Lower birth weight has been associated with lower cardiorespiratory fitness（CRF）and muscular strength in later life. The aim of this study was to examine whether physical activity may modify associations of birth weight with CRF and muscular strength in children, as well as the relative contributions of birth weight and current physical activity levels to the fitness. The subjects were 535 elementary school children aged 7-12 years. Data were collected on current weight and height. Birth weight was reported according to the Maternal and Child Health Handbook records. CRF was measured by a 20 metre shuttle run test and muscular strength was assessed by grip strength. Information on physical activity was obtained by a questionnaire filled by parents, which asks weekly time spent on habitual exercise activity（min/week).We calculated the time spent in exercise activity higher than 3 METs as moderate to vigorous physical activity（MVPA）and higher than 6 METs as vigorous physical activity（VPA). Lower birth weight was associated with lower CRF（β = 0.087, p = 0.010), after adjustment for sex, months of age, schools, and current height and body mass index（kg/m2). This association was little changed by MVPA（β = 0.090, p＝0.008）or VPA（β = 0.088, p = 0.007), however, VPA was a stronger predictor of CRF than birth weight（β = 0.237, p ‹ 0.001). Birth weight was not associated with muscular strength. The results showed that the association of birth weight with CRF was not modified by physical activity levels in childhood. However, VPA was found to be a stronger predictor of CRF than was birth weight, suggesting that physically active lifestyle which focus on vigorous intensity activity may have a much more important role in development of CRF than an individualʼs low birth weight.
