Multivariate analysis of simultaneously active motor units in human skeletal muscle

The joint activity of simultaneously active motor units was analyzed, using several methods including interunit interval and expectation density functions. The results show that there is no significant interaction between any motor units and, except for a slight coupling, the units fire independently of one another. The coupling is weak but pervasive and affects motor units overlapping and disjoint territories. Synchronous action potentions were recorded at sites whose distance is greater than the acknowledged diameter of motor unit territories. It is proposed that these expansive motor units and the viscoelastic properties of muscle are responsible for smooth contractions in human muscle.This work was supported in part by the Social Rehabilitation Service through the Krusen Research Center, Temple University, Philadelphia, Pennsylvania, while Dr. Shiavi was with the Biomedical Engineering and Science Program, Drexel University, Philadelphia, Pennsylvania, and the Krusen Research Center.