Interunit and Intraunit Reliability of the RT3 Triaxial Accelerometer

in Journal of Physical Activity and Health
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Interest in the quantification of physical activity is on the rise. Tri-axial accelerometry has frequently been used; however, research on the reliability of these devices is limited. We examine the interunit and intraunit reliability of 22 RT3 triaxial accelerometers using a performance-documented laboratory agitator.


The RT3 units were tested while moving in 2 directions (antero-posterior, medio-lateral) and speeds (150 and 275 RPM) on a shaker with simultaneous documented performance output for three 24-hour periods.


Minimal shaker variance was recorded for all trials (coefficients of variation [CVs] < 0.52%). Our data demonstrate good reliability within RT3s (CVs < 1.81%) but poor reliability among the 22 units (CVs range = 9.5% to 34.7%).


In longitudinal studies, each subject should use the same RT3 unit at each assessment. The use of multiple RT3 units in cross-sectional studies is not recommended because data interpretation would be compromised by the high between-unit variability.

Krasnoff is with the Exercise Physiology and Body Composition Laboratory, Dept of Medicine, and Kohn the Dept of Epidemiology and Biostatistics, University of California at San Francisco, San Francisco, CA 94143. Choy is with NCIRE Biomedical Engineering, and Doyle and Johansen the Division of Nephrology, San Francisco Veterans Administration Medical Center, San Francisco, CA 94121. Painter is with the University of Minnesota Dept of Medicine, Division of Renal Diseases, Minneapolis, MN 55455.