Timmons et al. BMC Public Health 2012, 12:284 http://www.biomedcentral.com/1471-2458/12/284 are answered; written informed consent is then obtained from the parent. Parents then provide demographic infor- mation and complete questionnaires. The remainder of this first visit is used to assess body composition, aerobic fitness, short-term muscle power and motor skills. At the end of visit 1, an accelerometer to monitor physical activity is given to the child and their parents are provided detailed instructions on its use for the next 7 days. For their final visit, indices of vascular structure and function are assessed and the accelerometer returned. Measurements Physical activity Accelerometers are now quite common in physical ac- tivity research. The Actigraph is the most commonly used accelerometer to assess physical activity in children. It has excellent intra- and inter-instrument reliability across a wide range of accelerations [22], and has been validated for use in 3- to 5-year-old children [23]. We use the Actigraph GT3XE activity monitor which weighs 27 g with dimensions of 3.8 × 3.7 × 1.8 cm (about the size of a matchbox) and measures accelerations ranging in magni- tude from ~0.05 to 2 G’s. The acceleration signal is sampled and digitized by a 12-bit Analog to Digital Con- verter at a rate of 30 Hz. The signal then passes through a digital filter that band-limits the accelerometer to the fre- quency range of 0.25 to 2.5 Hz, which detects normal human motion and rejects motion from other sources. The GT3XE is secured to the child by a belt worn around the waist. The Actigraph GT3XE accelerometer is ideal for the HOPP study because it can program short epochs (sampling intervals) allowing more accurate measurement of children’s physical activity behaviour [18]. We use a 3-s epoch because this duration approximates the mean and median duration of vigorous activity bouts observed in young children [24] and, based on our pilot work, ac- curately captures physical activity intensity [25]. Over the 7 days of monitoring, the device is only removed when the child is sleeping or when exposed to water (e.g., bathing or swimming) since it is not waterproof. Parents keep a log book to record the time of day when the accelerometer is placed on the child and taken off and the reasons why, in- cluding any activities that occur during this time (e.g., a nap or swimming lessons). In our pilot work, we achieved a mean monitoring (wearing) time of 11 h per day. The data are visually inspected and only discarded when corresponding to a time the child was not wearing the accelerometer, based on log books. How do we determine physical activity prevalence and pattern? Accelerometer output (counts) is usually analyzed according to cut-off values or thresholds reflecting different intensities of physical activity, and there are numerous thresholds to choose from. We use cut-off values developed and validated specifically with healthy 3- to 5-year-old Page 3 of 7 children using Actigraph accelerometers [23]. These are: 420 counts/15 s (moderate intensity) and 842 counts/15 s (vigorous intensity). Because we use 3-s epochs, we divide these counts by 5 to establish counts per 3 s, as previously published [18]; thus, the physical activity intensity categor- ies used in the HOPP study are: sedentary (<8 counts/3 s); light (≥8 but <84 counts/3 s); moderate (≥84 but <168 counts/3-s); and vigorous (≥168 counts/3-s). To determine physical activity prevalence, we calculate each intensity cat- egory as minutes per day and as % of time when the accel- erometer was worn, to account for individual differences in monitoring time. To determine physical activity patterns, we calculate the number of bouts of continuous activity at each intensity category, as previously described [18]. To ac- commodate these analyses, we developed custom Microsoft EXCEL™ macros. Health outcomes Body Composition We perform standard measures of anthropometry, including standing height measured without shoes to the nearest 0.1 cm using a calibrated stadiometer and body mass measured without shoes and in light clothing to the nearest 0.1 kg using a digital scale. A bioelectric impedance analyzer is used to assess body composition while children are in a supine position. Fat free mass (FFM) is calculated using an age-specific equation [26], which has been validated in 4- to 6-year–old children using doubly-labelled water. Percent body fat is calculated as [(Body weight – FFM)/body weight] x 100. As an additional estimate of body fat distribution, we measure waist circumference as a surrogate of central adiposity in preschoolers [27]. Measurements are made in duplicate at 3 different locations – uppermost lateral border of the iliac crest, midpoint between the lower margin of the last palp- able rip and the top of the iliac crest, and 4 cm above the umbilicus. In a subset of participants (n = 100), we are also assessing body composition using dual energy x-ray ab- sorptiometry (DXA). Participants are scanned in a supine position using a Hologic QDR 4500A scanner. In addition to total body analysis, we perform regional lean mass and fat mass calculations with the pediatric software (Version 12.3) accompanying the DXA scanner. Health-related fitness Aerobic fitness is assessed on a treadmill using the Bruce protocol, a progressive treadmill test, as previously described [28]. Participants are given several minutes of familiarization on the treadmill at a comfortable walking pace. During the test, participants are instructed to hold onto the handrails to assist with coordin- ation, with a researcher positioned directly behind the child to ensure safety. Heart rate (HR) is recorded continuously during the test using a HR monitor (Polar Electro, Kempele, Finland). The test is terminated when the child can no