INCREASE IN RECIPROCAL IA INHIBITION DURING ANTAGONIST CONTRACTION INTHE HUMAN LEG - A STUDY OF MOTOR UNITS AND THE H-REFLEX

1. The change in reciprocal Ia inhibition of soleus motoneurones produ ced by stimulation of the common peroneal nerve was investigated by th e use of twenty-three soleus motor units as well as the soleus H refle x 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 u nit with the probability of 50%. The conditioning effect, assessed fro m the change in the CFS, was expressed as a percentage of the uncondit ioned CPS. 3. At a conditioning intensity of 0.95 times the motor thre shold value, there was Ia inhibition in sixteen of the twenty-three mo tor units (69.6%) at rest. Of these sixteen motor units, twelve showed increases in inhibition at intervals below 2.0 ms during pretibial co ntraction. In four of the remaining seven units, inhibition first appe ared during contraction. There was no significant decrease in inhibiti on at any time during contraction. 4. Based on the conventional H refl ex, reciprocal Ia inhibition increased during very weak (below 2% of t he maximum) voluntary dorsiflexion and continued to increase at a slig htly stronger (3-8% of the maximum) contraction, then decreased contin uously when contraction was strengthened further. Maximal inhibition o ccurred at a relatively strong contraction when a weak conditioning st imulus was used, and vice versa. 5. We conclude that the activity of r eciprocal Ia inhibitory interneurones increases during tonic antagonis t contraction. The previous controversy about this inhibition is the r esult of occlusion at the Ia interneuronal level.