Muscle is one of the most efficient "machines" on earth. The energy source of muscle contraction is ATP hydrolysis by actin and myosin. One of the research efforts is to understand how the energy of ATP hydrolysis is transformed to physical displacement. Since the function of muscle is to produce force and to move an object, some structural changes at the molecular level are expected to take place, coupled with the ATP hydrolysis. As myosin (acting as an enzyme) hydrolyzes ATP, it passes through several intermediate states including the binding of the nucleotide, the hydrolysis step, and the release of the hydrolysis products. It is thought that force is generated (and hence movement) during the release of the hydrolysis products. An ideal way to study structural changes associated with the force generation is to study structures of each individual state within the ATPase cycle. By characterizing the structures of the intermediate states, structural transitions involved in force generation might be understood. To achieve this, we have used ATP analogues to simulate the intermediate states or chemically modified the myosins, which are "trapped" in one of the intermediate states. X-ray diffraction is the technique of choice, since it is one of the few non-invasive techniques that allow direct observation of structural changes while the muscle remains fully functioning. To shorten the exposure time as much as possible so as to maintain viability of the muscle tissue, intense synchrotron radiation at the Brookhaven National Laboratory has been routinely used as the X-ray source. We have identified that those myosin heads arranged in an ordered helical array on the surface of the myosin filament are in the M.ADP.Pi state. Recently, we have moved on to characterize another actomyosin state in the ATPase cycle - that of the low affinity state A.M.ATP. The weakly bound cross-bridge states (A.M.ATP and A.M.ADP.Pi) were discovered by a group of scientists at NIH, which were shown to be on the pathway to force generation, i.e. myosin must first bind to actin in the low affinity states before going onto force generation. A major accomplishment of FY00 is that the structure of the isolated state of A.M.ATP is finally being determined. Our data indicated that while weakly bound to actin, the myosin heads (cross-bridges) still maintain a helical structure following the symmetry of the thick filament backbone, contrary to the situation where cross-bridges are tightly bound to actin such as A.M.ADP or A.M. In the latter cases the cross-bridges follow the symmetry of the actin filament. The binding site on actin was shown to be different from the site for rigor binding. With the conclusion of studying the A.M.ATP structural state, we have now identified distinct actin and myosin filament structures associated with seven out of eight major biochemical states. Work is in progress to study the remaining intermediate state A.M.ADP.Pi state. These findings provide a significant step towards delineating the structure-function relationship of muscle. Studies of the structure of a sonic muscle has been continued by X-ray diffraction and modeling. The unusually large Z-band provides an excellent opportunity for studying one of the major cytoskeleton systems of striated muscle. Modeling based on the X-ray data revealed the organizational symmetries of various proteins including actin filaments and alpha-actinin.