Longitudinal associations between 24-hour movement behaviors and physical fitness in preschoolers: a compositional isotemporal substitution analysis

This study investigated the prospective associations between changes in 24-h movement behaviors and physical fitness (PF) components in preschool children; and examined how the reallocation of time between movement behaviours was associated with longitudinal changes in PF. This is a two-year longitudinal study of 200 Chinese children aged 3-4 years old. At baseline and follow-up, 24-h movement behaviors including moderate-to-vigorous physical activity (MVPA), light physical activity (LPA), screen-based sedentary behavior (SSB), non-screen-based sedentary behavior (NSB), and sleep were assessed. Balance, cardiorespiratory fitness, flexibility, muscle strength, muscular endurance, and speed-agility were measured using a balance beam test, 20-m shuttle run test, sit and reach test, handgrip test, sit-ups, and 4 × 10 m shuttle run test respectively. The robust compositional regression was employed to explore the association between changes in movement behaviours and PF components, and compositional longitudinal isotemporal substitution analysis was used for the time reallocation. LPA was positively associated with flexibility (β = 3.25, p < 0.05), while sleep was negatively associated with flexibility (β = -4.39, p < 0.05). MVPA was positively associated with muscular endurance (β = 1.36, p < 0.05) at follow-up. The addition of MVPA at the expense of other movement behaviors during the two years was associated with increased muscular strength at follow-up. Reallocations between sleep, NSB, SSB, and LPA showed significant associations with flexibility at follow-up. The impact of MVPA replacing other behaviors on muscular strength was smaller than other behaviors replacing MVPA. Our findings highlight the importance of maintaining PA time to promote PF levels in preschool children. Programs on encouraging a range of light to vigorous active play opportunities and balancing between sleep and PA are crucial for enhancing PF development in preschool children.