Antagonist inhibition during rest and precontraction

The excitability of antagonist soleus motoneurons was tested during fast voluntary contractions of tibialis anterior (TA). Contractions of TA started either from zero or higher levels of tonic contraction of ankle extensors. Results showed that H reflex depression under 3 different conditions: 25% (R25) or 50% (R50) of the maximal isometric dorsiflexion from the resting state and the sequential isometric response (SW) appeared about 20-40 msec prior to the EMG onset of TA. In particular, H reflex depression was clear in the SW response, and the stronger the prior contraction of extensors the greater the H reflex depression. There was a significant difference in the integrated EMG between R25 and SW but none between R50 and SW. The different amounts of inhibition found for dorsiflexion from the resting state and sequential movement is most probably explained by presynaptic inhibition. If presynaptic inhibition were increased throughout the sequential movement, in comparison with the resting state, change in the H reflex gain would occur.