All the models discussed so far treat crossbridge binding as a relatively simple phenomenon; either crossbridges are bound or they are not, and there is no consideration of the possibility that each crossbridge might have a number of different bound states. As we have seen, such assumptions do a good job of explaining muscle behavior on the time scale of tens of milliseconds. However, as the development of new experimental techniques allowed the measurement of muscle length and tension on much shorter time scales, the initial models were improved to take this high time-resolution data into account. One of the first models to do so was that of Huxley and Simmons (1971). The Huxley-Simmons model is quite different from models discussed above, giving a detailed description of how the force exerted by an attached crossbridge can vary with time over a short period, but it does not take into account the kinetics of crossbridge binding and unbinding to the thin filament.
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