Interinstrument Reliability of the ActiGraph GT3X+Ambulatory Activity Monitor During Free-Living Conditions in Adults

Background: Currently, no studies have investigated interinstrument reliability of the ActiGraph (AG) GT3X+ in free-living conditions. Methods: Nineteen adults (11 males, 8 females; aged 36.8 +/- 11.9 years) wore a pair of AG's (one on each hip), during all waking hours for 1 day. Raw outputs were generated for total counts, steps, wear time and mean counts per minute. Intensity outputs were derived for time (minutes) spent in <moderate, moderate, vigorous, very vigorous and moderate-to-vigorous physical activity (MVPA). Intraclass correlation (ICC), absolute percent difference (APD), coefficient of variation (CV), Bland-Altman plots, and paired t tests were used to evaluate reliability. Results: Interinstrument reliability was high (CV < 5%) for raw count and derived intensity outputs, except vigorous and very vigorous activity. ICC, CV, and APD values for vigorous and very vigorous were.97, 12.28, 17.36% and.98, 18.15, 25.67%, respectively. Amalgamating moderate, vigorous, and very vigorous into a single MVPA category reduced the CV and APD values to 2.85 and 4.02%, and increased the ICC value to.99. No significant differences were found between contralateral units for any outputs (P >.05). Conclusion: Reliability decreases beyond moderate intensities. MVPA displays superior interinstrument reliability than individual intensity categories. Research question permitting, reporting time in MVPA may maximize reliability.