Quaternion-Based Predictive Framework for Scapulohumeral Coordination

TL;DR

This paper introduces a quaternion-based predictive control framework for shoulder motion analysis, improving physiological accuracy and aiding understanding of shoulder pathologies.

Contribution

It combines a quaternion shoulder model with EMG calibration, reducing artifacts and enabling realistic simulation of shoulder coordination and impairments.

Findings

Quaternion formulation eliminated kinematic singularities.

EMG-informed calibration reduced muscle excitation discrepancies by up to 60%.

Simulations produced SHR values within experimental ranges.

Abstract

Scapulohumeral rhythm (SHR), the coordinated motion between the scapula and humerus during arm elevation, is frequently altered in rotator cuff pathologies, yet the mechanical principles underlying coordination redistribution remain difficult to explain from experimental data alone. This study presents a predictive optimal control framework for investigating scapulohumeral coordination, combining a quaternion-based shoulder model with EMG-informed muscle parameter calibration. The quaternion formulation eliminated kinematic singularities and associated non-physiological activation artifacts observed in the Euler-angle model, while maintaining comparable tracking accuracy. EMG-informed calibration reduced discrepancies between predicted and measured muscle excitations by up to 60% on independent validation tasks. In predictive simulations where only thoracohumeral elevation was prescribed, scapular and clavicular kinematics emerged from musculoskeletal mechanics and minimization of muscular effort, producing SHR values consistent with established experimental ranges. Simulated rotator cuff deficiency resulted in increased reliance on glenohumeral rotation. The proposed framework may serve as a tool for understanding impaired coordination patterns across a broad range of shoulder pathologies, with potential to inform personalized rehabilitation strategies and the design of assistive and prosthetic devices.