This paper is the first stage of an iterative process aiming at the (re)design of a training device for swimming. The purpose of this study was to evaluate the usability of the Measuring Active Drag (MAD) system, a technical device for biomechanical evaluation and performance analysis. To do so, this study examines real activity of elite swimmers using this system. It was conducted within an activity-centred approach: the course-of-action technological research programme. Three international male swimmers volunteered to participate in the study. Two types of data were collected: (a) video recordings, and (b) verbalisations during post-protocol interviews. The data were processed in two steps: (a) reconstructing each swimmer's course of action, and (b) comparing these courses of action. The results are presented in two stages: (a) the concerns and modalities of using, and (b) use sensations. One of the most important results was that these components changed according to the swimmer's speed when using the MAD system. The discussion is organized in two sections: (a) usability of the MAD system, and (b) design proposals to insert MAD system into training sessions