Jendrassik maneuver vs controlled contractions conditioning the excitability of soleus monosynaptic reflexes.

To analyze the factors that influence the conditioning of monosynaptic reflexes by the Jendrassik maneuver, the latter was replaced by a rapid, isolated, reproducible contraction of the wrist extensors; this procedure facilitated reflexes as effectively as the classical Jendrassik maneuver. The results were expressed quantitatively in relation to maximal motor response to allow comparisons to be made when different test reflexes were used. Identical selective contractions produced results that were reproducible in the same subject from one experiment to another. The facilitation depends on the time interval between the onset of the signal to contract and elicitation of the reflex. It develops in 3 distinct phases: the 1st, of moderate intensity, begins before electromyographic activity in the conditioning muscle; the 2nd, of much greater intensity, comes after the beginning of electromyographic activity, rapidly attains a maximum and then decreases progressively; the 3rd phase, of medium intensity, is stable until the end of the contraction. Facilitation also depends on the amplitude of the conditioned reflex and is most marked for one-half the maximum amplitude value and maximum tendon reflexes. The H-reflex is facilitated to the same extent as a tendon reflex of the same amplitude, but only during the 2nd phase. During the 3rd phase, facilitation of the H-reflex is no longer significant, but that of the tendon reflex is. Facilitation of alpha motoneurones may be assumed to explain the increases in amplitude during the 2nd phase but this mechanism alone cannot account for all the phenomena observed.