The Performance of Ankle- and Waist-Based Accelerometry in Quantifying Physical Activity Intensity Among 6- to 24-Month-Year-Olds: A Semistructured Laboratory Study.

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We aim to quantify the performance of accelerometry in objectively measuring physical activity (PA) intensity among infants and toddlers. Thirty-eight 6- to 24-month-olds participated in a 30-min, semistructured lab visit. Twenty-three (61%) children could walk independently. Children engaged in a variety of activities while wearing accelerometers on each ankle and at the waist. Visits were video recorded, and study team members independently coded the first three 5-s epochs of each minute for PA intensity using a 5-level scale ranging from 1 = completely sedentary to 5 = moderate-to-high intensity. Interrater agreement for PA classifications was excellent (median kappa per child = 0.85). A series of logistic regression models were fit to find the vector magnitude threshold per 5-s epoch that differentiated activity intensity above each PA level with ≥ 80% sensitivity. Analyses included 3191 epochs; a median of 88 epochs per child. The classification performance applying all thresholds concurrently for the five PA intensity levels was poor for each wear location (agreement < 50%, kappa < 0.25). Classification improved when concatenating intensity levels, with the best performance comparing sedentary (levels 1-2) to nonsedentary (levels 3-5) and using data from the left ankle device: agreement ≥ 77.6%, kappa ≥ 0.44. Applying those novel thresholds to predict the total time spent in level 3-5 activities over all coded epochs using linear regression performed as well as using the sum of vector magnitude across epochs when using data from the left ankle device. Overall, the performance of the left ankle wear location was similar to the right ankle wear location and superior to the waist location. Ankle-worn accelerometry had adequate validity to classify in-the-moment nonsedentary behaviours and total time spent nonsedentary over a time interval among this sample of infants and toddlers. While caution is warranted when generalizing these lab-based findings to naturalistic settings, findings provide insight into objective measures of PA for this age range.