The posture-related interaction between Ia-afferent and descending input on the spinal reflex excitability in humans.

The separate and combined depressive effects induced by vibration and standing on the soleus H-reflex have been studied by administering Achilles' tendon vibration in prone position and during stance. Without vibration, H-reflex amplitude was larger under prone than standing condition. Vibration reduced the reflex both in prone position and even more during stance. When vibration was superimposed to inclined stance (greater EMG background), the reflex was reduced of the same absolute amount as when it was superimposed to normal stance. When vibration was superimposed on stance with minimal or no background EMG, the reflex disappeared. These results confirm that both upright posture and vibration have a strong depressive effect on the H-reflex. They also show that muscle activity during stance is enough for overcoming the reflex depression. These findings provide information about the origin of the disfacilitatory effects on the monosynaptic reflex pathway, contribute to the understanding of the posture-related mechanisms responsible for the modulation of the spinal reflex excitability, and allow arguing in favour of a minor but adaptable role for the short latency stretch reflex in the control of quiet unperturbed stance.