Interinstrument Reliability of the Actigraph GT3X+ Ambulatory Activity Monitor During Free-living Conditions in Adults

Background: Currently, no studies have investigated inter-instrument 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 one 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: Inter- instrument 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>0.05). Conclusion: Reliability decreases beyond moderate intensities. MVPA displays superior inter-instrument reliability than individual intensity categories. Research question permitting, reporting time in MVPA may maximise reliability.