Physical education (PE) is often the subject chosen for applying multilingual initiatives based on content and language integrated learning (CLIL). Some research literature (Lynott, F. J. 2008. "Integrating Other Subject Matter without Jeopardizing Physical Education Goals: The Content Linkage Approach." Strategies 22 (1): 10-17; Merino, J. 2016. "Non-Linguistic Content in CLIL: Is Its Learning Diminished?" In "CLIL Experiences in Secondary and Tertiary Education: In Search of Good Practices," edited by D. Lasagabaster, and A. Doiz, 17-43. Bern: Peter Lang) suggests that this subject could lose its essence if too much emphasis is placed on language learning, as it could lead to a reduction in moderate-vigorous physical activity (MVPA), which is directly related to healthy lifestyles. Given that PE needs to increase MVPA, while also allowing students to learn new languages, the purpose of this study is to analyse whether CLIL affects the levels of MVPA and, if so, to understand how and why. Through a mixed methodological approach based on a sequential explanatory design, data were collected and analysed in two consecutive phases: a quantitative one (n = 49) based on a quasi-experimental design in which mixed and factorial ANOVA were applied; and a qualitative one based on a subsequent analysis of interviews with 13 participants. The results show that levels of MVPA are higher in the experimental group (CLIL) due to the greater attention paid and the use of certain communicative teaching strategies.