Objective: We investigated to what extent the facilitation of the soleus (Sol) Hoffmann (H-) reflex during a phasic voluntary wrist flexion (Jendrássik maneuver, JM) can be modulated by graded plantar flexion force and conditioning wrist flexion force. Methods: The subjects were asked to perform phasic wrist flexion under a reaction time condition. Sol H-reflex was evoked by stimulating the right tibial nerve at various time intervals (50-400 ms) after the 'Go' signal for initiating JM while the ankle was at rest and while plantarflexing. The level of tonic plantar flexion force (isometric contraction of 10, 20 and 30% of maximal EMG) and conditioning wrist flexion (isometric contraction of 30, 50 and 80% of maximum voluntary contraction) during JM was graded systematically. Results: Although JM facilitation could be seen 80-120 ms after the flexor carpi radialis (FCR) EMG onset even while plantarflexing, the magnitude of JM facilitation under plantar flexion was significantly decreased compared to that at rest. The degree of decrease in JM facilitation did not depend on the level of plantar flexion force. In contrast, the degree of JM facilitation was proportional to the level of wrist flexion force while the ankle was at rest and while plantarflexing, though the amount of JM facilitation significantly decreased while plantarflexing. Conclusions: JM facilitation of Sol H-reflex is decreased while performing tonic voluntary contraction of the homonymous muscle. The degree of decrease in JM facilitation is independent of the level of homonymous muscle contraction, but depends on the level of remote FCR contraction. In clinical application, when we intend to elicit a maximum stretch reflex by JM, full relaxation of homonymous muscle should be carefully confirmed. Significance: Our results provide evidence for better understanding of the features of JM and insight into its clinical application.
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