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BMC Medical Research Methodology

, 7:26

First Online: 25 June 2007Received: 11 December 2006Accepted: 25 June 2007


BackgroundUnderstanding the relationships between physical activity PA and disease has become a major area of research interest. Activity monitors, devices that quantify free-living PA for prolonged periods of time days or weeks, are increasingly being used to estimate PA. A range of different activity monitors brands are available for investigators to use, but little is known about how they respond to different levels of PA in the field, nor if data conversion between brands is possible.

Methods56 women and men were fitted with two different activity monitors, the Actigraph™ Actigraph LLC; AGR and the Actical™ Mini-Mitter Co.; MM for 15 days. Both activity monitors were fixed to an elasticized belt worn over the hip, with the anterior and posterior position of the activity monitors randomized. Differences between activity monitors and the validity of brand inter-conversion were measured by t-tests, Pearson correlations, Bland-Altman plots, and coefficients of variation CV.

ResultsThe AGR detected a significantly greater amount of daily PA 216.2 ± 106.2 vs. 188.0 ± 101.1 counts-min, P < 0.0001. The average difference between activity monitors expressed as a CV were 3.1 and 15.5% for log-transformed and raw data, respectively. When a conversion equation was applied to convert datasets from one brand to another, the differences were no longer significant, with CV-s of 2.2 and 11.7%, log-transformed and raw data, respectively.

ConclusionAlthough activity monitors predict PA on the same scale counts-min, the results between these two brands are not directly comparable. However, the data are comparable if a conversion equation is applied, with better results for log-transformed data.

AbbreviationsPAPhysical activity

AMActivity monitor

AGRActigraph activity monitor

MMActical activity monitor

CVcoefficient of variation

SDstandard deviation

Electronic supplementary materialThe online version of this article doi:10.1186-1471-2288-7-26 contains supplementary material, which is available to authorized users.

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Autor: David R Paul - Matthew Kramer - Alanna J Moshfegh - David J Baer - William V Rumpler


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