Reliability of Passive Physical Activity Assessment for Inpatients Using Marker-Free Motion Tracking System

Evidence-based practices of public health will benefit from quantification of passive physical activity assessment. This study aims to investigate the reliability of marker-free system (MFS) such as Microsoft Kinect in measuring upper extremity motion from different angles. Ten healthy participants performed elbow and shoulder extension/flexion along frontal and median anatomical planes for ten pace-controlled repetitions, during which the spatiotemporal positions of upper extremity joints were concurrently recorded by two sensors from 0° and 45° viewing angles. Reliability between the two sensors were evaluated using Pearson correlation coefficient, intra-class correlation coefficients, and 95% limits of agreement and coefficient of variation. Worse reliability was observed when possibility of occlusion was higher. However, better reliability was found when longer observation interval (10 s) was used as elementary measuring unit than shorter observation interval (2 s). The overall angular reliability of activity as displacement or changes in angle was not satisfactory. The results are expected to inform the industry for the extension of MFS to clinic applications.