Purpose: Guided by the declarative-procedural knowledge framework, the study attempts to identify middle school students' declarative (knowing what) and procedural (knowing how) fitness knowledge and the relationship between the two. Methods: A sample of students (n = 291, age 11-14 years) from 24 middle schools took a grade-relevant standardized knowledge test on declarative fitness knowledge and received a semistructured interview designed to clarify their declarative and procedural knowledge. Results: Most students were lacking in procedural knowledge to conduct fitness-enhancing physical activities. A few students who had mastered declarative fitness knowledge demonstrated a high level of procedural knowledge consistent with personal fitness goals. Discussion: The findings suggest that incapability to engage in fitness-enhancing physical activities could be a result of lacking procedural fitness knowledge. Future school-based interventions may prioritize procedural knowledge learning for actual physical activity participation.