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Abstract
Some hypotheses suggest that stretch reflex threshold regulation may be an essential
element of motor control. Disturbances in this mechanism may lead to motor dysfunction.
We investigated this possibility by comparing stretch reflex threshold regulation
in 11 spastic hemiparetic and 6 normal subjects. Subjects sat with their arms fully
supported in a forearm and hand mold attached to a manipulandum mounted on and controlled
by a torque motor. They remained completely passive while their elbow was extended
from 30 degrees flexion through an arc of 100 degrees. Displacement and velocity of
the forearm were measured as well as EMG signals from 2 elbow flexors and 2 elbow
extensors, when the elbow flexors were stretched at each of 7 velocities. Velocities
ranged from 8 to 160 degrees/s for hemiparetic subjects and from 32 to 300 degrees/s
for normal subjects. Phase diagrams (velocity versus angle) were plotted and the threshold
angles (lambda) for muscle activation at each velocity of stretch were used to determine
the static stretch reflex threshold (lambda) and the slope (mu) of the relationship
between the lambda s and velocity. Our main findings were that static and dynamic
stretch reflex thresholds were decreased in spastic hemiparetic compared to normal
subjects and that the thresholds depended on velocity. The static threshold value
correlated with the severity of clinically measured spasticity. In addition, the range
of regulation of lambda was decreased in the patients compared to normal. This may
explain some of the problems of force and position regulation as well as hypertonus
(and weakness) common to these patients.