In the early twentieth century, fatigue research marked an area of conflicting scientific, industrial, and cultural understandings of working bodies. These different understandings of the working body marked a key site of political conflict during the growth of industrial capitalism. Many fatigue researchers understood fatigue to be a physiological fact and allied themselves with Progressive-era reformers in urging industrial regulation. Opposed to these researchers were advocates of Taylorism and scientific management, who held that fatigue was a mental event and that productivity could be perpetually increased through managerial efficiency. Histories of this conflict typically cease with the end of the First World War, when it is assumed that industrial fatigue research withered away. This article extends the history of fatigue research through examining the activities of the Harvard Fatigue Laboratory in the 1920s and 1930s. The Laboratory developed sophisticated biochemical techniques to study the blood of exercising individuals. In particular, it found that exercising individuals could attain a biochemically "steady state," or equilibrium, and extrapolated from this to assert that fatigue was psychological, not physiological, in nature. In contrast to Progressive-era research, the Laboratory reached this conclusion through laboratory examination, not of industrial workers, but of Laboratory staff members and champion marathon runners. The translation of laboratory research to industrial settings, and the eventual erasure of physiological fatigue from discussions of labor, was a complex function of institutional settings, scientific innovation, and the cultural meanings of work and sport.