Recent X-ray diffraction studies of muscle contraction and their implications.

Recent studies on the interference fringes in the myosin meridional reflections provide a new source of structural information on cross-bridge movement during mechanical transients and steady shortening. Many observations can be interpreted satisfactorily by the tilting lever-arm model, with some assumptions, including the presence of fixed repeating structures contributing to the M3 and higher-order meridional reflections. In isometric contraction, the lever arms are oriented near the start of the working stroke, with a dispersion of ca+/-20-25 degrees . Upon a rapid release of 10-12 nm, they move to the end of the stroke, with a well-known T2 delay of 1-2 ms. This delay must represent additional processes, which have to occur even in tension-generating heads, or activation of attached heads, which initially do not develop force. Surprisingly, in muscles shortening at moderate loads (0.5-0.6 P0), the mean position of the heads moves only 2-3 nm closer to the M-line than in the isometric case, reminiscent of the Piazzesi-Lombardi model.