A state-variable model for skeletal muscle, termed the “Distribution-Moment Model,” is derived from A. F. Huxley’s 1957 model of molecular contraction dynamics. The state variables are the muscle stretch and the three lowest-order moments of the bond-distribution function (which represent, respectively, the contractile tissue stiffness, the muscle force, and the elastic energy stored in the contractile tissue). The rate equations of the model are solved under various conditions, and compared to experimental results for the cat soleus muscle subjected to constant stimulation. The model predicts several observed effects, including (i) yielding of the muscle force in constant velocity stretches, (ii) different “force-velocity relations” in isotonic and isovelocity experiments, and (iii) a decrease of peak force below the isometric level in small-amplitude sinusoidal stretches. Chemical energy and heat rates predicted by the model are also presented.

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