Validation of the GT3X ActiGraph in children and comparison with the GT1M ActiGraph

The GT1M ActiGraph has been replaced by the triaxial GT3X which includes an inclinometer to detect postures. The purpose of this study was to investigate comparability of the GT3X to the GT1M and to develop activity intensity cut-points for the vector magnitude of the GT3X (VM(GT3X)) in children. Additionally, the study aimed to assess the validity of the GT3X inclinometer for detecting postures.

Forty-nine children aged 10-15 were tested during semi-structured activities in a laboratory setting (lying, sitting, standing, Nintendo Wii boxing, walking and running).

Expired respiratory gases were measured continuously using the Cosmed K4b(2) portable metabolic system. Simultaneously, ActiGraph counts by a GT3X and a GT1M were recorded.

Significantly higher counts were found for GT3X vertical mean counts per second (vcps) and antero-posterior counts per second (apcps) during running, relative to the GT1M. Cut-points for the VM(GT3X), developed using Receiver Operator Characteristics (ROC) curves (development group N=32, validation group N=17), were <3 counts s⁻¹ for sedentary (cross-validation: 87% correctly classified), 3-56 counts s⁻¹ for light (cross-validation: 70% correctly classified) and >56 counts s⁻¹ for moderate to vigorous (mvpa) (cross-validation: 86% correctly classified). The inclinometer correctly classified standing 20%, lying 15%, sitting 94% and the off position 45% of the time.

The vcps from the two monitors differ for certain activities. Cross-validated cut-points for the classification of VM(GT3X) counts into sedentary, light and mvpa are presented. Posture classification by the GT3X should be interpreted with care, as misclassifications are common.