INFLUENCES OF MUSCLE STRETCH REFLEXES ON VOLUNTARY, VELOCITY-CONTROLLED MOVEMENTS IN SPASTIC PARAPARESIS

We studied voluntary, velocity-controlled knee movements in 22 patient s with spastic paraparesis (11 male, 11 female) and 22 healthy control s (11 male, 11 female). Torque and EMG activity of the quadriceps and the hamstring muscles were determined in maximal voluntary concentric (shortening) and eccentric (lengthening) actions of knee extensor and flexor muscles at constant movement velocities of 30, 60, 120 and 180 degrees/s, using an active, isokinetic dynamometer. In the spastic pat ients, the voluntary strength and the agonist EMG activity were reduce d in all movements. The reduction was largest in concentric actions at high velocity. The antagonist EMG activity was reduced in the same pr oportion as the agonist EMG activity in eccentric actions, In concentr ic actions when stretch is imposed upon antagonists, the antagonist EM G activity increased with the velocity of stretch, indicating stretch reflex activation. In parallel with the stretch reflex activation of a ntagonists, there was reduced activation of the agonists compatible wi th Ia reciprocal inhibition of agonist motoneurons. When agonists were stretched in eccentric actions, stretch reflexes appeared to support the voluntary, agonist activation of knee flexor muscles but not knee extensors.