The long term goal of the Heidelberg muscle group is to understand the structure and function of muscle by systematically building up the structure from that of its components. By fitting models composed of the crystal structures of the individual proteins found in muscle to fiber diffraction patterns obtained from skinned muscle fiber bundles, one can see how these structures change when they are in a macromolecular assembly in situ. We used the BioCAT facility to obtain very high quality fiber patterns of over-stretched rabbit muscle labeled with exogenous myosin heads (S-1) and catalytic domains from Dictostelium in the presence and absence of ADP to mimic the force producing "power stroke". We were able to detect intensity changes in the fiber pattern that can be related to changes in the domain structure of these species. (Poole et al., 1999). We are presently refining our models in preparation for publication.