The effect of the ankle joint angle in the level of soleus Ia afferent presynaptic inhibition.

The factors that are responsible for the relationship between motoneuron excitability and muscle length may have both mechanical and/or neurophysiologic origins. The aim of the study was to investigate the changes in the level of presynaptic inhibition, as measured with a soleus H-reflex conditioning protocol, and muscle length. Ten healthy volunteers were measured at three different ankle angles: 30 degrees plantar flexion, neutral position (0 degrees) and 15 degrees dorsiflexion. At each position the soleus H-reflex and the maximum M-wave were measured while the limb was relaxed. The H-reflex was conditioned by a stimulation of the common peroneal nerve, 100 ms prior to the tibial nerve stimulation. The results revealed that the level of presynaptic inhibition was higher at the neutral position in comparison to the dorsiflexed or plantarflexed positions. Additionally, the HMAX/MMAX ratio was significantly decreased when the joint position was set at dorsiflexion. Further, there was a significant correlation, independent of ankle joint angle, between presynaptic inhibition levels and the HMAX/MMAX ratio. The above findings support the concept that peripheral feedback from passive, static modifications in the joint angle and consequently in muscle length, can modify the input/output threshold of the motoneurons on a presynaptic level.