389 Journal of Physical Activity and Health, 2012, 9, 389-393 © 2012 Human Kinetics, Inc. Ekblom is with the Åstrand Laboratory of Work Physiology, The Swedish School of Sport and Health Sciences, Stockholm, Sweden. Nyberg and Marcus are with the Dept of Clinical Sciences, Intervention, and Technology, Stockholm, Sweden. Ekblom Bak is with the Åstrand Laboratory of Work Physiology, The Swedish School of Sport and Health Sciences, Stockholm, Sweden. Ekelund is with the MRC Epidemiology Unit, Institute of Metabolic Science, Cambridge, United Kingdom. Validity and Comparability of a Wrist-Worn Accelerometer in Children Orjan Ekblom, Gisela Nyberg, Elin Ekblom Bak, Ulf Ekelund, and Claude Marcus Background: Wrist-worn accelerometers may provide an alternative to hip-worn monitors for assessing physi- cal activity as they are easier to wear and may thus facilitate long-term recordings. The current study aimed at a) assessing the validity of the Actiwatch (wrist-worn) for estimating energy expenditure, b) determining cut-off values for light, moderate, and vigorous activities, c) studying the comparability between the Actiwatch and the Actigraph (hip-worn), and d) assessing reliability. Methods: For validity, indirect calorimetry was used as criterion measure. ROC-analyses were applied to identify cut-off values. Comparability was tested by simultaneously wearing of the 2 accelerometers during free-living condition. Reliability was tested in a mechanical shaker. Results: All-over correlation between accelerometer output and energy expenditure were found to be 0.80 (P < .001).Based on ROC-analysis, cut-off values for 1.5, 3, and 6 METs were found to be 80, 262, and 406 counts per 15 s, respectively. Energy expenditure estimates differed between the Actiwatch and the Actigraph (P < .05). The intra- and interinstrument coeficient of variation of the Actiwatch ranged between 0.72% and 8.4%. Conclusion: The wrist-worn Actiwatch appears to be valid and reliable for estimat- ing energy expenditure and physical activity intensity in children aged 8 to 10 years. Keywords: Actigraph, Actiwatch, cut-off values, energy expenditure, indirect calorimetry Accelerometers are now commonly used to quantify physical activity in adults, adolescents, and children. The ability to quantify frequency, intensity, and duration makes it possible to estimate energy expenditure and to estimate the amount of time accumulated and frequency of bouts of activity in prespeciied, absolute intensities in large cohorts. However, loss of data is a common problem in epidemiological research when accelerometers are used, not least in children. 1 In many studies, individuals are usually instructed to wear a motion sensor between 4 and 8 days. In some studies, authors are forced to exclude a relatively large number of individuals due to loss of data (ie, too few days or too few hours per day 2,3 ). Wrist-worn motions sensors seem to be more feasible compared with hip worn monitors and thus facilitate long-term record- ings in children. 4,5 There are several different models of accelerometers, and they may be worn on different positions on the body (ie, wrist, waist, and chest). The Actiwatch (AW) is a uniaxial accelerometer usually intended to be worn on the wrist of the nondominant arm. The placement on the wrist makes it possible to study low-intense physical activities such as arm movements during household work or when playing interactive games, as well as monitoring physical activity during sleep for sleep duration and quality assess- ment. Frequent movements of low intensity have been shown to be of great importance for weight control. 6,7 Previous validation studies 8–10 have examined the valid- ity of the AW when the monitor was placed on the hip and have reported good validity (correlation coeficients between 0.66–0.89) and proposed cut-off values for dif- ferent activity intensities. A previous study suggested that activity monitors placed on hip, ankle, or wrist may provide similar physical activity output (although dif- ferent absolute counts) in adults. 11 Results from another study in adult women suggested that the wrist-worn AW accurately estimated activity energy expenditure (r = .73). 12 However, the validity of the wrist-worn monitor in children has not yet been established. It is likely that estimated energy expenditure and cut-off values for light (1.5 METs), moderate (3 METs), and vigorous (6 METs) physical-activity intensity varies depending on the placement and age. The aims of the current study were, therefore, a) assessing validity of a wrist-worn AW for estimating energy expenditure using indirect calorimetry as reference method, b) determin- ing cut-off values for light, moderate, and vigorous activities, c) examining the agreement between the AW