A multibody biomechanical model of the upper limb including the shoulder girdle

The aim of this work is to propose a robust musculoskeletal model of the upper limb to serve as the basis for the study of different types of shoulder pathologies, including the use of anatomical or reverse prostheses. The multibody biomechanical model is defined by seven rigid bodies constrained by the sternoclavicular, acromioclavicular, and glenohumeral joints, each modeled as a three d.o.f. spherical joint; the humeroulnar and radioulnar joints, each modeled as one d.o.f. hinge joint; and the scapulothoracic articulation, modeled by two holonomic constraints that allow the scapula to glide over the thorax. The muscle system includes 21 muscles described by 37 individual segments using the obstacle-set method. The muscle contraction dynamics is represented by the Hill-type muscle model, being the activation of each muscle unknown. The muscle force sharing is a redundant problem in which an optimization technique is applied to find the muscle activations, and the corresponding muscle forces, by minimizing an objective function that represents muscle energy consumption. The fulfillment of the equations of motion of the biomechanical model are enforced and the stability of the glenohumeral joint and the scapulothoracic articulation is also imposed, thus providing two sets of constraints for the optimal problem. The validation of the model is carried out by comparing the results from an acquired motion, the abduction of the arm, with available data in the literature and with EMG data.