Recurrent inhibition in the soleus motor pool of elderly and young adults.

Recurrent inhibition in the spinal cord has been suggested to serve as a variable gain regulator to allow for optimal muscle force control, to influence alpha-motoneuron firing rate, and to contribute to task related motor synergies between muscles at the same or different joints. The purpose of this study was to examine the resting recurrent inhibition levels in the soleus motoneuron pool of 20 elderly and 21 young adult subjects. To assess recurrent inhibition, a conditioning electrical stimulus was used to activate group Ia afferent fibers and elicit a reflex response in some of the a-motoneurons innervating the soleus muscle; producing both activation of Renshaw interneurons excited by those involved soleus a-motoneurons via a recurrent branch of the a-motoneuron axon, and an H-reflex response in the soleus muscle. A H' test reflex elicited by a successive supramaximal stimulus to the same nerve 10 ms after the conditioning stimulus evaluated the resulting inhibitory effect. There was no difference in the H' test reflex amplitude when comparing the young and elderly adult subjects. This result was found following two different methods employed to control for a possible effect on the H' test reflex amplitude of a smaller maximum H-reflex amplitude in the elderly subjects. These results indicate that the level of recurrent inhibition in the motoneuron pool of the resting soleus muscles of the young and elderly adults examined was not significantly different.