Validity, reliability, and calibration of the Tritrac accelerometer as a measure of physical activity

Purpose: The purposes of this study were to assess the validity and reliabi lity of the Tritrac R3D accelerometer during treadmill walking and running and then to calibrate the instrument. Methods: The Tritrac was assessed on 60 young adults (23.4 +/- 2.9 yr) during treadmill walking and running at 3 .2, 6.4, and 9.7 km.h(-1). The calibration was carried out by identifying r anges of Tritrac raw data (vector magnitude) values corresponding to light (2-3.9 MET), moderate (4-7 MET), and vigorous (>7 MET) physical activity. E nergy expenditure (EE), measured by indirect calorimetry, served as the cri terion measure. Results: Interinstrument intraclass reliability coefficient s for Tritracs worn on the right and left hip ranged from 0.73-0.87, while intersession coefficients demonstrated high reliability for all speeds (R = 0.87-0.92). Paired t-tests comparing mean accelerometer counts at 6.4 km.h (-1), 0% grade (2647 +/- 456), and 6.4 km.h(-1), 5% grade (2635 +/- 435) de monstrated no significant difference (P > 0.05). Mean differences between E E measured by indirect calorimetry and that estimated by the Tritrac ranged from 0.0082 kcal.kg(-1).min(-1) at 3.2 km.h(-1) to 0.0320 kcal.kg(-1).min( -1) at 9.7 km.h(-1), with the Tritrac consistently overestimating EE during horizontal treadmill walking. The relationship between vector magnitude an d EE across all speeds was highly linear (R-2 = 0.90, SEE = 0.014 kcal.kg(- 1).min(-1)), with little overlap between light, moderate, and vigorous cate gories. The mean vector magnitudes at 2, 4, and 7 MET were 650, 1772, and 3 455, respectively. Conclusions: These data indicate that the Tritrac is hig hly reliable from day to day and is sensitive to changes in speed but not g rade. Furthermore, the Tritrac accurately distinguishes various intensities of walking and jogging on level ground. With limitations, these cut-points can be used to categorize light, moderate, and vigorous physical activity and to estimate EE.