On the use of inertial measurement units and musculoskeletal modeling in the calculation of muscle and joint loading at the shoulder.

Inertial measurement units (IMUs) are wearable motion sensing devices that support low-cost human kinematics measurement outside of the laboratory; however, the impact of IMU motion measurement errors on estimates of upper limb muscle and joint force using musculoskeletal modeling remains poorly understood. The aims of this study were to measure upper limb kinematics using IMUs and optical motion analysis, and evaluate the differences in shoulder musculoskeletal model estimates of muscle and joint forces when using IMU-based motion analysis and optical motion analysis data. Thirty healthy adults performed activities of daily living at fast and slow speeds while upper limb motion measurement was simultaneously acquired using an optical motion analysis system and self-placed IMUs. Kinematics measurement accuracy using IMUs was highest in humerothoracic joint elevation and scapular lateral rotation, and lowest in humerothoracic plane of elevation and humeral axial rotation. Musculoskeletal model calculations of a muscle's force were most sensitive to IMU motion measurement errors about the degree of freedom primarily actuated by that muscle. For example, subscapularis force calculations were more sensitive to errors in humeral axial rotation than humeral elevation. Consequently, errors in humeral axial rotation motion measurement affected infraspinatus and subscapularis muscle forces, and subsequently, estimates of the compressive component of the glenohumeral joint force. During fast flexion, a mean difference of 4.5 %BW in the compressive joint force was observed when using IMU motion measurement compared to optical motion analysis data (p < 0.001). The findings of this study provide guidance in the use of IMUs for muscle and joint force calculation during activities of daily living.