(see also "Several Insights into muscle contraction mechanism" part 1 and 2)

 

        The cross-bridges theory of muscle contraction claims that the sliding process of myofilaments is driven by myosin cross-bridges that extend from the thick filaments and cyclically interact with the thin filaments as adenosine triphosphate (ATP) is hydrolysed.

         According to the cross-bridges theory, the thick filament lateral expansions (cross-bridges) perform the following four actions during contraction:

         1. attachment to one neighbor  at an angle of 90°

        2. the power stroke (whereby the angle changes from 90° to 45°)

          3. detachment from the thin filament

        4. reorientation  to the initial 90° angle relative to thin filament

 

        The Lymn-Taylor cross-bridge model presumes that the myosin cross-bridge complexed with ADP and Pi successively binds actin molecules (in the thin filament) with a rate constant of 100 s-1, releases the products of ATP hydrolysis with a rate constant of 10-30 s-1 in solution and 1.5 s-1 in muscle fiber, binds a molecule of ATP at a very fast rate, detaches from the actin molecules (in the thin filament) (with a rate constant 1000 s-1) and hydrolyses the bound ATP (with a rate constant 150 s-1) so that the overall actomyosin ATP-ase cycle is as long as 50-125 ms in solution and about 700 ms in muscle fiber. There is only one power-stroke of the cross-bridge during an actomyosin ATP-ase cycle, and, because the length of the myosin head is 15 nm, two successive attachment points to the actin molecules (in the thin filament) of an individual cross-bridge are maximum 11 nm apart. However, it is known that the maximum rate of the relative sliding of filaments in intact muscle under zero load is 6 micrometer/s, so that the time available for a cross-bridge to perform its mechanical cycle is no longer than 2 ms, i.e. 25-350 times smaller (depending mainly on the rate constant of the products-release step in muscle fiber) than the time needed for a myosin head to go through a complete ATP-ase cycle.