Study design and participants

Data from the Childhood Health, Active, and Motor Performance School Study Denmark (CHAMPS Study-DK) were used. The study sample and procedures of this quasi-experimental study have been published elsewhere^21,22. Between 2008 and 2011, 10 public schools participated in the study. The current analysis was restricted to (1) participants who performed a minimum of 60 min of daily MVPA at 12 months and 22 months, which represents 43.2% of the original sample, and (2) participants who have a minimum of three time point measurements for health trajectories variables (cardiorespiratory fitness, waist circumference, and body mass index). Anthropometric variables were measured at baseline and at 6, 12, 18, 24, 30 months, while cardiorespiratory fitness was measured at baseline, 6, 12, 18, and 30 months. The rationale for differences in the above measurements’ time points was to accommodate logistic challenges related to the original investigation. Reporting of this secondary analysis of the CHAMPS Study-DK follows the STROBE statement²³.

Physical activity was measured using Actigraph GT3X accelerometers (Actigraph, Pensacola Florida). Trained research staff instructed children and parents on how to wear the device. Children wore the accelerometer on the right hip using a customized elastic belt from waking in the morning until they went to bed, except when bathing or swimming.

A customized software program (Propero, version 1.0.18, University of Southern Denmark, Odense, Denmark) processed all accelerometry data. Accelerations were recorded every 2 s and subsequently collapsed into 10-s epochs. Although, some data suggest shorter epochs length to quantify PA in children, a10-second epoch was selected based on the rationale that other data using shorter epochs length showed an overestimation of sedentary behaviour in children. Therefore, to optimize the accuracy of the sedentary data without compromising the vigorous PA data, we used the 10-s epochs. Digitalized accelerometer signals were filtered with 0.25–2.5 HZ band limits. This filter process is a mathematical weighting function that contribute to alter movement of low or high frequency above or below these limits and therefore help eliminating accelerations not associated with human movement (e.g., vibration). To distinguish inactivity from periods of non-wear, we interpreted readings of zero activity lasting at least 30 consecutive minutes as ‘accelerometer non-worn’. Although, other suggest length of 45–60 min, others have been suggesting 30 min of zero activity as non-wear time in children for more accuracy²⁴. Therefore, we decided to use the 30 min to enhance accuracy. Data included in this study were limited to children who wore the accelerometer for at least 10 h per day on 4 or more days during each week of measurement. To represent overall PA, average counts per minute (CPM) were calculated by the following formula: (total counts/minutes of wear time). Proportion of the day spent in sedentary time (0–25 counts), light (26–573 counts), moderate (574–1002 counts), and vigorous (≥ 1003 counts) PA intensities were identified using pre-established and validated cut-points according to Evenson et al.²⁵ As our data were collected in 2-s epochs, they were re-integrated in 10-s epochs and Evenson’s cut-points were scaled to ensure they mirrored the cut-points proposed by Evenson et al. All PA outcomes were averaged across the two measurement periods.

Cardiorespiratory fitness was measured with the Andersen test. This is an intermittent maximal indirect indoor running test developed for children and youth²⁶. Briefly, children ran as fast as possible down a 20 m lane, touched behind the line with one hand, and turned and ran back in the opposite direction. After 15 s, children stopped immediately when hearing a whistle. Following 15 s of rest, the process repeated, with children attempting to cover the greatest distance possible. The test outcome was total distance run by each child in 10 min, and this was monitored by trained research staff. This test has good test–retest reliability and concurrent validity when compared with direct VO_2max testing in children²⁷.

Anthropometric measurements including height and weight were measured with children barefoot, wearing light clothes, without hats or helmets, and with empty pockets. Height was measured to the nearest 0.5 cm with a portable stadiometer (SECA 214, Seca Corporation, Hanover, MD, USA), and weight to the nearest 0.1 kg using a calibrated Tanita BWB-800S digital scale (Tanita Corporation, Tokyo, Japan). Body mass index (BMI) was calculated as: weight (kg)/height (m)² and categorized as normal weight, overweight, or obese according to International Obesity Task Force criteria²⁸.

Waist circumference was measured to the nearest 0.5 cm with a tape measure placed at the level of the umbilicus following normal expiration. The measure was taken twice, and if differences greater than 1 cm were observed, a third measure was obtained. The mean of the two closest measurements was reported. Waist circumference outcomes were used to classify children as normal weight, overweight, or obese using sex and age-adjusted criteria²⁹.