Abstract : This work focuses on three research projects in which technologies are worn on the body and produce information about it. At the crossroads of sociology of the body and sociology of technology, we investigate forms of coupling that are never taken for granted but constantly evolving. The experimental situation is like a trial for the body and for technology. They both carry a socio-technical network of objects, places, experiences, people, routines and expertise; their confrontation reveals multiple definitions of the body and of technology. We show how bodily expertise – an intimate knowledge of one's own body and that of others, a profound experience of interaction and mediation – reappears as a resistance within action. Bodily expertise arbitrates these sometimes contradictory definitions. Therefore, we analyse these trials and how they question the body and the technological design process.