Quantifying shoulder motion in the free-living environment using wearable inertial measurement units: Challenges and recommendations

Understanding function and dysfunction of the shoulder may be best addressed by capturing the motion of the shoulder in the unstructured, free-living environment where the magnitudes and frequencies of required daily motion can be quantified. Miniaturized wearable inertial measurement units (IMUs) enable measurement of shoulder motion in the free-living environment; however, there are challenges in using IMU-based data to estimate traditionally used measures of shoulder motion from lab-based motion capture. There are limited options for IMU placement/fixation that minimize soft tissue effects and there are significant challenges in developing the algorithms that can accurately estimate shoulder joint angles from IMU measurements of acceleration and angular velocity. In an effort to collate current knowledge and highlight solutions to addressable challenges, in this paper, we report the results of a focused search of research articles using IMUS for kinematic measurements of the shoulder in the free-living environment, discuss the basic steps required for quantifying shoulder motion in the non-laboratory field-based setting using wearable IMUs, and we discuss the challenges that must be overcome in the context of the shoulder joint and the literature review. Finally, we suggest some IMU-based measures that are less sensitive to experimental design and algorithm choices, make recommendations for the information documented in manuscripts describing studies that use IMUs to quantify shoulder motion, and propose directions for future research.