To analyze the intra- and inter-instrument reliability of the ActiGraph GT3X accelerometer using a vibration table on each orthogonal axis and at five frequencies of motion. Ten GT3X units were subjected to a specific vibration using a motorized vibration table along the vertical, horizontal right-left and horizontal front-back axis, and at 1.1, 2.1, 3.1, 4.1 and 10.2 Hz. The 5 min data for each frequency were analyzed separately for frequency, axis effects, and inter- and intra-instrument variability. We found overall high intra-and inter-instrument reliability for the GT3X accelerometer at frequencies between 2.1 and 4.1 Hz. For frequencies ranging between 2.1 and 4.1 Hz, the intra-instrument coefficient of variation was ≤2.5%. The inter-instrument coefficient of variation ranged widely along axes and frequencies, with the lowest values (≤9%) corresponding to 2.1-4.1 Hz. The intra-class correlation coefficient for activity counts across frequencies and for all axes was 0.97. Overall, our findings support the use of the GT3X accelerometer as an accurate tool to estimate free-living physical activity, at least within those frequencies that are common to most types of human daily activities.
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