The shoulder joint is under an incredible amount of stress due to the many forces that act upon it throughout the typical day. The shoulder must support the entire weight of the arm and whatever the arm moves, while also providing a wide range of motion. The deltoid muscle abducts the arm, which is a common motion performed while lifting heavy objects. As the deltoid muscle contracts superior to the shoulder, it rubs against the head of the humerus and the joint capsule, which would cause irritation to the delicate structures of the joint. The subdeltoid bursa absorbs the shock and friction at the joint to protect the underlying structures.