The H-Reflex Changes During Wrist Flexion and Wrist Extension

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The aim of this study was to find the optimal H-reflex testing conditions regarding the wrist position and forearm muscle contractions. We investigated the effects of weak (at 10% and 20 % of maximal voluntary contraction) isometric wrist flexion and wrist extension on the Hreflex amplitudes in flexor carpi radialis (FCR). In addition, the H-reflexes during passive wrist flexion and passive wrist extension were also evaluated and compared with that at the neutral wrist position. Thus, the factors regarding contractions and mechanical properties to the changes of FCR H-reflex could be scrutinized. The H-reflex amplitudes were significantly different in the seven experimental conditions used in this study, while M-responses were not. Compared to resting neutral wrist position, the H-reflex amplitudes were significantly larger during 10% and 20% wrist flexion. On the contrary, the H-reflex amplitudes during passive and active wrist extensions were all smaller and significantly different from those at neutral positions. However, the H-reflex amplitudes during passive wrist flexion could not be distinguished from that at neutral position. The results indicated that wrist flexion itself exerted a major excitatory input on the alpha motoneurons, while the wrist extension was the contrary. In addition, passive stretch of FCR significantly reduced the H-reflex amplitude, though the passive flexion of the wrist had no effect. This study showed that the FCR H-reflex was strongly modulated in the wrist isometric contraction; high during wrist flexion, and low during wrist extension. This may be functionally important. The results also indicated that minimal tonic contraction (10% MVC) in the wrist flexion position was sufficient to obtain the best test condition. These findings also extend the diagnostic and therapeutic utility of H-reflex testing. In order to maximize the value of reflex studies, the effects of contraction level and joint position must be taken into consideration when setting the test conditions.