Comparison of Physical Activity Assessed by Uniaxial and Triaxial Accelerometry in Adolescents under Free-living Conditions: The HELENA Study

Introduction Different types of accelerometers are available to assess physical activity (PA) patterns levels, and the choice about which to use depends on various factors: cost, physical characteristics (weight, size, and battery life), performance (number of axes, possible epochs, system of data transfer, recording duration, function of the epochs, and thememory capacity), the validity and the intra and interinstrument reliability. Given the large number of studies that have used uniaxial or triaxial devices, it is of interest to know whether the different devices give similar information about PA levels and patterns. The aim of this study was to compare PA levels and patterns obtained simultaneously by uniaxial accelerometry and triaxial accelerometry in adolescents in free-living conditions (FLC).

Methods Sixty-two participants, aged 13–16 years, were recruited to participate in this ancillary study, which is a part of the Healthy Lifestyle in Europe by Nutrition in Adolescence Study (HELENA). All participants wore a uniaxial accelerometer (ActiGraph GT1M®, Pensacola, FL) and a triaxial accelerometer (RT3®, Stayhealthy, Monrovia, CA) simultaneously for seven days. The levels of PA were expressed as counts per minute. The patterns were calculated by converting accelerometer data output as a percentage of time spent at sedentary, light, moderate, and vigorous PA per day, using thresholds defined previously for each accelerometer. Analysis of output data from the two accelerometers were assessed by two different statistics tests: Equivalence Test and Bland & Altman method.

Results Mean duration of data record available for analysis was 11.5 ± 2.5 hours·day^-1. The mean PA was 204.1 counts·h^–1 for the RT3 and 353.2 counts·h^–1 for the ActiGraph. The equivalence test confirmed equivalence between the data from the triaxial accelerometer and uniaxial accelerometer at each intensity level (P < 0.001). The differences between the two devices were 2.1% ± 1% for sedentary activity, 1.4% ± 1.7% for light activity, 0.6% ± 0.5% for moderate activity, and 0.02% ± 0.09% for vigorous activity. The Bland & Altman method showed good agreement between data obtained between the both accelerometers (p < 0.05).

Discussion and Conclusion In summary, this study demonstrates no differences between PA levels and patterns obtained by a uniaxial accelerometer (ActiGraph) and triaxial accelerometer (RT3) in adolescents in FLC. The choice of a uniaxial or triaxial accelerometer makes little difference in the assessment of PA pattern in free-living conditions. Therefore, both uniaxial or triaxial accelerometers can be used in clinical practice to quantify PA.