My project examines the role that research on the mechanism of muscle contraction played in scientists’ efforts to understand biological energy transformations across the twentieth century. Since the latter half of the nineteenth century, animal muscle had been at the center of inquiries into how living organisms process the chemical energy derived from ingested food and transform it into the mechanical energy of movement. By the dawn of the twentieth century, studies of the “muscle machine,” as it was often called, were taken up in various laboratories as physical, physiological, and increasingly (bio)chemical problems. Over the next several decades, researchers interested in understanding the properties of the contractile muscle cell—known as the muscle fiber—developed a variety of approaches, from working with isolated muscle preparations, to microscopic sections of muscle, to solutions of muscle extracts. By the middle of World War II, researchers using biochemical methods had determined that muscle fibers consisted primarily of two interlocking sets of protein filaments, which were termed “actin” and “myosin.” While the energy-carrying molecule adenosine triphosphate (ATP) had been suspected to fuel muscle contraction for decades, it was not until the postwar era that the combination of biochemical and structural studies of muscle fibers prompted researchers to theorize about how ATP might interact with actin and myosin to furnish energy for muscle contraction. By tracking these and other developments in the history of muscle research, my project aims to use approaches to the study of muscle contraction as a window into researchers’ changing conceptions of biological energy as a physical, physiological, and biochemical problem throughout the twentieth century.