With millions of people around the world spending weeks and months in quarantine, new questions emerged during the COVID-19 pandemic about the opportunities, benefits, and risks of physical activity. Health organisations, governments and the media alike advocated the importance of physical activity for health and wellbeing. While exercise was being encouraged, options for engagement were increasingly constrained. With gyms, fitness studios, recreational centres, and parks and outdoor facilities closed, many created new fitness rhythms and routines. In this chapter we draw upon feminist new materialist theory, and particularly the work of Karen Barad, to critically explore new questions about the risks of physically active bodies and the ‘trails’ of contagion that they may disperse in and through the ebbs and flows of the natural (i.e. air, wind) and built (i.e. gym and fitness studios) environment. Drawing upon Barad’s conceptualisation of bodily boundaries, we explore new ethical considerations and concerns of aerosol particles (i.e. breath) and bodily secretions (i.e. sweat). In so doing, we diffractively read media releases, scientific reports, and public commentaries through our own embodied experiences of physical activity. Ultimately this chapter offers a critical and creative commentary on the new noticings of bodily boundaries in times of pandemic where the body – any and every body – was a site of possible contagion.