Musculoskeletal design of the human shoulder: implications for neuromuscular control

Abstract

The shoulder complex is a unique musculoskeletal structure capable of versatile motor behaviour yet requiring delicate control. The purpose of our work was to better understand the nature of musculoskeletal redundancy at the shoulder accounting for biomechanical demands and motor control strategies. Using a biomechanical model of the shoulder, we simulated a series of static exertions. Joint moment results from inverse dynamics were combined with an iterative sampling method to survey the landscape of feasible muscle activity patterns. By repeating the sampling process across different numbers of degrees of freedom at the shoulder, we demonstrate how emergent solutions are shaped by the biomechanical demands at each of the shoulder joints. Furthermore, we observed that the degree of musculoskeletal redundancy appears to be higher among the scapulohumeral muscles than the thoracohumeral and thoracoscapular muscles. Finally, we found that many of the muscle activity patterns requiring similar effort costs as the minimal effort solution have similar activation profiles, but there can be a wide range of possibilities especially at greater task intensities. Altogether, the simulations provide insight into neuromuscular control and musculoskeletal model decision-making process for the shoulder.