Purpose: To determine how factors are related to change in cardiorespiratory fitness (CRF) across time in middle school girls followed through high school.
Methods: Adolescent girls (N = 274, 59% African American, baseline age = 13.6 ± 0.6 yr) performed a submaximal fitness test (PWC170) in 8th, 9th, and 12th grades. Height, weight, sports participation, and physical activity were also measured. Moderate-to-vigorous physical activity (MVPA) and vigorous physical activity (VPA) were determined by the number of blocks reported on the 3-Day Physical Activity Recall (3DPAR). Individual differences and developmental change in CRF were assessed simultaneously by calculating individual growth curves for each participant, using growth curve modeling.
Results: Both weight-relative and absolute CRF increased from 8th to 9th grade and decreased from 9th to 12th grade. On average, girls lost 0.16 kg·m·min−1·kg−1·yr−1 in weight-relative PWC170 scores (P < 0.01) and gained 10.3 kg·m·min−1·yr−1 in absolute PWC170 scores. Girls reporting two or more blocks of MVPA or one or more blocks of VPA at baseline showed an average increase in PWC170 scores of 0.40-0.52 kg·m·min−1·kg−1·yr−1 (weight relative) and 22-28 kg·m·min−1·yr−1 (absolute) in CRF. In weight-relative models, girls with higher BMI showed lower CRF (~0.37 g·m·min−1·kg−1·yr−1), but this was not shown in absolute models. In absolute models, white girls (~40 kg·m·min−1·yr−1) and sport participants (~28 kg·m·min−1·yr−1) showed an increase in CRF over time.
Conclusion: Although there were fluctuations in PWC170 scores across time, average scores decreased during 4 yr. Physical activity was related to change in CRF over time; BMI, race, and sport participation were also important factors related to change over time in CRF (depending on expression of CRF-weight-relative vs absolute). Subsequent research should focus on explaining the complex longitudinal interactions between CRF, physical activity, race, BMI, and sports participation.