Characteristics of Soleus H-Reflex Variability During the Standing Position Estima Ted by HRV

It is well known that the successive H-reflexes, evoked by stimulation of constant intensity, show considerable variability. To investigate physiological factors affecting this H-reflex variability, we elicited the successive soleus H-reflexes by stimulating the post-tibial nerve at a constant strength during the standing and supine position. Comparison between the soleus H-reflex amplitudes, recorded from 7 subjects showed that the variability of H-reflexes on standing were greater than those in the supine position. Then, spectral analysis was used to evaluate the degree of the H-reflex variability and the effect of autonomic activation. The total power (TP) value of the H-reflex on standing (0.2961±0.0618 mV⋅mV) was significantly greater (p<0.001) than that in a supine position (0.0565±0.0080mV⋅mV), which may indicate that the soleus H-reflexes while standing have a large variability. Such variation of the H-reflex may be explained by a change in the soleus motoneuron activity. The values of peak power frequency (PPF) obtained from the soleus H-reflex while both standing and supine, were about 0.17Hz. This would suggest that variations in H-reflexes were influenced by respiration. The electrocardiogram (ECG) and respiration curves were recorded simultaneously when successive soleus H-reflexes were evoked. Variations of RR interval obtained from ECG recordings were analyzed as well as H-reflex variations and the TP values in the supine position (1254.4676±150.3057 msec⋅msec) were significantly (p<0.001) larger than those while standing (566.0482±60.4576 msec⋅msec) . Furthermore, we examined the relationship between the variability of H-reflex and RR interval. There were significant correlations between the H-reflex and RR interval when standing in TP (r= -0.3886, p=0.0254) and PPF (r=0.5042, p=0.0028) values. Besides, the amplitudes of H-reflex while standing that evoked during both the inspiratory and expiratory period were increased. This increase in H-reflex might be due to the effect of postural elements of the respiratory impulse. These results indicated that the variability of soleus H-reflexes at standing might imply autonomic activation and descending effects from central nervous system depending on postural control.