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Changes in presynaptic inhibition of Ia fibres at the onset of voluntary contraction in man.

1. Two independent methods were used, in man, to assess changes in presynaptic inhibition of I a terminals at the onset of selective voluntary contractions: (1) measurement of the amount of heteronymous monosynaptic I a facilitation (from the quadriceps muscle to soleus motoneurones) to provide an assessment of the amount of ongoing presynaptic inhibition exerted on the I a fibres responsible for the facilitation; (2) measurement of the inhibition of H reflexes 40‐60 ms after a short vibration to the tibialis anterior tendon to estimate the excitability of the interneurones mediating presynaptic inhibition from tibialis anterior I a afferents to the I a afferents of the test H reflex (soleus or quadriceps). 2. At the onset of an isolated voluntary plantar flexion of the foot (gastrocnemius‐soleus contraction) the heteronymous facilitation from quadriceps to soleus was increased, reflecting a decreased presynaptic inhibition of the quadriceps I a terminals on soleus motoneurones. Vibratory inhibition of the soleus H reflex was decreased, reflecting an inhibition of transmission of presynaptic inhibition to homonymous soleus I a afferent terminals. 3. At the onset of the same gastrocnemius‐soleus contraction there was, on the contrary, an increased vibratory inhibition of the quadriceps H reflex indicating a facilitation of transmission of presynaptic inhibition to homonymous quadriceps I a afferent terminals. 4. At the onset of an isolated voluntary knee extension (quadriceps contraction) the opposite pattern was seen: the heteronymous facilitation from quadriceps to soleus was decreased and the vibratory inhibition of a soleus H reflex was increased, whereas the vibratory inhibition of the quadriceps H reflex was decreased. 5. These results indicate that presynaptic inhibition of I a afferent terminals on motoneurones of contracting muscles is decreased, permitting I a activity to contribute to excitation of voluntarily activated motoneurones. On the contrary, presynaptic inhibition of I a fibres to motoneurones of muscles not involved in the contraction is increased. It is argued that the former must be supraspinal in origin. 6. It is concluded that the control of presynaptic inhibition of I a fibres at the onset of movement may be organized so as to aid in achieving selectivity of muscle activation, i.e. so as to increase motor contrast.