The aim of this study was to examine and quantify the cross-sectional associations of body composition (BC), physical activity (PA) and sedentary time (ST) with physical fitness (PF) in children and adolescents. A sample of 594 Finnish students (56% girls), aged 9-15 (12.4 ± 1.3 years) were selected for a study performed in 2013. The measurements of the Move! monitoring system for physical functional capacity were used to measure cardiorespiratory and musculoskeletal fitness and fundamental movement skills. Moderate-to-vigorous PA (MVPA) and ST were measured objectively with an accelerometer and BC by a bioelectrical impedance analysis. Fat mass index (FMI) and fat-free mass index (FFMI) were calculated to represent height-adjusted BC. Associations were explored with a linear regression model. In general, FMI had statistically significant negative associations, while FFMI and MVPA had positive associations with PF. No statistically significant associations were observed between ST and PF. In general, FMI had the strongest association with PF, although some variation occurred with sex and PF component. However, associations were practically relevant only in 20-m shuttle run, push-up, curl-up and 5-leaps test. For example, approximately 5 kg increase in fat mass in 155 cm tall children was estimated to correspond to  8 laps in 20-m shuttle run. Similar increase in fat-free mass corresponded to +4 and +6 laps, and 10 min increase in daily MVPA +3 and +2 laps in 20-m shuttle run, in boys and girls, respectively. Understanding these associations is necessary when interpreting children's PF and designing interventions.

The immediate postexercise decreases in H- and stretch reflexes are probably partially caused by activation of group III and IV afferent fibers by high lactate concentration in combination with possible increases in potassium outflow. Both of these parameters recovered quickly (i.e., 2 h after exercise). The events after the 2-h postexercise point are very likely to be related to muscle damage and associated inflammation. Group III and IV afferent fibers are probably reactivated during this period by mechanical factors.

Background: This study investigated the associations of subjectively and objectively measured physical activity (PA) and sedentary behavior with academic achievement. We further examined whether aerobic fitness, obesity, and bedtime mediate these associations. Methods: This study included 970 children aged 9-15 years (52.3% girls) from 9 schools throughout Finland. Register-based academic achievement [grade point average (GPA)] as well as self-reported and accelerometer-measured PA/ sedentary behavior were assessed during spring 2013. Aerobic fitness (assessed via a maximal shuttle run test), body composition (assessed via bioimpedance analysis), and self-reported bedtime were collected. Structural equation modeling was applied to examine the associations. Standardized regression coefficients are presented. Results: Self-reported PA had a direct positive [β = 0.084; 95% confidence interval (CI), 0.023 to 0.145] and an indirect positive association with GPA through higher aerobic fitness (β = 0.061; 95% CI, 0.033 to −0.087). Accelerometer-based PA was not associated with GPA. Self-reported screen time had an indirect negative association with GPA through later bedtime (β = −0.071; 95% CI, −0.096 to −0.035) and lower aerobic fitness (β = −0.039; 95% CI, −0.059 to 0.019). Nonscreen sedentary time had a direct positive (β = 0.193; 95% CI, 0.101 to −0.289) and an indirect negative association with GPA through lower aerobic fitness (β = −0.040; 95% CI, −0.063 to −0.016). Conclusions: Participating in PA, avoiding excessive screen time, and going to bed earlier may benefit academic achievement.Keywords: academic performance, moderate to vigorous physical activity, screen time, sedentary time, adolescents Globally, children spend extensive amounts of time being sedentary, 1 and only one-third of children are physically active enough to gain health benefits. 2 Accompanying these trends, the rates of childhood overweight and obesity, along with poor fitness, are escalating. 3,4 These trends have raised concerns about the effects of a physically inactive lifestyle on children's health during maturation and across the lifespan, as physical activity (PA) continues to decrease and sedentary time (ST) continues to increase from childhood to adolescence and adulthood. 5,6 The association of a physically active lifestyle with learning outcomes has also recently received considerable attention. Previous studies have suggested that excessive screen time 7,8 and excess adiposity 9,10 may predict poorer academic achievement (AA), whereas regular PA and higher aerobic fitness 11,12 benefit AA.Sufficient sleep has also been related to better AA. 13,14 The periods of sleep, together with time spent in PA and sedentary behavior (SB), have been suggested to constitute a composite whole, as the time spent in one of these behaviors necessarily displaces time spent in another. 15 Although these behaviors are closely related to each other, the associations with AA have thus far been studied only in isolation or with a partial adjustment for ...

Objectives The objective of this study was to determine the gender‐specific and time‐segmented changes in accelerometer measured physical activity (PA) and sedentary time (ST) during adolescence. Methods The study population (N = 970) consisted of children from nine schools throughout Finland. At the baseline, the children were in grades 4‐7 (ages 10‐13). Five times during the two‐year follow‐up period, hip‐worn accelerometers (ActiGraph GT3X+) were deployed for seven consecutive days in order to monitor the pupils’ PA. The daily time spent in vigorous (VPA), moderate to vigorous (MVPA), and light physical activity(LPA), as well as sedentary time(ST), were assessed. Results Significant gender differences were observed in terms of the changes in MVPA and ST during the follow‐up period. The total MVPA declined significantly in boys (by 2.2min/day/year from 60 min/d at baseline) but not in girls (49 min/d at baseline). The total ST increased both in boys (by 20.7%‐points/y) and in girls (by 16.1%‐points/y, P < .001). However, when we compared the results during the weekdays and weekend days separately, we observed that the declines in MVPA and increases in ST were greater in boys than in girls during the weekend days. Conclusion A greater decrease in PA and a greater increase in ST during adolescence were observed among boys than among girls, especially during weekend days. In order to diminish these unfavorable behavioral changes during adolescence, we encourage the separate tailoring of interventions for boys and girls and for weekdays and weekends.

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