Activewear refers to casual clothing designed specifically for both function (enables and supports movement) and fashion. Since the early 2000s (and arguably earlier), activewear has seen a dramatic increase in popularity amongst women described, by some, as a fundamental change in women’s dress. Although often dismissed as simply the latest fashion trend, the activewear phenomenon can be understood as a complex entanglement of branding, fabric, skin, fat, muscle, consumption, environmentalism, gender, healthism, neoliberalism, and politics. While extremely prevalent in popular culture, there is limited socio-cultural scholarship on this complex phenomenon. This research seeks to better understand the connection between activewear, women’s fitness, and broader ideas around women’s moving bodies and femininity using a new materialist theoretical framework. New materialisms refer to a series of theories and concepts that are part of the posthumanist turn in social sciences. Although the approaches vary, in general, feminist new materialisms emphasise the vitality and liveliness of nonhuman matter, centralise the relationship between humans and nonhumans, see research processes as co-implicated in the production of knowledge, and maintain a political orientation towards the doing of gender. While many have discussed the theoretical implications of new materialisms, only recently have scholars begun exploring how to put new materialist concepts into practice in the research process. Therefore, this thesis contributes to a growing body of literature on the empirical possibilities of new materialisms. In particular, I use new materialist feminist physicist, Karen Barad’s theory of agential realism. Bringing together an array of humanist-based (interviews, focus groups, photo diaries) and creative methods, this research explores the potential of Baradian theory for studying the activewear phenomenon. Using three Baradian concepts (spacetimemattering, intra-action, and entanglement), I examine the material-discursive production of gender, fit femininity, and feminist politics within the activewear phenomenon. More specifically, I show how Barad’s spacetimemattering allows for an understanding of the ways activewear is entangled with previous iterations of femininity and works to rearticulate femininity as strong and powerful. Next, I bring together the material (i.e., clothing and bodies) with the discourses around activewear to emphasise how fit femininity and idealised bodies are produced through an entanglement of leggings, clothing, bodies, advertisements, and discourses. Finally, I use Barad’s concept of entanglement and writings on bodily boundaries to reimagine women’s moving bodies as phenomenon comprised of human and nonhuman entities. The thesis concludes with a discussion of the possibilities and challenges of using Baradian theory within feminist sport sociology. In particular, I describe how Barad’s agential realism allows for an understanding of activewear as a vital force in the production of boundaries around acceptable femininity. I also speak to some of the tensions and challenges scholars may experience when working with Baradian theory, such as issues with accessibility, representation, and time. In using new materialisms to understand activewear, this thesis contributes to the socio-cultural study of women’s fitness using a novel and innovative approach, in addition to advancing the literature on new materialisms and its empirical implications for feminist scholars of the moving body.