Shoulder abduction is the act of lifting the arm away from the body in the coronal plane. While seemingly simple, this movement is a complex, coordinated process. The shoulder is the most mobile joint in the human body, allowing for a vast range of motion required for daily tasks. This mobility requires precise muscular control and joint synchronization to prevent instability and injury.
Defining the Movement
The full range of shoulder abduction involves a total arc of approximately 180 degrees. This movement is achieved through the coordinated action of the glenohumeral joint and the scapula, known as the scapulohumeral rhythm. The glenohumeral joint is the primary ball-and-socket connection between the humerus and the scapula.
This rhythm follows a 2:1 ratio. For every three degrees of arm elevation, two degrees occur by movement of the humerus and one degree occurs by the upward rotation of the scapula. This coordination keeps the glenoid socket optimally positioned beneath the head of the humerus. Without the scapular movement, the humerus would contact the acromion, limiting the lift to about 90 to 120 degrees.
The movement is often broken down into phases based on joint action. The initial phase, from 0 to about 60 degrees, primarily involves movement at the glenohumeral joint. Beyond this point, scapular rotation becomes increasingly necessary to continue lifting the arm.
Key Muscles Driving Abduction
Shoulder abduction is a sequential process driven by a group of muscles working in a precise, timed manner. The initiation of the movement is handled by the supraspinatus muscle, a member of the rotator cuff group. This muscle is responsible for lifting the arm for the first 15 to 20 degrees of abduction.
Once the arm moves past this initial phase, the middle fibers of the deltoid muscle take over as the primary power source. This large, cap-like muscle provides the main force for the middle range of motion, continuing the lift from approximately 15 to 90 degrees. The deltoid’s position on the side of the humerus gives it the leverage needed to pull the arm upward against gravity.
To achieve full overhead abduction beyond 90 degrees, the scapula must rotate upward. This movement is facilitated by two large muscles: the serratus anterior muscle and the upper and lower fibers of the trapezius muscle. They work together to rotate the shoulder blade, repositioning the glenoid socket and allowing the humerus to complete the lift toward the 180-degree mark.
Common Causes of Limited Movement
When the shoulder’s range of motion is restricted, the cause often relates to pathology involving the muscles or the joint space. Rotator cuff injuries are a frequent cause of pain and weakness during abduction. Specifically, a tear or tendonitis of the supraspinatus tendon can make it painful or impossible to initiate the lift of the arm.
Shoulder impingement syndrome is another common issue directly affecting the abduction arc. This condition occurs when soft tissues, such as the supraspinatus tendon or the subacromial bursa, are pinched between the head of the humerus and the acromion. Pain is felt in a “painful arc” between 60 and 120 degrees of abduction as the structures are compressed during this movement range.
A more severe, generalized restriction is adhesive capsulitis, commonly known as frozen shoulder. This condition involves the thickening and tightening of the capsule surrounding the joint, which severely limits all shoulder movement, including abduction. Frozen shoulder typically results in a global loss of range of motion.