Actiheart

For the characterization of autonomic nerve activity, electrocardiogram during sleeping hours was obtained using a heart rate monitor (ActiHeart; CamNtech Inc., TX, USA) with 1mS resolution. The data were transferred to the ActiHeart software (CamNtech Inc.), and heart rate variability and inter-beat interval were analyzed. The heart rate (HR), the low frequency spectrum (LF); (0.05–0.15 Hz), the high frequency spectrum (HF); (0.15–0.4 Hz), and total power (TF) for 4 h (2–6 h after asleep) were calculated. HF and LF/HF were adopted as indices of parasympathetic nerve activity and sympathetic nerve activity, respectively [9 (link)]. TF was adopted as the total activity of autonomic nerves.

Data collection took place between June 2011 and November 2012 including the Australian winter and summer. Under researchers’ supervision, students completed a paper-based survey at school, either in class or during lunch recess. The paper-based survey administered to children assessed socio-demographics, travel and independent mobility behaviours. A paper-based survey administered to parents included information on socioeconomic status. Furthermore, children wore an Actiheart, a combined heart rate and accelerometer monitor (CamNtech Actiheart, Cambridge, UK), for four consecutive days, except during periods of heavy contact sports and water-based activities (e.g. showering, swimming). The Actiheart monitoring period included weekdays and weekend days, either consecutively from Thursday until Monday morning or from Friday until Tuesday morning. The Actiheart was attached directly onto the skin on the lower left side of the chest using two electrocardiograph (ECG) electrodes (model Red Dot 2238 soft cloth, 3 M)
[33 (link),34 ]. One electrode was placed at the base of the child’s sternum and the other electrode placed horizontally to the left so the wire of the Actiheart was straight with tension, but not tight. Researchers demonstrated to the students how the Actiheart should be worn. The students then fitted the units and researchers checked the positioning.

Body composition analysis was performed using dual-energy x-ray absorptiometry (Lunar Prodigy X-ray Tube Housing Assembly, Brand BX-1L, Model 8743; GE Medical Systems, Madison, WI, USA) at the Clinical Research Centre at Umeå University Hospital. $\stackrel{\cdot }{V}{\mathrm{O}}_{2max}$ was determined during a standard cardiopulmonary exercise test on a cycle ergometer at the Department of Clinical Physiology at Umeå University Hospital. Resting energy expenditure was measured by indirect calorimetry (Datex-Ohmeda Deltatrac II; Datex-Ohmeda, Madison, WI, USA) and adjusted by subtracting 5% during 8 h of sleep. Physical activity energy expenditure was estimated using a combined heart rate monitor and accelerometer for 7 consecutive days (Actiheart; CamNtech, Cambridge, UK) as previously described [28 (link)]. Total energy expenditure was calculated as the sum of resting energy expenditure and physical activity energy expenditure with added 10% for diet-induced thermogenesis.
Liver fat and tibialis anterior/soleus muscle fat were analysed by proton magnetic resonance spectroscopy as described in the electronic supplementary material (ESM) Methods.

Heart rate will be monitored using Actiheart (Camntech, Cambridge, United Kingdom), which is based on electrocardiography (ECG). Actiheart is a waterproof, compact and lightweight device designed to measure ECG Inter-Beat-Intervals and physical activity levels. The Actiheart can remain on the subject 24 hours a day and collect data for up to 72 h in total. The Actiheart software given by manufacturer will be used for set up, reading of data and charging the Actiheart. Recommended placements will be applied, attaching electrodes at the apex of the sternum and at the left intercostals at the level of the 6^th and 7^th costae [69 (link)]. The sensor has been found to be reliable and valid for use both in the laboratory [70 (link)] and free-living situations [71 (link)], and can be used as a measure for work ability [72 (link)].