Public Health Nutrition: 14(12), 2124–2133 doi:10.1017/S1368980011001868 Measuring physical activity using accelerometry in 13–15-year-old adolescents: the importance of including non-wear activities Femke De Meester 1, *, Ilse De Bourdeaudhuij 1 , Benedicte Deforche 1,2 , Charlene Ottevaere 3 and Greet Cardon 1 1 Department of Movement and Sport Sciences, Faculty of Medicine and Health Sciences, Ghent University, Watersportlaan 2, B-9000 Ghent, Belgium: 2 Department of Human Biometry and Biomechanics, Faculty of Physical Education and Physiotherapy, Vrije Universiteit Brussel, Brussels, Belgium: 3 Department of Public Health, Faculty of Medicine and Health Sciences, Ghent University, Ghent, Belgium Submitted 2 February 2011: Accepted 16 June 2011: First published online 11 August 2011 Abstract Objective: The present study aimed to examine the impact of non-wear activities registered in diaries when using accelerometers to assess physical activity (PA) in young adolescents. Design: Data arise from a large-scale cross-sectional study on PA. PA was objectively assessed using Actigraph TM accelerometers (Actigraph MTI, Manufacturing Tech- nology Inc., Pensacola, FL, USA) during seven consecutive days. Non-wear time activity diaries were provided to register the activities for which the accelerometer was removed. After correction to deal with over-reporting, the registered minutes of PA were used to replace periods of non-wear time measured by the accelerometer. Setting: Between October 2008 and May 2009 adolescents were recruited by home visits in Ghent (Belgium). Subjects: Young adolescents (n 513; 48?6 % boys) aged 13 to 15 years. Results: Of the total sample, 49?9 % registered at least one activity of moderate to vigorous intensity in the non-wear time activity diary. More adolescents registered an activity performed on a weekday than on a weekend day and the registered mean number of minutes of moderate to vigorous PA were higher on weekend days. Repeated-measures (M)ANOVA tests revealed a significant difference between the mean minutes with and without non-wear activities for all PA intensities, regardless of adolescents’ socio-economic status or gender. More adolescents achieved the PA recommendations after inclusion of the non-wear activities irrespective of accelerometer thresholds used. Conclusions: The collection of information regarding non-wear time by non-wear time activity diaries when using accelerometers in 13–15-year-old adolescents can lead to different PA outcomes at the individual level and therefore can improve the ability to accurately measure PA. Keywords Accelerometry Activity diary Adolescents Physical activity Regular physical activity (PA) is a key protective factor for health. Among youngsters, a physically active lifestyle can contribute to concurrent (1–4) and future health bene- fits (4,5) . Based on these findings, PA recommendations for school-aged adolescents subscribe participation in PA of at least moderate to vigorous intensity (MVPA) for 60 min/d (3,6) . Consequently, accurate assessment methods for the amount and intensity of PA are necessary to examine the relationship between PA and specific health outcomes and to evaluate the effectiveness of PA interventions (7) . During the last decade, accelerometers have emerged as one of the most commonly used tools to objectively assess the frequency, duration and intensity of free-living PA. Recent reviews have designated accelerometry as valid and reliable for quantifying PA levels in adolescents (8–10) . Furthermore, accelerometers are small, lightweight and thus unobtrusive. Following the advantages of the device, sev- eral large-scale observational and intervention studies used accelerometers in children and adolescents (11–15) . Never- theless, different challenges are related to accelerometry, of which several are inherent to methodological considerations concerning the data-reduction analysis process (16) . When reducing and cleaning accelerometer data, investigators are often confronted with missing data arising from accelerometers that are not worn consistently during the entire measurement period (9,17,18) . Non-wear time can be due to participants’ non-compliance to the study protocol or to legitimate reasons for removing the accelerometer, such as aquatic activities (e.g. swimming, bathing) and activities where safety regulations prohibit *Corresponding author: Email Femke.DeMeester@UGent.be r The Authors 2011