Heteronymous monosynaptic Ia facilitation from supine to standing and its relationship to the soleus H-reflex

To measure changes in presynaptic inhibition, 10 subjects (5 male, 5 female) were tested under two conditions: supine and standing. This study utilized the heteronymous facilitation protocol, as described by Hulborn et al. (1987a), to measure presynaptic inhibition of the Ia afferent pathway onto the soleus alpha-motoneuron pool. The magnitude of the facilitation produced by the conditioning stimulus provides an indirect assessment of presynaptic inhibition from supine to standing. Maximal soleus H-reflex (H-max) and motor response (M-max) amplitudes were determined prior to testing at each condition. Subjects received 24 test H-reflex stimuli (approximately 15% M-max), and 24 conditioned stimuli at each body position. Results demonstrated a significant decrease in H-max/M-max ratio from supine (68.7%) to standing (54.8%). This was the result of changes in H-max between the two body positions with no significant changes in M-max. Significant inhibition of the conditioned H-reflex was also demonstrated from supine to standing (30.7% M-max vs 17.5% M-max). Furthermore, it was demonstrated that a strong correlation (r = .85) existed between individual changes in H-max/M-max ratio and the changes in facilitation of the conditioned H-reflex from supine to standing. This relationship helps explain the modulation of the H-reflex during static changes in body position, and it could also provide insight into the reflex modulation associated with more functional activities such as walking or running. These results are consistent with the hypothesis that presynaptic inhibition increases as body position is changed from supine to standing.