The shoulder is a complex structure that allows the human arm an unparalleled range of motion. This articulation, known as the glenohumeral joint, is a ball-and-socket joint where the head of the humerus (upper arm bone) meets the glenoid cavity of the scapula (shoulder blade). Its unique design permits movement in multiple planes, including flexion, extension, abduction, adduction, and rotation. Rotation involves the humerus spinning around its own long axis, allowing for a wide variety of functional tasks.
Defining Internal Rotation
Internal rotation, also referred to as medial rotation, describes the movement of the humerus toward the center line of the body. When the elbow is bent at a 90-degree angle and held close to the torso, internal rotation is the action of swinging the forearm inward across the stomach. The normal range of motion for true internal rotation, measured with the arm at the side, is about 70 to 90 degrees. It is the opposite action of external rotation, which spins the humerus away from the body’s midline.
Muscles Responsible for the Movement
Executing this powerful inward movement requires the coordinated effort of several muscles, collectively known as the internal rotators. The subscapularis, one of the four rotator cuff muscles, is the primary and most powerful internal rotator, covering the front surface of the scapula and pulling the humerus medially. Other major contributors include the pectoralis major and the latissimus dorsi, massive muscles of the chest and back. The teres major and the anterior fibers of the deltoid muscle also assist in this rotational action.
Why Internal Rotation Matters in Daily Life
Shoulder internal rotation is foundational to a vast number of everyday movements, allowing the hands to interact with the torso and the opposite side of the body. Activities of daily living (ADLs) that require this motion include tucking a shirt into the back of pants or reaching to scratch the middle of the back. It is the movement used when securing a seatbelt across the chest or putting an arm into the sleeve of a jacket. The ability to perform personal hygiene tasks, such as reaching the perineum, is heavily dependent on having adequate internal rotation range. In sports and athletic endeavors, internal rotation is a high-speed, dynamic component of many actions, such as throwing a baseball or swinging a racket.
Causes of Restricted Internal Rotation
A limitation in shoulder internal rotation is a common clinical finding that often indicates an underlying issue within the glenohumeral joint or surrounding soft tissues. One of the most frequent causes is adhesive capsulitis, commonly known as frozen shoulder, which involves the thickening and tightening of the joint capsule. This condition progressively restricts all shoulder movements, with rotation often being one of the most severely affected. The loss of motion can also be due to stiffness and contracture of the posterior joint capsule.
This posterior stiffness is particularly prominent in overhead and throwing athletes, leading to a condition called Glenohumeral Internal Rotation Deficit (GIRD). GIRD is the loss of internal rotation compared to the non-throwing shoulder, which occurs because the repetitive stress of throwing causes the posteroinferior capsule to thicken and tighten. This tightness can push the head of the humerus forward during movement, potentially leading to a problem known as internal impingement, where soft tissues get pinched. Other causes of restricted internal rotation include arthritis, post-surgical stiffness, and muscle imbalances where the external rotators become excessively tight.