Muscle Dynamics, Size Principle, and Stability

A mathematical model of skeletal muscle activation during small or isometric movements is discussed with the following attributes. A direct relationship between the stimulus rate and the active state is established. A piecewise linear relation between the length of the contractile element and the isometric force is considered. Hill's characteristic equation is maintained for determining the actual output force during different shortening velocities. A physical threshold model is proposed for recruitment which encompasses the size principle and its manifestations and also exceptions to the size principle. Finally, the role of spindle feedback in stability of the model is demonstrated by studying a pair of muscles.