Recurrent inhibition of soleus alpha-motoneurons during a sustained submaximal plantar flexion.

During 10 min of sustained isometric plantar flexion at 20% of maximal voluntary contraction, recurrent inhibition of soleus alpha-motoneurons was studied in 9 healthy subjects (age 22-37 years). Recurrent inhibition was brought about by a conditioning H-reflex and assessed by a test H-reflex delivered 10 ms later. The amplitude of the test H-reflex during the tenth minute of the contraction (16.9 +/- 13.2% of the maximal compound motor action potential) was significantly increased as compared to that during the first minute (9.8 +/- 7.6%), while the conditioning H-reflex remained unchanged. Concomitantly, muscle fatigue was evidenced by a significant increase in amplitude of the soleus electromyogram. The increase of the test-H-reflex amplitude implies that a decrease in recurrent inhibition occurred during the sustained submaximal contraction, which contrasts results from studies on maximal voluntary contractions. These results indicate a modulation of soleus Renshaw interneurons, which is likely to serve the purpose of optimising motor unit recruitment and firing rates of this muscle during a sustained submaximal contraction.