Effect of the Jendrassik manoeuvre on a phasic stretch reflex in normal human subjects during experimental control over supraspinal influences

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Abstract
Electromyographic experiments with the patellar reflex were conducted on 4 human subjects. A Jendrassik Manuver was executed briefly during normal relaxation, active contraction of the rectus femoris, intake of Myanesin, an interneuronal blocking agent, and normal conditions. Mean amplitude of reflex response increased during normal trials and pre-strain trials. Variability of the impulse of the muscle action potential during Jendrassik maneuver (1 sec.) trials significantly increased when compared with controls. No increase in variability occurred when fusimotor activity was assumed to be attenuated during pre-strain Jendrassik maneuver (10 sec.) and peak drug effect. Results indicate that in addition to spindle biasing, direct potentiation of alpha spinal cells by supraspinal influences play some role in the facilitatory mechanism for the phasic stretch reflex.