The ability to touch the opposite shoulder with the palm is a common, informal test used to gauge the general health and flexibility of the shoulder joint. This movement requires a combination of motions at the shoulder, elbow, and shoulder blade. Difficulty achieving this action often signals a limitation in the shoulder’s range of motion, which is the greatest of any joint in the human body. Understanding the mechanics offers insight into the complex interplay of bones, muscles, and connective tissues that govern arm movement.
Anatomical Feasibility: The Joint Movements Required
The act of bringing the palm to the opposite shoulder is a multi-joint action primarily centered on the glenohumeral joint, the main ball-and-socket connection of the shoulder. Two major movements are required at this joint to bring the upper arm across the body. The first is adduction, which involves moving the arm toward the midline of the body in the coronal plane, effectively bringing the elbow across the chest. The second is internal rotation, where the humerus, or upper arm bone, pivots inward around its long axis, turning the hand toward the body’s center.
Normal glenohumeral internal rotation typically ranges between 70 to 90 degrees, a large arc that is necessary for the hand to reach the shoulder. Adduction, when combined with internal rotation, is often referred to as horizontal adduction or transverse flexion, which is the specific motion of reaching across the chest. The elbow joint must also contribute by flexing, or bending, to bring the hand the final distance. Furthermore, the motion requires the shoulder blade, or scapula, to move smoothly across the rib cage, which is known as scapulothoracic rhythm.
Key Muscles and Soft Tissues Involved
The power for this movement comes from a coordinated group of muscles that work to both mobilize and stabilize the joint. The primary muscles responsible for the required internal rotation and adduction are the pectoralis major, which spans the chest, and the latissimus dorsi, the large muscle of the back. Other strong internal rotators that contribute include the subscapularis, one of the four rotator cuff muscles, and the teres major. These muscles must contract effectively to pull the upper arm across the front of the chest.
While these muscles facilitate the movement, the soft tissues on the back of the shoulder often act as the limiting factors. The posterior shoulder capsule, a fibrous layer of connective tissue encasing the joint, and muscles like the infraspinatus and teres minor, which are primary external rotators, must lengthen significantly. If the posterior capsule is stiff or contracted, it physically restricts the inward rotation of the arm, making it difficult to complete the motion. The coordinated relaxation of the external rotators is just as important as the contraction of the internal rotators for the arm to achieve the full range of motion.
Common Causes of Restricted Range of Motion
A common reason for limited movement is non-traumatic stiffness in the soft tissues surrounding the joint. Muscular tightness in the latissimus dorsi and posterior shoulder muscles creates a physical barrier to full internal rotation and adduction. This tightness results from repetitive activities or a lack of regular movement through the full joint capacity. Poor posture, such as sitting with rounded shoulders, can also alter the resting position of the scapula and humerus.
When the shoulder is held in a protracted position, it can shorten the posterior joint capsule, leading to a condition known as glenohumeral internal rotation deficit (GIRD). Although GIRD is often associated with throwing athletes, a similar tightness can develop in the general population, reducing the available internal rotation. More severe limitations may be caused by adhesive capsulitis, commonly called frozen shoulder, where the entire joint capsule thickens and tightens. This condition leads to a significant and progressive reduction in all shoulder movements, severely hindering the ability to reach the opposite shoulder.