The commitment of participants to a project under extreme conditions, such as a polar expedition, has a vital bearing on its end result. This finding is consistent with many studies (Baron 1993; Garel 2003; Picq 2005) which suggest that in a project an individual's performance depends more on his actual motivation than on his inherent skills and their suitability for the functions the project needs. But how can one apprehend the very notion of commitment? Its meaning is more complex than it seems. In fact, we have to distinguish the actors' "declared" motivations from what actually mobilizes them in the situation (Récopé et al., 2006). Work from the field of motivation psychology (Nuttin, 1985; Lazarus, 2001) and more broadly from the philosophy of experience (Merleau-Ponty, 1942; Canguilhem, 2003) has made a decisive contribution to this distinction and to establish the importance of the mobilization, norms, and sensibilities underlying it. Having defined and situated these notions we shall show how they throw a light on the participants' behaviour throughout the polar expedition and the project's progress. To account for this result empirically we shall use a case study involving within the same expedition two participants from which personal sensitivity are different. Our conclusions lead to a reconsideration of how to recruit for project teams, highlighting as they do, on the one hand, the importance of commitment not just in terms of intensity but also of meaning, and on the other, the difficulty of using this criterion for recruiting in so far as this commitment may be separate from the motivation expressed.