Association of physical activity and sedentary behavior with cardiorespiratory fitness among middle school students in Lhasa

Objective To explore the relationship of physical activity (PA) and sedentary behavior (SB) with cardiorespiratory fitness (CRF) among middle schoold students in Tibet, so as to provide empirical references for improving the cardiorespiratory fitness and health levels of adolescents in Tibet. Methods From August to December 2020, 1 225 junior and senior high school students were selected from 2 middle schools in Lhasa, Tibet Autonomous Region, using the stratified cluster random sampling method. Triaxial accelerometers were used to evaluate PA and SB behaviors, and the 20-meter shuttle run was employed to assess CRF among the middle school students. Isochronous substitution modeling was used to analyze the associations of SB, low-intensity physical activity (LPA), and moderate-vigorous physical activity (MVPA) with CRF, and the saturation threshold effect in the dose-response relationship between MVPA and CRF was analyzed through restricted cubic spline and two-stage linear regression. Results After adjusting for covariates such as gender, body mass index and sleep quality score, isotemporal substitution analysis showed that among junior high school students aged 13–15, replacing 30 minutes of SB (B = 1.73) or LPA (B = 2.38) with MVPA were positively associated with CRF (both P < 0.05). Among senior high school students aged 16–18, replacing SB (B = 0.99) or LPA B = 1.38) with MVPA were also positively associated with CRF (both P < 0.05). Restricted cubic spline and two-piecewise linear regression analyses indicated that only middle school girls aged 13–18 exhibited a saturation threshold effect between MVPA and CRF (logarithmic likelihood ratio test=0.03), with the optimal CRF improvement observed at 60 minutes of MVPA per day (B = 0.13 P < 0.01). Conclusions Reducing SB and LPA while increasing MVPA can improve CRF in Tibetan middle school students. To maximize CRF improvement, middle school girls should engage in at least 60 minutes of MVPA daily.