[Quantitative analysis of the change in length of active muscle under conditions of varying external load]

Processes of length changes evoked by ramp-and-hold alterations in the external load were studied in ankle extensor of anesthetized cats under conditions of constant frequency of efferent stimulation. Basing on the experimental data a phenomenological theory of muscle dynamics was elaborated. It was shown that the system under study could be presented as a system with nonlinear statics and linear dynamics. The nonlinear statics was shown to be determined mainly by hysteretical effects of muscle contraction. A hypothesis was proposed to explain the hysteresis in active muscle on the basis of functioning of the troponin-tropomyosin regulatory complex. Dynamic element in the model was described by the first order linear differential equation corresponding to a three-component visco-elastic muscle model. The suggested phenomenological theory of muscle dynamics can be used to describe lengthening processes but satisfactory results were obtained only for rather slow load increments.