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

University students spent the majority of their 24-hour day in sedentary behavior and sleep, with very little time in moderate or vigorous physical activity.

• Participants spent an average of 645.78 minutes (46.05% of the day) in sedentary behavior and 561.31 minutes (40.21%) in sleep.
• Time in moderate-intensity physical activity averaged only 16.42 minutes (1.14% of the day) and vigorous-intensity physical activity averaged 26.57 minutes (1.85%).
• Light-intensity physical activity accounted for 150.92 minutes (10.48%) of the day.
• Sample consisted of 325 undergraduate students (167 males, 158 females) from Tianjin University of Science & Technology, recruited May–June 2023.
• Mean age was 19.74±1.16 years for males and 19.51±1.29 years for females.

Replacing sedentary time with moderate-intensity physical activity (MPA) was associated with improvements in all measured health-related fitness outcomes.

• A 15-minute isotemporal substitution of sedentary behavior with MPA was associated with reductions in BMI by 0.07–0.19 units, body fat percentage by 0.53–0.59 units, and waist circumference by 0.16–0.27 cm.
• The same substitution was associated with increases in vital capacity by 119.18–152.67 mL, VO2max 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.
• MPA had the most significant effect on improving BMI, body fat percentage, and waist circumference among all activity substitutions.
• Models were adjusted for covariates including sex and age.

Replacing sedentary time with vigorous-intensity physical activity (VPA) was associated with improvements across all health-related fitness outcomes, particularly cardiorespiratory fitness, muscular strength, and flexibility.

• Substituting sedentary behavior with VPA was associated with decreases in BMI by 0.14–0.16 units, body fat percentage by 0.49–0.54 units, and waist circumference by 0.12–0.23 cm.
• VPA substitution was associated with increases in vital capacity by 127.45–160.84 mL, VO2max 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.
• VPA was more effective than MPA in enhancing cardiorespiratory fitness, muscular strength, and flexibility.

Increasing light-intensity physical activity (LPA) was associated with modest increases in BMI, handgrip strength, and sit-and-reach performance.

• Increasing LPA (substituting from sedentary behavior) resulted in BMI increases of 0.11–0.12 units.
• LPA substitution was associated with handgrip strength increases of 0.65 kg and sit-and-reach increases of 0.21 cm.
• LPA was generally considered unfavorable for health-related physical fitness alongside sedentary behavior.

Increasing sleep duration (SLP) was associated with modest reductions in BMI and waist circumference, but was less impactful than MPA and VPA.

• Substituting sedentary behavior with sleep was associated with a BMI reduction of 0.04 units and waist circumference reduction of 0.09 cm.
• Sleep had a modest positive effect on BMI and waist circumference but was less impactful than MPA and VPA.
• No significant associations were found between sleep substitution and cardiorespiratory fitness, muscular strength, or flexibility outcomes.

Compositional data analysis was used to model 24-hour activity behaviors as a composition, applying a 15-minute isotemporal substitution model to estimate substitution effects.

• Daily 24-hour activity behaviors were measured using a triaxial accelerometer (ActiGraph GT3X+), capturing MPA, VPA, LPA, sedentary behavior, and sleep duration.
• Body composition was assessed via BMI, waist circumference, and body fat percentage; muscular strength by handgrip strength; cardiorespiratory fitness by vital capacity and VO2max; and flexibility by the sit-and-reach test.
• A compositional isotemporal substitution model was applied, adjusting for sex and age as covariates.
• Sampling combined convenience and random methods to recruit 325 undergraduates.