Reliable recognition of lying, sitting, and standing with a hip‐worn accelerometer

TLDR

Hip‑worn accelerometers are widely used to estimate physical activity, yet they struggle to distinguish stationary postures such as lying, sitting, or standing, and walking is often used as a reliable reference for posture determination. This study aimed to develop a novel method for accurately classifying body posture from hip‑mounted triaxial accelerometer data and to evaluate its performance in free‑living conditions against a thigh‑worn accelerometer. The method uses the constant Earth gravity vector and upright walking posture as a reference, computing an angle for posture estimation (APE) from the accelerometer orientation relative to the walking reference, and was tested on thirty healthy adults who performed randomized lying, sitting, standing, and walking tasks. Receiver‑operator‑characteristic analysis identified optimal APE cut‑points of 64.9° for lying/sitting and 11.6° for sitting/standing, achieving 100% sensitivity and specificity for the former and 94.2%/94.5% for the latter, and free‑living validation showed 89.2%–90.4% agreement between hip and thigh accelerometers for sedentary periods, confirming the method’s accuracy in.

Abstract

Hip-worn accelerometers are widely used to estimate physical activity (PA), but the accuracy of acceleration threshold-based analysis is compromised when it comes to identifying stationary and sedentary behaviors, let alone classifying body postures into lying, sitting, or standing. The purpose of this study was to devise a novel method for accurate classification of body posture using triaxial data from hip-worn accelerometer and to evaluate its performance in free-living conditions against a thigh-worn accelerometer. The posture classification rested on 2 facts: constant Earth's gravity vector and upright walking posture. Thirty healthy adults wore a hip-mounted accelerometer and underwent an array of lying, sitting, standing, and walking tasks. Task type, their order, and length were randomly assigned to each participant. During walking, the accelerometer orientation in terms of gravity vector was taken as reference, and the angle for posture estimation (APE) was determined from the incident accelerometer orientation in relation to the reference vector. Receiver operating characteristic (ROC) curve yielded an optimal cut-point APE of 64.9° (sensitivity 100% and specificity 100%) for lying and sitting and 11.6° (94.2%; 94.5%) for sitting and standing. In free-living conditions, high agreement (89.2% for original results and 90.4% for median-filtered results) in identifying sedentary periods (sitting and lying) was observed between the results from hip- and thigh-worn accelerometers. Walking provides a valid reference activity to determine the body posture. The proposed APE analysis of the raw data from hip-worn triaxial accelerometer gives accurate and specific information about daily times spent lying, sitting, and standing.