Comparison of electrically and mechanically induced soleus H-reflex depression

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.

The soleus H-reflex was tested under three different stimulus intensities (25% Hmax, 50% Hmax, and Hmax), 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.

A 3 x 2 (stimulus intensity x treatment condition) repeated measures ANOVA indicated significant main effects for stimulus intensity (FGG = 16.23; p = .003) and condition (FGG = 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% Hmax, 50% Hmax, and Hmax, respectively.

There appears to be more mechanically than electrically stimulated H-reflex depression when testing at 25% Hmax, 50% Hmax, and Hmax. 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.