Comparison of electrically and mechanically induced soleus H-reflex depression

Objectives: The aim of the present investigation was to compare the responses of mechanically-evoked and electrically-evoked conditioning among three different stimulus intensities and provide possible insight in determining whether these two types of inhibitory conditioning demonstrate the same response across the motor pool. Methods: The soleus H-reflex was tested under three different stimulus intensities (25%H_max, 50%H_ma'x, and H_max), and peak to peak amplitudes were recorded after either mechanical or electrical conditioning. For electrical conditioning, the ankle was held at 90 degrees, whereas mechanical conditioning utilized a 10 degree passive dorsiflexion at 15 degrees/sec back to the 90 degree position. All H-reflex measurements were recorded with the ankle at 90 degrees. Results: A 3 × 2 (stimulus intensity x treatment condition) repeated measures ANOVA indicated significant main effects for stimulus intensity (F_GG=16.23, p=.003) and condition (F_GG=28.48; p<.001), as well as an interaction (F_(2,18)=6.59, p=.008). Simple main effects identified significantly more H-reflex depression for mechanical conditioning (94%, 91%, and 44%) compared to electrical conditioning (65%, 55%, and 19%) at 25%H_max, 50%H_max, and H_max, respectively. Conclusions: There appears to be more mechanically than electrically stimulated H-reflex depression when testing at 25%H_max, 50%H_max, and H_max. These results demonstrate the influence of Ia discharge properties on H-reflex depression and may suggest differences in the affinity of Ia terminals to intrinsic presynaptic inhibition at different stimulus intensities.