A M-IMU-to-Segment Alignment Procedure for Shoulder Angles Estimation: a Preliminary Study

Monitoring shoulder joint angles is a topic of great interest in clinical settings. Wearable systems integrating magneto-inertial measurement units (M-IMUs) could be a valuable solution in quantifying movement features that are otherwise unmeasurable in real-life contexts. This study aimed to implement a static M-IMUs-to-segment alignment procedure that could be easily performed independently by patients with shoulder musculoskeletal pathologies and to perform a preliminary validation study by comparing shoulder elevation angles estimated with M-IMUs versus an optical motion capture system. Seven healthy volunteers with no shoulder pathologies were enrolled. For the M-IMU-to-segment alignment procedure, each participant initially holds the N-pose in an anatomic stance with arms positioned alongside the body and palms facing inwardly. Then, participants performed two functional tasks of clinical relevance, namely elevation in the scapular plane and elevation in the frontal plane. For shoulder elevation angles estimated during movements in the scapular plane, results showed an RMSE value equals to 6.7°. For shoulder elevation angles estimated during movements in the frontal plane, results showed an RMSE value equals to 7.5°. For both tasks, an excellent correlation was found (r > 0.9). The MIMU-body segment alignment procedure implemented for estimating shoulder elevation angles appears to be a viable alternative to more complex calibration procedures that are not easily executable independently by patients with shoulder musculoskeletal disorders. However, further investigations are needed, e.g., including a larger sample size and evaluating more complex functional tasks.