The shoulder is a very mobile joint. Because of the mobility, the shoulder is considered to have an inherent weakness. The joint consists of three major bones, the clavicle, scapula and humerus. These bones are more commonly called the collarbone, shoulder blade, and upper arm bone, respectively. Collectively, the shoulder is referred to as the scapula-humeral-clavicle complex. The joint between the humerus and scapula is a ball-socket joint. The joint between the scapula and acromial process allows for some movement but is primarily fixed. The ligaments, tendons, and muscles surround the shoulder to provide stability, movement, and limit the amount of rotation.
In this paper, a multibody dynamics model of the shoulder-upper arm complex is presented. Three major bones clavicle, scapula, and Humerus in the shoulder-upper arm complex are represented by rigid bodies. The soft tissues such as tendons, ligaments, and muscles are modeled as springs and actuators respectively attached to the rigid bodies. The joints between the bones are expressed as ideal kinematic joints. Kane’s equations are then used to derive equations of motion of this multibody system. Based on the model, an elder who uses a cane with his or her shoulder-upper arm complex force to support his or her upper body weight during walking is analyzed. Commercial computer software is used to create the multibody shoulder-upper arm complex computational model and then carry out simulation. The model may be utilized in motion analysis of elderly people and sports medicine to study fatigue mechanism and prevent injuries of the shoulder-upper arm complex.