Behind every champion athlete are scores of physiologists studying his or her performance. This is no new phenomenon. Since the late 19th century, physiologists have been bringing ‘well-trained’ athletes into their labs to produce knowledge about how human bodies work, to determine the causes of human fatigue, and to probe human limits. In this article, I argue that the athlete, like the fruit fly or the mouse cress plant, can be considered a model organism. Ethnographic data are presented from 7 months of participant-observation and semistructured interviews in three human performance laboratories located in South Africa, the United Kingdom, and the United States. Ethnographic data suggest that the athlete functions as a model organism in physiology for two reasons. First, athletes ‘accommodate’ the particular experimental instruments and practical demands of fatigue research. Second, a distinct ‘biosociality’ drives the choice of athlete as model organism.