International Society for Posture and Gait Research edition:19 location:Bologna- Italy date:21-25 June 2009
INTRODUCTION: In studies exploring the interaction between upper and lower limbs it was found that Soleus H-reflexes are suppressed by rhythmic arm movements[1-4]. Furthermore in some of these studies[2,3] it was shown that there was a phase-dependent modulation of the reflexes while others, using cycling arm movements, did not find this[4,5]. In order to be under the control of central gating the modulation of the reflexes should differ from the modulation of the background. Hence, the question remains whether phase-dependency is present due to arm movements and whether the modulation is premotoneuronal (independent of modulation of background activity).
METHODS: To reexamine this issue a study is needed in which the variations in background EMG are compared to those seen in the H-reflexes sampled over many different phases of the cycle. Subjects were sitting while they made rhythmic arm movements of different types either in the sagittal plane (in one arm only, bilateral in phase or alternating) or in the frontal plane (bimanual abducting), guided by a metronome. Background EMG in soleus was measured as RMS in a 50 ms period prior to stimulation and compared to the peak to peak H-reflex amplitude. Stimuli were given in one of eight phases during the arm cycle.
RESULTS: Soleus H-reflexes were suppressed by the arm movements as expected. In addition in some conditions (ipsilateral arm swing) there was a phase-dependent modulation of the H-reflex amplitude. This modulation did not follow the changes in soleus background activity which was kept constant.
CONCLUSIONS: The present findings confirm the general observation that H-reflexes are suppressed by arm movements. Evidence for a phasic modulation of arm rhythm generators on these reflexes was also present during sitting. This premotoneuronal gating of H-reflexes may be due to phasic presynaptic inhibition, as suggested by others.
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