[Isotemporal substitution analysis of 24-hour activity behaviors and health-related physical fitness in university students].

OBJECTIVE: This study examined the associations between 24-hour movement behaviors and health-related fitness in university students, and estimated the substitution effects using a compositional isotemporal substitution model. METHODS: This study was conducted between May and June 2023, using a combination of convenience and random sampling to recruit 325 undergraduate students from Tianjin University of Science & Technology as participants, including 167 males and 158 females. Daily 24-hour activity behaviors were measured using a triaxial accelerometer(ActiGraph GT3X+), including moderate-intensity physical activity(MPA), vigorous-intensity physical activity(VPA), light-intensity physical activity(LPA), sedentary behaviors(SB), and sleep(SLP) duration. Body composition was assessed via body mass index(BMI), waist circumference, and body fat percentage. Muscular strength was measured by handgrip strength, cardiorespiratory fitness was measured by vital capacity and maximum oxygen uptake(VO_(2 max)), and flexibility was assessed by the sit-and-reach test. Compositional data analysis was used to investigate the associations between activity behaviors and health-related physical fitness. A 15-minute isotemporal substitution model was applied to predict the effects of replacing one activity with another on outcome variables. RESULTS: The mean age of male participants was(19.74±1.16) years, and that of female participants was(19.51±1.29) years. Based on 24-hour compositional activity behavior analysis, college students spent an average of 16.42 minutes(1.14% of the day) in MPA, 26.57 minutes(1.85%) in VPA, 150.92 minutes(10.48%) in LPA, 645.78 minutes(46.05%) in SB, and 561.31 minutes(40.21%) in SLP. After adjusting for covariates including sex and age, isotemporal substitution models revealed that replacing an equivalent amount of sedentary time with MPA was associated with a reduction in BMI by 0.07-0.19 units, body fat percentage by 0.53-0.59 units, waist circumference by 0.16-0.27 cm, an increase in vital capacity by 119.18-152.67 mL, VO_(2 max) by 1.76-1.88 mL/(kg·min), handgrip strength by 0.86-1.46 kg, and sit-and-reach performance by 0.19-0.38 cm. Similarly, increasing VPA led to decreases in BMI by 0.14-0.16 units, body fat percentage by 0.49-0.54 units, waist circumference by 0.12-0.23 cm, increases in vital capacity by 127.45-160.84 mL, VO_(2 max) by 1.91-2.03 mL/(kg·min), handgrip strength by 0.98-1.56 kg, and sit-and-reach by 0.14-0.32 cm. Increasing LPA result ed in BMI increases of 0.11-0.12 units, handgrip strength increases of 0.65 kg, and sit-and-reach increases of 0.21 cm. Increasing SLP was associated with BMI reduction of 0.04 units and waist circumference reduction of 0.09 cm. CONCLUSION: MPA had the most significant effect on improving BMI, body fat percentage, and waist circumference, while VPA was more effective in enhancing cardiorespiratory fitness, muscular strength, and flexibility. SLP had a modest positive effect on BMI and waist circumference but was less impactful than MPA and VPA. SB and LPA were generally unfavorable for health-related physical fitness.