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Detecting Indoor and Outdoor Environments Using the ActiGraph GT3X+ Light Sensor in Children

Flynn, Jennifer I.; Coe, Dawn P.; Larsen, Chelsea A.; Rider, Brian C.; Conger, Scott A.; Bassett, David R. Jr.

Medicine & Science in Sports & Exercise: January 2014 - Volume 46 - Issue 1 - p 201–206
doi: 10.1249/MSS.0b013e3182a388c0
SPECIAL COMMUNICATIONS: Methodological Advances

Introduction Experts recommend children spend more time playing outdoors. The ambient light sensor of the ActiGraph GT3X+ provides lux measurements. A lux is the International System’s unit of illumination, equivalent to 1 lm·m−2. Few studies have established a lux threshold for determining whether a child is indoors or outdoors.

Purpose This study aimed 1) to assess the reliability of the ActiGraph GT3X+ ambient light sensor, 2) to identify a lux threshold to accurately discriminate between indoor and outdoor activities in children, and 3) to test the accuracy of the lux threshold in a free-living environment.

Methods In part 1, a series of reliability tests were performed using 20 ActiGraph GT3X+ monitors under different environmental conditions. Cronbach’s alpha was used to determine interinstrument reliability. In part 2, 18 children performed 11 different activities (five indoors and six outdoors) for 6 min each. The optimal threshold for detecting indoor/outdoor activity was determined using a receiver operator characteristic curve analysis. In part 3, 18 children at a preschool wore the monitor during a school day. Percent accuracy was determined for all conditions.

Results In part 1, the devices had Cronbach’s alpha values of 0.992 and 1.000 for indoor and outdoor conditions, respectively, indicating high interinstrument reliability. In part 2, the optimal lux threshold was determined to be 240 lux (sensitivity = 0.92, specificity = 0.88, area under the curve = 0.96, 95% CI = 0.951–0.970). In part 3, results of the school-day validation demonstrated the monitor was 97.0% accurate for overall detection of indoor and outdoor conditions (outdoor = 88.9%, indoor = 99.1%).

Conclusions The results demonstrate that an ActiGraph GT3X+ lux threshold of 240 can accurately assess indoor and outdoor conditions of preschool children in a free-living environment.

Department of Kinesiology, Recreation, and Sport Studies, University of Tennessee, Knoxville, TN

Address for correspondence: Jennifer I. Flynn, M.S., Department of Kinesiology, Recreation, and Sport Studies, The University of Tennessee, Knoxville, 1914 Andy Holt Ave., Knoxville, TN 37996; E-mail:

Submitted for publication April 2013.

Accepted for publication June 2013.

© 2014 American College of Sports Medicine