Associations between the timing of device-measured physical activity and health-related physical fitness in children and adolescents.

Objectives: Investigate the associations between the timing of moderate-to-vigorous physical activity and health-related physical fitness in children and adolescents.

Design: Cross-sectional study.

Methods: This is a secondary analysis of data from a surveillance of Portuguese children and adolescents' physical fitness and physical activity. Moderate-to-vigorous physical activity was determined using accelerometry whereas physical fitness was determined using the FITESCOLA® battery test, the handgrip strength test, and the body mass index. Moderate-to-vigorous physical activity was defined per morning, afternoon, and total moderate-to-vigorous physical activity.

Results: A total of 2641 participants (54.9 % girls) accumulated 37.7 ± 24.2 min·day of moderate-to-vigorous physical activity. Afternoon moderate-to-vigorous physical activity was associated with 20 m shuttle run, standing long jump, push-ups and curl-ups, while morning moderate-to-vigorous physical activity was associated with standing long jump and inversely associated with speed 40 m. Compared to morning and afternoon moderate-to-vigorous physical activity, total moderate-to-vigorous physical activity was more consistently associated with most physical fitness tests (20 m shuttle run, standing long jump, push-ups, curl-ups, speed 20 m, and speed 40 m). The body mass index was only inversely associated with morning moderate-to-vigorous physical activity.

Conclusions: The associations between moderate-to-vigorous physical activity and health-related physical fitness seemed to be related to total moderate-to-vigorous physical activity accumulated throughout the day rather than the timing of moderate-to-vigorous physical activity. The morning moderate-to-vigorous physical activity was inversely associated with the body mass index, suggesting that time-of-day moderate-to-vigorous physical activity may play a role in body composition.