Increase in reciprocal Ia inhibition during antagonist contraction in the human leg: a study of motor units and the H reflex.

1. The change in reciprocal Ia inhibition of soleus motoneurones produced by stimulation of the common peroneal nerve was investigated by the use of twenty-three soleus motor units as well as the soleus H reflex in six normal subjects during tonic pretibial contraction. 2. In the motor unit experiments, motoneuronal excitability was measured as the 'critical firing stimulus' (CFS), which is the difference between the test stimulus intensity needed to reach the threshold for the lowest threshold Ia fibres and the intensity which evokes firing of a motor unit with the probability of 50%. The conditioning effect, assessed from the change in the CFS, was expressed as a percentage of the unconditioned CFS. 3. At a conditioning intensity of 0.95 times the motor threshold value, there was Ia inhibition in sixteen of the twenty-three motor units (69.6%) at rest. Of these sixteen motor units, twelve showed increases in inhibition at intervals below 2.0 ms during pretibial contraction. In four of the remaining seven units, inhibition first appeared during contraction. There was no significant decrease in inhibition at any time during contraction. 4. Based on the conventional H reflex, reciprocal Ia inhibition increased during very weak (below 2% of the maximum) voluntary dorsiflexion and continued to increase at a slightly stronger (3-8% of the maximum) contraction, then decreased continuously when contraction was strengthened further. Maximal inhibition occurred at a relatively strong contraction when a weak conditioning stimulus was used, and vice versa. 5. We conclude that the activity of reciprocal Ia inhibitory interneurones increases during tonic antagonist contraction. The previous controversy about this inhibition is the result of occlusion at the Ia interneuronal level.