[The nonlinear properties of changes in length of an active muscle]

The main nonlinear properties of muscle movement under conditions of controlled changes of external load were studied in ankle extensors of anaesthetized cats. Summation of responses to successive short-lasting changes in external load was shown to be absent due to hysteresis effects, that was the reason for stabilizing effects which active muscle produced on the motor control. Trajectories of the movement were dependent on its prehistory and the velocity at the beginning of the movement was invariably higher when the preceding movement had the same direction. These important nonlinear properties of muscle dynamics were also provided by hysteresis of muscle contraction. Functional consequences of the nonlinear muscle dynamics are discussed.