Epistemic beliefs are deeply held convictions about the nature of knowledge, knowing, and learning. In this study, approximately 500 ninth and tenth-grade physical education (PE) students completed fitness-specific measures assessing their epistemic beliefs in the simplicity and stability of knowledge and the speed of its acquisition along with their effort regulation, fitness outcomes (grade, level, and frequency), and indices of fitness motivation (goal orientation, value, perceived autonomy support). Participants completed three surveys during their regular PE class. Exploratory and confirmatory factor analyses revealed two reliable epistemic belief factors similar to those previously reported in PE (the simple-integration of fitness knowledge; stable-and-useless fitness knowledge). This provided some support for the generality of epistemic beliefs across PE and fitness. As expected, epistemic beliefs related negatively to indices of motivation and effort regulation. An intrinsic goal orientation and a view for the simplicity of fitness knowledge predicted fitness outcome and a statistically significant pathway was observed from simple and stable epistemic beliefs and indices of motivation to fitness outcomes through effort regulation. The study provides particular fresh insights into the nature of epistemic beliefs, their relations to effort regulation and indices of motivation, and their potential influence on fitness outcomes in PE and beyond.