Spinal neuronal mechanisms explaining the modulation of soleus H-reflexes during sustained passive rotation of the hip joint.

OBJECTIVES

The aim of this study was to investigate the mechanism of modulation of soleus H-reflexes during sustained passive rotation of the hip joint.

METHODS

Healthy men participated as subjects in this study. In the supine position, soleus maximum H-reflexes were recorded during rotation of the hip joint. To examine the roles of muscle spindles, we recorded H-reflexes while applying vibration to muscles around the hip joint. Furthermore, to assess the modulation of pre- and postsynaptic inputs to soleus motoneurons, H-reflexes were conditioned by common peroneal nerve stimulation or transcranial magnetic stimulation (TMS).

RESULTS

The ratio of the maximum H-reflex to the maximum M-response was significantly reduced in externally rotated positions of the hip joint compared to neutral position. In these positions, presynaptic inhibition, assessed by D1 inhibition, was significantly increased. H-reflexes were reduced during vibration, irrespective of muscles involved and hip position. Facilitation of H-reflex induced by conditioning-TMS showed insignificant differences across all hip positions.

CONCLUSIONS

These findings suggest that Ia afferents induced by rotation of the hip joint presumably inhibit soleus H-reflexes presynaptically.

SIGNIFICANCE

Based on the results, therapists could efficiently modulate the spasticity of the lower legs of hemiplegic patients during rotation of the hip joint.