Timmons et al. BMC Public Health 2012, 12:284 http://www.biomedcentral.com/1471-2458/12/284 longer keep up with the increasing grade or slope of the treadmill, or by refusal to continue exercising despite verbal encouragement. Following test termination, the child is im- mediately seated and asked to remain as still as possible. HR is recorded every 30 s during this recovery period and HR recovery is calculated as the difference between HR at the end of exercise and at each time point of recovery, with higher values indicating faster recovery. In our pilot work we found evaluation of HR recovery according to these procedures to be feasible and reliable [19]. To determine short-term muscle power, we developed a 10-s modified Wingate anaerobic cycling test that is feasible and reliable for preschoolers [19]. To establish peak pedalling speed, the child first performs an initial “sprint” by pedaling as fast as they can for ~20 s against only the internal resistance of the ergometer (Pediatric Corival, LODE). Braking force on the ergometer is calculated as 0.55 N m kg-1 body mass. The child is instructed to pedal as fast as they can with a “ready. . . set. . . go” signal from the investigator. When 80 % of the peak pedalling speed is reached, the braking force is applied. During the test, the child is encouraged to keep pedalling as fast as he/she can. Peak power output is taken as the highest power output during the test; mean power output is taken as the average power output over the 10 s. Vascular structure and function Arterial stiffness and wall thickness are assessed using non-invasive methods, as previously described in our pilot work [21]. Participants rest in the supine position and single-lead electrocardiog- raphy (ECG) is continuously recorded. After ~10 min of rest, longitudinal images of the carotid artery are acquired using b-mode ultrasound. A linear array transducer is posi- tioned at the lateral side of the neck (Vivid-q, GE Medical Systems, Horten, Norway) and ECG-gated images repre- senting 10 consecutive heart cycles (sampling frequency: 23 frames/s) are obtained. To determine carotid artery stiff- ness, two additional 10-beat clips are recorded simultan- eous to acquisition of carotid artery blood pressure waveforms on the opposite common carotid artery, using a hand-held tonometer (model SPT-301, Millar Instruments Inc., Houston, TX). The carotid pressure waveforms are calibrated to discrete measures of brachial blood pressure using an automated device (Dinamap Pro 100, Critikon LCC, Tampa, FL.). Images are digitally stored for off-line analyses using an automated edge tracking system [AMS II (Artery Measurement System) Image and Data Analysis, Tomas Gustavsson], which provides measurements of ar- terial borders (intima to media and internal diameter) on all acquired frames. Intima-media thickness is assessed at the far-wall on end diastolic frames and carotid artery stiff- ness (distensibility) is calculated as: [(maximum diameter – minimum diameter)/(carotid pulse pressure x minimum diameter)]. Page 4 of 7 Arterial stiffness is also determined using “whole body” pulse wave velocity in a large segment of the arterial tree (heart to dorsalis pedis artery in foot). A photoplethysmo- graphic probe (MLT1020PPG, ADInstruments, Colorado Springs, CO, USA) is placed on the skin over the dorsalis pedis artery. The time delay from ventricular depolarization (R-spike on ECG) to the arrival of the pulse wave at the ar- tery is calculated and divided into the measured body sur- face distance between these sites (i.e., suprasternal notch to foot). Following supine measurements, children are seated and automated cuff measurements of brachial blood pres- sure are obtained after 2–3 min of seated rest. We have pre- viously established the reliability of these measures in 3- to 5-year-old children [21]. Health-related quality of life We measure health- related quality of life to capture domains of health such as emotional and social functioning, which are relevant to early child development. To determine the association between physical activity and health-relate quality of life, parents/guardians complete the 23-item PedsQL™ 4.0 Generic Scale. This instrument measures: Physical Func- tioning (8 items); Emotional Functioning (5 items); Social Functioning (5 items); and Daycare/School Functioning (5 items). Excellent feasibility, reliability, and validity for this instrument have been demonstrated on a sample of 1,257 three-year-olds, 1,198 four-year-olds and 984 five- year-olds [29]. Additional measures In addition to the above measurements, parents provide demographic (ethnicity, family income, childcare arrangements, etc.) and medical (child’s birth weight, duration of breastfeeding, etc.) information as well as in- formation on their child’s participation in organized and unorganized activities, sports, and screen time. Other measures include: Dietary intake To help interpret the relationship between physical activity and health outcomes, particularly body composition, dietary intake is also assessed. Parents record their child’s diet for 2 days during the week (Monday-Friday) and 1 day on the weekend (Saturday or Sunday). Food and drinks consumed are recorded for all meals and snacks, with quantities, brand names, and preparation methods included where appropriate. Infor- mation is entered into the ESHA Food Processor soft- ware (Version 10.8, Salem, OR) and analyzed. Children’s Behaviour Questionnaire To assess the child’s temperament, parents complete the Children’s Be- haviour Questionnaire (very short form), which deter- mines temperament across three broad dimensions: surgency, negative affect, and effortful control [30,31].