A large-scale musculoskeletal model of the shoulder and elbow: The Delft Shoulder

INTRODUCTION The shoulder is probably one of the most complex joint mechanisms of the human body. The shoulder consists of four bony elements: Thorax, clavicle, scapula and humerus. The connection thorax-claviclescapula is a closed chain. Due to the closed chain, the rotational and translational degrees of freedom of the scapula and clavicle are constrained, meaning that not every orientation can be achieved. In addition, the moments around the sternoclavicular (SC) and acromioclavicular (AC) joint are coupled, i.e. if a muscle exerts a moment around one of these joints, the scapulothoracic reaction force will change and the altered moment of the scapulothoracic reaction force around the other joint must be compensated by a second muscle. In total 20 muscles and muscle parts are crossing the shoulder joints (SC, AC and Glenohumeral (GH) joint). Additionally, 11 muscles are crossing the elbow joint (flexion-extension in the humero-ulnar (HU) joint and pro-supination around the ulno-radial (UR) joint). Many muscles are bior tri-articular, and have large attachment sites. Parts of the larger muscles can contract independently from each other, as shown by EMG. The goal of this presentation is to discuss a number of general properties that are important for a musculoskeletal model: Inverse vs. forward dynamic simulation, motion constraints, generic model vs. individual model, importance of length-tension properties, optimization algorithm and validation.