Intensity Thresholds on Raw Acceleration Data: Euclidean Norm Minus One (ENMO) and Mean Amplitude Deviation (MAD) Approaches
Objectives (1) To develop and internally validate Euclidean Norm Minus One (ENMO) and Mean Amplitude Deviation (MAD) thresholds for separating sedentary behaviours from common light intensity physical activities using raw acceleration data collected from both hip and wrist worn tri axial accelerometers; and (2) to compare and evaluate the performances between the ENMO and MAD metrics. Methods Thirty three adults [mean age (standard deviation (SD) 27.4 (5.9) years; mean BMI (SD) 23.9 (3.7) kg/m2; 20 females (60.6% wore four accelerometers; an ActiGraph GT3X+ and a GENEActiv on the right hip; and an ActiGraph GT3X+ and a GENEActiv on the non dominant wrist. Under laboratory conditions, participants performed 16 different activities (11 sedentary behaviours and 5 light intensity physical activities) for 5 minutes each. ENMO and MAD were computed from the raw acceleration data, and logistic regression and receiver operating characteristic (ROC) analyses were implemented to derive thresholds for activity discrimination. Areas under ROC curves (AUROC) were calculated to summarise performances and thresholds were assessed via executing leave one out cross validations. Results For both hip and wrist monitor placements, in comparison to the ActiGraph GT3X+ monitors, the ENMO and MAD values derived from the GENEActiv devices were observed to be slightly higher, particularly for the lower intensity activities. Monitor specific hip and wrist ENMO and MAD thresholds showed excellent ability for separating sedentary behaviours from motion based light intensity physical activities (in general, AUROCs >0.95) with validation indicating robustness. However, poor classification was experienced when attempting to isolate standing still from sedentary behaviours (in general, AUROCs <0.65) The ENMO and MAD metrics tended to perform similarly across activities and accelerometer brands. Conclusions Researchers can utilise these robust monitor specific hip and wrist ENMO and MAD thresholds, in order to accurately separate sedentary behaviours from common motion based light intensity physical activities. However, caution should be taken if isolating sedentary behaviours from standing is of particular interest.
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