Theoretical considerations on myofibril stiffness.

A discrete model of the interaction between individual myofilaments was developed to study the stiffness of a sarcomere for the case in which filament compliance is not negligible. Our model retains, in the limit, the characteristics of the previously published model by Ford et al. (Ford, L. E., A. F. Huxley, and R. M. Simmons. 1981. The relation between stiffness and filament overlap in stimulated frog muscle fibres. J. Physiol. 311:219-249). In addition, the model is able to model the interaction in cases in which few cross-bridges are attached, or when the distribution of attached cross-bridges is not uniform. Our results confirm previous indications that it might be impossible to calculate the number of attached cross-bridges by using only stiffness measurements in quick-stretch (or release) experiments.