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Department of Neurophysiology, University of Copenhagen, Denmark.

1. The inhibition of the soleus Hoffmann reflex (H reflex) during voluntary dorsiflexion of the foot--henceforth referred to as 'natural' reciprocal inhibition--was found to be initiated 50 ms before the onset of the EMG activity in the tibialis anterior muscle and to increase gradually during a ramp-and-hold dorsiflexion. There was a positive correlation between strength of tonic dorsiflexion and amount of 'natural' reciprocal inhibition. 2. The change of activity in the disynaptic and a long-latency group Ia inhibitory pathway and the change in presynaptic inhibition of the Ia fibres mediating the soleus H reflex were tested separately during ramp-and-hold dorsiflexion as well as during tonic dorsiflexion of the foot, and the results were compared with the development of the 'natural' reciprocal inhibition of the unconditioned soleus H reflex. 3. The disynaptic group I inhibition of soleus motoneurones was increased, as compared to rest, during the dynamic phase of a ramp-and-hold dorsiflexion movement, but the inhibition generally did not increase during tonic dorsiflexion of the foot. 4. The long-latency group I inhibition was seen only during dorsiflexion of the foot. It appeared around 50 ms before tibial anterior EMG activity and there was a positive correlation between strength of tonic dorsiflexion and amount of this long-latency inhibition. 5. Presynaptic inhibition of Ia afferents terminating on soleus motoneurones was estimated by an indirect method. The increase of presynaptic inhibition started soon after the onset of the ramp-and-hold dorsiflexion, and gradually became more pronounced during the ramp phase. The amount of presynaptic inhibition was positively correlated with strength of tonic dorsiflexion. 6. It is concluded that all investigated mechanisms may contribute to the 'natural' reciprocal inhibition and it seems that the different pathways are used differentially during different types of movement.

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