[Dynamics of efferent regulation of muscle contraction. Determination of transition processes: external load--muscle length].

Transition processes of the length changes were studied in the ankle extensors of anesthetized cats in case of abrupt transition from isometric to isotonic condition of muscle contraction which was evoked by distributed stimulation of efferents supplied these muscles. Dynamic properties of muscles were analyzed in the framework of the two-components (inertialess and aperiodic) approximation of the transition processes. These properties were nonlinear because of the complex dependence of time constants and weight of an aperiodic component on the amplitudes of input signals. Both of these parameters were significantly lower in the shortening processes comparing with the lengthening ones. When amplitudes of length changes were small, the average limit values of the time constants were 1.38s for slow soleus muscle shortening and 0.39s for its lengthening. The same parameters for fast muscles (gastrocnemius and plantaris) were 0.55 and 0.25s, respectively. The observed dynamical properties of muscle contraction are discussed for their functional significance.