE OF STRETCH REFLEX THRESHOLD REGULATION IN NORMAL AND IMPAIRED MOTOR CONTROL

Some hypotheses suggest that stretch reflex threshold regulation may b e an essential element of motor control. Disturbances in this mechanis m may lead to motor dysfunction. We investigated this possibility by c omparing stretch reflex threshold regulation in 11 spastic hemiparetic and 6 normal subjects. Subjects sat with their arms fully supported i n a forearm and hand mold attached to a manipulandum mounted on and co ntrolled by a torque motor. They remained completely passive while the ir elbow was extended from 30 degrees flexion through an are of 100 de grees. Displacement and velocity of the forearm were measured as well as EMG signals from 2 elbow flexors and 2 elbow extensors, when the el bow flexors were stretched at each of 7 velocities. Velocities ranged from 8 to 160 degrees/s for hemiparetic subjects and from 32 to 300 de grees/s for normal subjects. Phase diagrams (velocity versus angle) we re plotted and the threshold angles (lambda) for muscle activation at each velocity of stretch were used to determine the static stretch re flex threshold (lambda) and the slope (mu) of the relationship between the lambdas and velocity. Our main findings were that static and dyn amic stretch reflex thresholds were decreased in spastic hemiparetic c ompared to normal subjects and that the thresholds depended on velocit y. The static threshold value correlated with the severity of clinical ly measured spasticity. In addition, the range of regulation of lambda was decreased in the patients compared to normal. This may explain so me of the problems of force and position regulation as well as hyperto nus (and weakness) common to these patients.