A numerical study of the stiffness of a sarcomere

Relatively recent experimental findings of the significant compliance of the thin (actin) and thick (myosin) filaments have brought into question a number of conclusions based on the assumption of perfect myofilament rigidity. A new model based on a discrete representation of the relevant structures was used to calculate the theoretical stiffness of a sarcomere with compliant myofilaments. Because of the discrete nature of the model, it can be applied to situations in which either the number of links between filaments is low (i.e. partial overlap or partial activation) or the spatial distribution of links is not uniform. The results of this model are discussed and compared to the predictions given by the previously published model by Ford et al. (Ford, L. E., Huxley, A. F. and Simmons, R. M., Journal of Physiology, 1981, 311, 219--249). Although it can be shown that both models give identical results for an infinite number of links, our model consistently predicts a stiffer sarcomere for partial overlap. The differences in the stiffness values as calculated by a model with rigid filaments, the continuous model by Ford et al. and the discrete model presented here would lead to differences in the interpretation of experimental results, especially in the case of partial overlap and partial activation. An explanation for the discrepancies between models is presented together with a way to correct the continuous model to approximate discrete model results.