Mechanism of the vibration paradox: excitatory and inhibitory effects of tendon vibration on single soleus muscle motor units in man.

1. The parameters of presynaptic inhibition of the Ia spindle afferents from soleus muscle by vibration have been investigated. The inhibitory effects increase with the amplitude of vibration, but decrease when the vibration frequency is increased.2. The monosynaptic reflex threshold of twenty-one single soleus motor units activated in the H (Hoffmann) reflex by a single electrical stimulus to the posterior tibial nerve was estimated quantitatively and expressed in relation to the size of the simultaneously recorded H reflex.3. A parametric study of the effects of various Achilles tendon vibrations on the reflex threshold of the single soleus motor units indicated that their order of derecruitment is concordant with their rank order for activation in the phasic reflexes of the soleus. The last recruited motoneurones are the most susceptible to being silenced by steady vibration.4. Muscle vibration progressively recruits single motor units according to the motoneurone size principle through polysynaptic proprioceptive pathways. However the presynaptic inhibition of Ia spindle afferents simultaneously induced by the vibration works in reverse on the same rank order of motoneurones of the soleus spinal pool, thereby limiting the polysynaptic recruitment of units in the tonic vibration reflex while depressing the autogenic phasic proprioceptive reflexes. These mechanisms elucidate the so-called vibration paradox and extend the size principle of Henneman to presynaptic inhibitory effects.