Based on an ethnographical study, the authors have produced an on-the-spot analysis of a fitness instructor “at work.” Observing the sequences of the sessions reveals the instructor’s physical and psychological commitment, which defines, among other things, the ever-changing collective relationship of this professional to the symbolic and material objects of the gym environment, as well as with the social group the instructor has brought into being and shapes. The indeterminate and improvisational character of the situations the trainees and instructor go through in this particular setting–?in which people are there to see and meet others–?leads the fitness trainer to use abilities and techniques, play on spatial closeness and distance with the people working out, and, finally, build a sense of cooperation and sharing.