Effect of support deprivation on the order of motor unit recruitment

Abstract

Motor unit (MU) activity in the leg extensors has been tested by maintenance of a small isometric effort under the conditions of support deprivation enabled by dry immersion with simultaneous mechanic stimulation of the foot support zones. The analysis of MU interspike interval (ISI) histograms in the heads of two leg extensors (m. soleus and m. gastrocnemius lat.) revealed that the order of MU recruitment is extremely dependent on the support input activity. In immersion, the order of MU recruitment was rear-ranged during the isometric effort maintenance test, which revealed weaker involvement of small tonic MUs. Large MUs with longer ISIs (up to 260 ms) and high variability replaced small tonic MUs with relatively short ISIs (100 ms) and low variability. Daily support stimulation under the conditions of immersion was favorable for maintaining the normal pattern of MU recruitment.