The contraction of muscle cells as well as the motility of nonmuscle cells is known to result from the interaction of myosin, actin and ATP. The long-range goal of our research is to understand the molecular mechanism of this interaction. It is now thought that force is produced in a "power-stroke" when a myosin head, attached to an actin filament, changes its configuration from one in which it is perpendicular to the filament to one in which it makes an acute angle with the filament. Although the angular orientation of the myosin head may play a central role in the process of contraction and it has been investigated by a variety of techniques, we still lack a detailed picture of how this orientation changes during muscle contraction. Our recent work has shown that paramagnetic probes we can accurately measure the angular orientation of the portion of the myosin to which they are attached. These probes have been successfully applied to skeletal fibers in rigor and relaxation, where they produced a new picture of the orientations and dynamics of the myosin heads. We now propose to extend these techniques to contracting fibers where we will determine the orientation and motion of cross-bridges that are participating in the contractile cycle. This will be important application of these techniques, since it is during contraction that the orientation of the myosin is thought to play its crucial roles. We will also use these techniques to investigate a number of problems that are related to contraction including: the energetics of the contractile interaction, the mechanism of relaxation, and the structure of insect muscle. The data obtained from these studies will provide a more detailed picture of the mechanism of contraction than has been obtained by previous methods. These studies will be extended to elucidating how protein phosphorylation controls the contraction of skeletal and cardiac muscle. Knowledge of how force is produced and how it is controlled in skeletal muscle, and especially in cardiac muscle, will help define more rational therapies for treating disorders in these muscles.