Supraspinal Control of Recurrent Inhibition during Anisometric Contractions.

Abstract

Increase in recurrent inhibition was observed during eccentric compared with isometric and concentric maximal voluntary contractions but the neural mechanisms involved in this specific control of the Renshaw cell activity are unknown. This study was designed to investigate the supraspinal control of the recurrent inhibition during anisometric contractions of the plantar flexor muscles. To that purpose, the paired Hoffmann-reflex (H-reflex) technique permitted to assess changes in homonymous recurrent pathway by comparing the modulations of test and conditioning H-reflexes (H' and H1, respectively) in the soleus (SOL) muscle during maximal and submaximal isometric, concentric and eccentric contractions. Submaximal contraction intensity was set at 50% of the SOL electromyographic activity recorded during maximal isometric contraction. Fourteen volunteer subjects participated in an experimental session designed to assess the activity of the recurrent inhibition pathway. The results indicate that the amplitude of H1 normalized to the maximal M-wave were similar (P > 0.05) regardless of the muscle contraction type and intensity. Whatever the contraction intensity, the ratio between H' and H1 amplitudes was significantly decreased (P < 0.05) during eccentric compared with isometric and concentric contractions. Furthermore, this ratio was significantly smaller (P < 0.05) during submaximal compared with maximal contractions whatever the muscle contraction type. Together, the current results confirm the supraspinal control of the Renshaw cell activity when muscle contraction intensity is modulated and show that this control remains similar for isometric, concentric and eccentric contractions. Data further suggest that recurrent inhibition pathway may serve as variable gain regulator at motoneuronal level to improve resolution in the control of motor output for the SOL during eccentric contractions.