Inter-instrument Reliability Of GT3X Accelerometers In A Free-living Condition

Accelerometers have the advantage of providing objective measures of both physical activity performed at various intensities and of sedentary time. ActiGraph accelerometers are widely used, however with technological advancements new models are periodically released. The GT3X is the most recent model. Although research has shown previous ActiGraph models to be reliable, little information is available on the inter-instrument reliability of GT3X accelerometers.

Purpose To determine inter-instrument reliability of GT3X accelerometers measures of physical activity and inactivity in a free-living condition.

Methods Sixteen women and 11 men (age 24.6 ± 9.9 yr, BMI 24.1 ± 3.6 kg/m2) wore two accelerometers simultaneously, one on the left and one on the right hip, for >4 consecutive days. Accelerometers were initialized to collect step counts and activity counts/minute (cpm), simultaneously. Activity counts (60 sec epochs) were categorized into time spent in sedentary (<100 cpm), light-intensity (100 – 1951 cpm), and moderate-to-vigorous-intensity physical activity (≥ 1952 cpm) using previously validated cutpoints. Bland Altman (BA) plots, intraclass correlation (ICC), and paired t-tests were used to determine inter-instrument reliability.

Results The BA plot mean difference was 0.7 min/d (95% CI: -1.6 to 2.9), -0.8 min/d (95% CI: -2.8 to 1.2), -0.1 min/d (95% CI: -0.5 to 0.4), and 22 steps/d (95% CI: -37 to 81) for sedentary, light-intensity, moderate-to-vigorous-intensity, and step counts, respectively. ICCs were 0.99 (P < .001), 0.99 (P < .001), 0.99 (P < .001), and 0.99 (P < .001) for sedentary, light-intensity, moderate-to-vigorous-intensity, and step counts, respectively. Results from the paired t-test showed there were no significant inter-instrument differences across sedentary (t = 0.61, P = 0.55), light-intensity (t = -0.85, P = 0.40), moderate-to-vigorous-intensity physical activity (t = -0.18, P = 0.86), and steps/d (t = 0.76, P = 0.45).

Conclusion findings suggest the GT3X accelerometer provides acceptable inter-instrument reliability in measuring sedentary behavior, physical activity, and step counts. These findings provide support for the use of inter-instrument comparisons in studies that employ GT3X accelerometers to assess habitual activity in free-living conditions.