Actigraph GT3X: Validation and Determination of Physical Activity Intensity Cut Points

The aims of this study were: to compare energy expenditure (EE) estimated from the existing GT3X accelerometer equations and EE measured with indirect calorimetry; to define new equations for EE estimation with the GT3X in youth, adults and older people; and to define GT3X vector magnitude (VM) cut points allowing to classify PA intensity in the aforementioned age-groups. The study comprised 31 youth, 31 adults and 35 older people. Participants wore the GT3X (setup: 1-s epoch) over their right hip during 6 conditions of 10-min duration each: resting, treadmill walking/running at 3,5,7, and 9km<bold></bold>h(-1), and repeated sit-stands (30times<bold></bold>min(-1)). The GT3X proved to be a good tool to predict EE in youth and adults (able to discriminate between the aforementioned conditions), but not in the elderly. We defined the following equations: for all age-groups combined, EE (METs)=2.7406+0.00056<bold></bold>VM activity counts (counts<bold></bold>min(-1))-0.008542<bold></bold>age (years)-0.01380<bold></bold> body mass (kg); for youth, METs=1.546618+0.000658<bold></bold>VM activity counts (counts<bold></bold>min(-1)); for adults, METs=2.8323+0.00054<bold></bold>VM activity counts (counts<bold></bold>min(-1))-0.059123<bold></bold>body mass (kg)+1.4410<bold></bold>gender (women=1, men=2); and for the elderly, METs=2.5878+0.00047<bold></bold>VM activity counts (counts<bold></bold>min(-1))-0.6453<bold></bold>gender (women=1, men=2). Activity counts derived from the VM yielded a more accurate EE estimation than those derived from the Y-axis. The GT3X represents a step forward in triaxial technology estimating EE. However, age-specific equations must be used to ensure the correct use of this device.