Cardiorespiratory Fitness Growth in Middle School: Socio-Demographic Factor Associations.

: The purposes of this study were to (a) examine the three-year longitudinal growth trends of cardiorespiratory fitness among middle school students, and (b) examine the associations between individual and school-level socio-demographic variables and middle school student cardiorespiratory fitness performance. : A longitudinal multilevel analysis design was used. Participants were adolescents ( = 44,801; 48.1% girls) from 33 middle schools in an Eastern U.S. state. The grand mean age for the participants was 12.42 ± 0.98 years old. Individual-level variables included grade (testing year), sex, and the 15-m Progressive Aerobic Cardiovascular Endurance Run (PACER) performance, which was measured annually for 3 years. School-level variables included the percentage of students receiving a free and reduced-price meal, student-faculty ratio for physical education, and school-level academic performance. : A three-level hierarchical linear model was used to model the longitudinal performance change across 3 years accounting for individual and school-level factors. A quadratic growth curve was identified in PACER performance changes, with a positive first-order coefficient (= 3.05) and a negative second-order coefficient ( =-0.82) overall. Sex was significantly associated with PACER changes ( < .01), with boys and girls having divergent longitudinal growth curves. School-level academic performance was positively associated with PACER performance; however, the association was not statistically significant. : Results indicate that although cardiorespiratory fitness improved across years, on average, performance did not keep up with healthy fitness zone recommendations. In addition, sex-based discrepancies were evident as boys and girls demonstrated different quadratic growth curves in cardiorespiratory fitness.