Data produced by sensor-equipped artefacts is often associated with techno-utopian visions of a smooth path from data to insight. We examine how sensors are put into use in practice, through a case study on a preventative care initiative aiming to discourage seniors from a sedentary lifestyle using wrist-worn activity trackers. We examine processes of commensuration, especially in relation to how activity is represented by the activity tracker and how the material and social arrangements configuring activity and agency are reconfigured when a self-tracking system is put into practice. We show how the self-tracking practices are reconfigured by the institutional context introducing the devices, as well as by software updates and user settings. These commensuration practices and their reconfigurations have consequences for which activities become measurable, considered normal and abnormal, and what accounts are considered credible.