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Muscle contraction is a fundamental biological process. In the fifties, the molecular picture of muscle contraction was revolutionized by two complementary theories. First, the sliding filament theory states that muscle contracts via the relative sliding of two sets of filaments. Second, the cross-bridge theory states that projections from one filament to the other, so-called cross-bridges, power contraction. In 1957, Andrew Huxley formalized these ideas in a simple mathematical model that was able to predict whole muscle measurements. Since then, the molecular details of muscle contraction have become increasingly clear. As molecular details have become clearer, however, the connection between the molecular and cellular (and larger) scales of muscle contraction has become less clear.

In this talk, I will introduce these models of muscle contraction, and briefly discuss their evolution over the years. I will then argue that such models cannot explain cellular scale muscle contraction. Based on this result, one might wonder where the models break down. I will show that the models can, in fact, successfully bridge size scales from single molecules to large ensembles, at least under simplified experimental conditions. This result suggests that the models break down at higher levels of complexity. I will finish with some ideas about how to extend the model to these larger size scales.