Mechanisms of decreased motoneurone excitation during passive muscle stretching.

The effect of pre- versus postsynaptic mechanisms in the decrease in spinal reflex response during passive muscle stretching was studied. The change in the electromyographic (EMG) responses of two reflex pathways sharing a common pool of motoneurones, with (Hoffmann or H reflex) or without (exteroceptive or E reflex) a presynaptic inhibitory mechanism, was compared. The EMG activities were recorded in the soleus muscle in response to the electrical stimulation of the tibial nerve at the popliteal fossa (H reflex), and at the ankle (E reflex) for different dorsiflexion angles of the ankle. The compound muscle action potential (M wave) in the soleus and the abductor hallucis was recorded in order to control the stability of the electrical stimulation during stretching. The results indicate that in the case of small-amplitude muscle stretching (10 degrees of dorsiflexion), a significant reduction (-25%; P < 0.05) in the Hmax/Mmax ratio was present without any significant change in the Emax/Mmax ratio. At a greater stretching amplitude (20 degrees of dorsiflexion), the E reflex was found to be reduced (-54.6%; P < 0.001) to a similar extent as the H reflex (-54.2%). As soon as the ankle joint returned to the neutral position (ankle at 90 degrees), the two reflex responses recovered their initial values. In additional experiments, motor-evoked potential (MEP) induced by the magnetic stimulation of the motor cortex was recorded and showed a similar type of behaviour to that observed in the E reflex. These results indicate that reduced motoneurone excitation during stretching is caused by pre- and postsynaptic mechanisms. Whereas premotoneuronal mechanisms are mainly involved in the case of small stretching amplitude, postsynaptic ones play a dominant role in the reflex inhibition when larger stretching amplitude is performed.