This paper investigates the role activity trackers play for their users, focusing on the transformative role of wearable technology in shaping our perceptions of our bodies and health. Drawing on autophenomenography, in-depth interviews and online content analysis, it questions the neutrality of tracking data and interpretation, by drawing on Annemarie Mol’s concept of ontonorms and highlighting the built-in normativity of the assemblage that makes it constantly prone to enacting a certain kind of optimal performance. Using Katherine
Hayles’ concept of cognitive assemblages, it defines trackers as being specific types of cognizers, entering alongside humans in assemblages in which their role sometimes ends up dethroning the body’s own perceptual systems and changing the way we are aware of our own bodies. Throughout the research, three different types of users are identified and described, for whom the cognitive assemblage takes a different shape: regular users, users with medical issues and power users.