Are Accelerometry-determined Cut-points For Activity Intensity Stable Within People?

Background: Cut-points for physical activity intensity are developed from the relationship between oxygen uptake (VO2) and rate of activity counts determined with accelerometry. This practice assumes that the relationship between VO2 and activity-count rate is stable within people, but this has not been directly examined.

Purpose: To examine whether the VO2 to activity-count rate relationship and the cut-points for moderate- and vigorous-intensity physical activity are reliable within people.

Methods: Twenty eight healthy persons (22 ± 4 yrs; 13 women) completed two sessions with identical procedures within 7 days. VO2 and tri-axial activity-count rate were respectively measured with open-circuit spirometry and an ActiGraph accelerometer (GT3X+) placed on the dominant hip during sitting and 7 physical activities: walking at 2 different intensities, jogging, ascending and descending a 20-step staircase, washing dishes, vacuuming, and moving a box. Individual linear regressions of VO2 against activity-count rate allowed determination of these dependent variables for each participant: the slope, intercept, and R2 of the relationship between VO2 and activity count rate, and the cut-points for moderate- and vigorous-intensity activity defined as activity-count rate at 3 and 6 Metabolic Equivalent units, respectively. Differences in the above variables between sessions were evaluated with paired t-tests, Intra-class Correlation Coefficients (ICCs; 2-way random model with absolute agreement), and Bland-Altman plots.

Results: There were no significant mean differences between sessions in any of the dependent variables (p > .05). All ICCs were significant (p ≤ .001). ICCs were strong for the slope and R2 (0.83 and 0.88, respectively), but moderate for the intercept and the moderate- and vigorous-intensity cut-points (0.75, 0.73, and 0.70, respectively). Bland-Altman plots showed negligible mean error, but wide variation in the difference between sessions for moderate- and vigorous-intensity cut-points.

Conclusions: The relationship between VO2 and activity-count rate and the cut-points for moderate- and vigorous-intensity activity are moderately reliable within people. Advances in calibration procedures are needed to improve the prediction of physical activity intensity from accelerometry.