Original researchValidation and comparison of ActiGraph activity monitors
Introduction
ActiGraph (Pensacola, FL) activity monitors are widely used in physical activity (PA) research. The uniaxial ActiGraph 7164 model was extensively used in the mid 1990s and early 2000s.1, 2, 3 The 7164 model was subsequently replaced with the technologically superior, Micro-Electro-Mechanical System (MEMS) uniaxial GT1M model. Studies have shown that activity counts measured in the vertical (VT) plane using the 7164 and GT1M monitors are not significantly different from each other.4, 5, 6 This allows inter-monitor (7164 and GT1M) comparisons of results among different studies. In October 2008, ActiGraph enabled dual axes measurement capability in the GT1M by unlocking the antero-posterior (AP) axis. Users can now obtain activity counts from the VT, AP, and a composite vector magnitude of these two axes (VM2) from the GT1M.
In 2009, ActiGraph discontinued the GT1M and released the triaxial GT3X activity monitor. The GT3X measures acceleration in three individual orthogonal planes (VT, AP, and medio-lateral (ML)) and provides activity counts as a composite vector magnitude of these three axes (VM3). Signal processing specifications of the GT1M and the GT3X are identical and ActiGraph states that there are no intra-axis differences in activity counts from these two activity monitors (Personal Communication with John Schneider, ActiGraph Vice-President for research and development, November 2009). To date, no study has empirically examined if there are differences in activity counts between these two monitors. This needs to be investigated since researchers potentially apply PA classification techniques developed on the older GT1M to data collected using newer models. Additionally, determining output similarities between the GT1M and the GT3X may also allow the comparability of results among different studies that did not use the same model of activity monitor. Therefore, the primary aim of this investigation was to determine if there are differences in activity counts obtained from the multiple axes of the GT1M and the GT3X. A secondary aim was to develop VM3 activity count cut-points to classify PA intensity. It is possible that advanced PA classification techniques using pattern recognition will be available for the GT3X in the future. However, these methods remain in the early stages of development. Therefore, despite the limitations of using simple accelerometer PA cut-points, this analytic technique remains the method of choice to classify PA intensity from accelerometer output.
Section snippets
Methods
Fifty healthy participants (28 men and 22 women; mean age ± SD = 26.9 ± 7.7 years) from Amherst, MA and surrounding areas volunteered to participate in this study. Participants were given detailed information about study procedures and provided researchers with written informed consent. This study was approved by the University of Massachusetts Institutional Review Board.
Activity monitors were initialized to collect data in 1 s epochs. The GT3X was initialized to collect data in the VT, AP, and ML
Results
The final sample for the comparison analysis was comprised of 32 participants [mean ± SD age, height, weight, and body mass index: 28.0 ± 9.0 years, 173.2 ± 8.5 cm, 71.6 ± 12.3 kg, and 23.8 ± 3.6 kg m−2]. No significant differences among VT activity counts from the GT1M and GT3X were observed during any of the 4 speeds (Fig. 1a). The Bland–Altman plot showed a high agreement between VT activity counts from the GT3X and GT1M monitors (Fig. 2a). The mean bias for VT activity counts (GT3X − GT1M) was −50 ± 382 counts
Discussion
This study compared activity counts between the ActiGraph GT1M and GT3X activity monitors during treadmill walking and running. Significant inter-monitor differences in AP and VM2 activity counts were found (Fig. 1b and c). These differences resulted in poor agreement between AP activity counts from the two activity monitors and moderate agreement between VM2 activity counts from the GT1M and the GT3X (Fig. 2b and c).
Inter-monitor discrepancy in AP activity counts may be attributable to
Conclusion
The present study showed that VT activity counts from the GT1M and the GT3X are comparable, while those from the AP axis and VM2 are dissimilar. Our results suggest that if data have been collected in more than just the VT axis, a direct comparison of findings among studies must be avoided when one study used the GT1M and the other the GT3X. This study provides new VM3 cut-points for the ActiGraph GT3X, allowing researchers to use a superior activity monitor to classify PA intensity.
Practical implications
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Physical activity prediction models developed from previous uniaxial ActiGraph accelerometers (e.g. Freedson cut-points) can be used with GT3X vertical axis activity counts.
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Due to dissimilarities in activity counts between the GT1M and the GT3X in the antero-posterior axis, direct comparisons of findings among studies using different monitors to measure physical activity must be avoided.
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Until advanced techniques are developed using triaxial data from the GT3X, the VM3 activity count cut-points
Acknowledgements
We thank all of the participants of the study and Cheryl Howe, Kate Lyden, Natalia Petruski, and Sarah Kozey-Keadle who assisted with various aspects of the project.
The present study did not receive any external funding.
Patty Freedson is a paid member of the Scientific Advisory Committee for Actigraph, Inc. (Pensacola, FL).
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