As fitness professionals, group fitness instructors play an important role in promoting recommended public health initiatives to various communities. While their practices are connected to public health policy, their certifications are often provided by commercially operated agencies in the North American context. The interrelationship of commercial and governmental control creates a complex environment where certain ways of understanding health and the fit body become dominant. In this paper, consequently, we were interested in what fitness knowledges informed Canadian group fitness instructors’ practices. Approaching our topic from a Foucauldian perspective, we conducted semi-structured interviews with five instructors who had a provincially governed fitness qualification. The findings revealed that two knowledges – health as illness prevention and the aesthetics of health – strongly directed the instructors’ class design despite their willingness to think, particularly about health, differently. Their certification reinforced the medical, physiological, and psychological knowledges that tend to assign health as individual responsibility, but left the social issues behind the healthy, fit body unproblematized. From a Foucauldian perspective, these dominant knowledges locked the instructors within dual control of anatomo-political and bio-political neoliberal power relations. To assist the instructors to problematize the control mechanism behind the dominant knowledge structure, we call for inclusion of broader knowledge base in their certification training through which serving the multiple needs of their various clientele can be constructively negotiated.