Purpose: Guided by the model of domain learning theory, this study was intended to determine the extent to which integrating a 10-min physical activity (PA) break in elementary school mathematics lessons would enhance the learning of mathematics content. Methods: A sample of 22 intact classes from a random sample of eight schools in Taiwan were randomly assigned to an experimental condition with PA breaks (n = 11) and a control condition (n = 11) for 12 weeks. Students' (N = 420) mathematics knowledge was assessed using the standardized aptitude tests accompanying the curriculum, which encompassed five knowledge domains. Results: Analysis on class means showed that adding the PA breaks did not benefit mathematics learning, which lends support to the model of domain learning theory. Discussion/Conclusion: The findings seem to question the theoretical foundation for adding PA breaks to classroom-taught content, including mathematics. Although interdisciplinary or cross-disciplinary instruction and learning may be necessary to advance students' knowledge, including PA in classroom-taught subjects needs extensive research.