Obesity Epidemiology

PHYSICAL ACTIVITY MEASUREMENTS 129A literature review by Trost et al. 38 on methodological issues related to accelerometerbasedassessments of physical activity in free-living individuals found no convincingevidence of greater validity or reliability for one make or model of accelerometer overanother. Thus, they suggested that accelerometer selection be based on cost, technicalsupport, and comparability with other studies. They also noted that the use of multipleaccelerometers to estimate energy expenditure produced only marginal improvements inprediction power compared with estimates from a single accelerometer. In addition, therewas no clear evidence that multiaxial accelerometers (detecting accelerations in multipledimensions) are better for detecting physical activity than uniaxial devices (detectingmotion in one plane, usually vertical). However, multiaxial accelerometers are more sensitiveto light activities (e.g., slow walking), and provide more accurate measurements ofstatic trunk movement in activities like cycling and rowing. 37 A newly developed accelerometer(Intelligent Device for Energy Expenditure and Activity [IDEEA]) has shownpromising results in measuring type, duration, and intensity of physical activity andimproving energy expenditure estimation in laboratory-based settings, 47,48 but field-basedvalidation studies are needed.Standardized placement of accelerometers on the body (typically mounted to thewaist) can help to reduce extraneous variations in measurements, although evidence islacking regarding the superiority of one position versus another. 36 Output of measurementscan be expressed in multiple ways including total movement counts, energy expenditurecalculated by prediction equations, or time spent performing individual activities.The estimated energy expenditure largely depend on the accuracy of prediction equationsderived from various calibration studies, 49 which appear to be population-specificand can yield different activity cut points for defining various intensities of physicalactivity. 50 How to enhance the comparability of different calibration studies remains achallenge in the field.To account for day-to-day variations and reliably estimate habitual physical activity inadults, Mathews et al. 51 suggested 3 to 5 days of monitoring (optimally 7 days, including1 weekend day). Trost 52 recommended 4 to 9 days of monitoring for children and adolescents.In a subsequent study, Trost et al. 38 found that repeated measures across differentseasons might be required to estimate long-term physical activity.Accelerometer-based devices are an attractive way to measure physical activity in children.53 In laboratory conditions with structured activities, accelerometer activity countsamong children were found to be strongly correlated with energy expenditure measuredby DLW and indirect calorimetry. 54,55 As in adult studies, accelerometer validity hasbeen found to vary by type and intensity of activities. 56 Lopez-Alarcon et al. 57 reportedthat accelerometry had limited value in measuring free-living energy expenditure amongvery young children. An 8-day field test of Actiwatch activity monitors in children 4 to 6years old showed that activity counts did not reflect total energy expenditure assessed byDLW and were not correlated with percentage fat mass.Heart Rate MonitoringBecause of the linear relationship between heart rate increases and oxygen consumptionof contracting skeletal muscles during moderate to vigorous physical activity, heartrate monitoring can provide a continuous record of physical activity energy expenditurein free-living populations. 58 Currently available heart rate monitors are lightweight,small, and have large data-storage capability. Heart rate data are used to predict energyexpenditure by calibrating heart rate against the maximum volume of oxygen consumed(VO 2 max). There are several approaches for estimating energy expenditure, including the