Motor units activity of dog postural muscles in standing position.

Discharges of separate motor units (MU) of dog muscles are studied in standing position and under conditions of passive stretch and shortening of the muscles. In standing position the average discharge frequency in the fore leg extensors is found to be of the order of 25-30 imp/s. A doublet pattern of firing and recruitment and derecruitment of single MUs over different periods of time is observed. The transient processes of the instantaneous discharge frequency of single MUs are observed upon passive stretch and shortening of the muscles. Different types of transient processes are described, both during recruitment and derecruitment in standing-position, and during the passive changes in muscle length. A dependence is found of the transient processes in the velocity of the passive changes in length and the probably physiological mechanisms of these phenomena are discussed.