Mechanisms relating force and high-frequency stiffness in skeletal muscle.

Muscle stiffness increases faster than muscle force during the rising phase of a tetanic contraction, and decreases more slowly during the falling phase. Different models of the stiffness arising from series, parallels, and crossbridge elasticity were compared to determine whether they could account quantitatively for the observed time course of force and stiffness. Data for slow and fast twitch mouse muscles at temperatures from 6 to 37 degrees C (Stein and Gordon, Can. J. Physiol. Pharmacol. 64, 1236-1244, 1986) and for single frog muscle fibers (Cecchi et al., Contractile Mechanisms in Muscle, pp. 641-655. Plenum, New York, 1984) were compared. The results showed that a good fit to the data for mouse muscles could be obtained with a model in which: (1) a nonlinear series elasticity contributed significantly to stiffness; (2) the attached crossbridges went from a stiff, force-generating state to a stiff, non-force-generating state; and (3) the rate of transition between these two states increased abruptly at the onset of relaxation. The increased transition rate probably arises from the internal rearrangement in which some sarcomeres shorten at the expense of other sarcomeres, once the muscle begins to relax. A significant series elasticity was not required for the frog data, but a pre-tension state was then needed to obtain a good fit.