Scapulohumeral rhythm preserves glenohumeral stability: Insights from a biomechanical simulation

Abstract

The shoulder’s dynamic function is largely influenced by scapulohumeral rhythm (SHR), a coordinated movement of the scapula and humerus that facilitates a safe range of motion. While SHR has been described and quantified in terms of shoulder kinematics, its specific contribution to glenohumeral joint stability. This study aims to estimate the impact of SHR on glenohumeral stability using a biomechanical model. A five-segment musculoskeletal model based on the work of Wu et al. (2016) was implemented in OpenSim. Three SHR patterns and two loading scenarios were evaluated: a fixed scapula, a humeral-to-scapular motion ratio, and an experimentally measured SHR with free abduction or abduction while holding a 2 kg weight in the hand. Muscle forces and glenohumeral stability ratios were calculated using static optimization, and the model predictions were compared to electromyography and in vivo joint force data. While glenohumeral contact forces showed minimal variation across different SHR conditions, the stability ratio analysis revealed that the absence of SHR significantly increased the risk of joint instability. In scenarios without SHR, even small shoulder elevations resulted in overloading of the superior glenoid. The addition of weight further destabilized the joint, while substantially increasing glenohumeral force. SHR does not reduce the overall glenohumeral load but plays a critical role in maintaining glenohumeral stability, particularly during early phases of shoulder elevation and when holding additional weight. These findings highlight the importance of scapular kinematics in shoulder joint function and may have implications for managing shoulder pathologies such as rotator cuff tears and impingement, where scapular motion is often compromised.