The infraspinatus is the primary external rotator of your shoulder, responsible for the motion of turning your arm outward, away from your body. It also plays a critical role in stabilizing the shoulder joint during almost every arm movement, keeping the ball of the upper arm bone centered in its shallow socket. As one of the four rotator cuff muscles, it works constantly during overhead activities, throwing, and even simple tasks like reaching behind your back.
Where the Infraspinatus Sits
The infraspinatus is a thick, triangular muscle that covers most of the back surface of your shoulder blade. It originates from a broad area called the infraspinous fossa, which is the large, slightly concave region below the bony ridge (the spine) that runs across the back of the scapula. From there, the muscle fibers converge into a tendon that crosses the back of the shoulder joint and attaches to the greater tuberosity of the humerus, a bony bump near the top of the upper arm bone.
This attachment point is shared real estate. The supraspinatus (above) and the teres minor (below) also connect to the greater tuberosity, forming a continuous cuff of tendon tissue that wraps around the humeral head. The infraspinatus sits between these two muscles, and the teres minor lies just beneath it, running roughly parallel.
External Rotation: Its Main Job
When you rotate your arm outward, like cocking your arm back to throw a ball or reaching to the side to open a car door, the infraspinatus is doing most of the work. It generates more external rotation force than any other shoulder muscle. The teres minor assists with this same motion, but the infraspinatus is the dominant contributor due to its larger size and more favorable line of pull.
External rotation matters more than most people realize. It’s essential for throwing and racquet sports, but also for everyday movements like brushing your hair, tucking in a shirt, or simply positioning your hand in space. When the infraspinatus is weak or injured, people typically notice they can’t rotate their arm outward against any resistance, and reaching behind the head becomes difficult or painful.
How It Stabilizes the Shoulder Joint
The shoulder is the most mobile joint in the body, but that mobility comes at a cost: the socket is remarkably shallow. The humeral head sits against the glenoid (the socket on the scapula) like a golf ball on a tee. Without active muscular control, the joint would be unstable in almost every position.
The infraspinatus is one of the key muscles preventing the humeral head from sliding out of place. Cadaver studies have shown that the infraspinatus and teres minor together are the most effective muscle group at controlling external rotation of the humerus and reducing strain on the shoulder ligaments. During the cocking phase of throwing, for example, these muscles act as a dynamic brake, preventing the arm from rotating too far and stressing the ligaments that hold the front of the joint together. This is why strengthening the infraspinatus is a cornerstone of treatment for anterior shoulder instability.
The infraspinatus also compresses the humeral head into the socket during arm elevation. Every time you lift your arm, the rotator cuff muscles fire to keep the ball centered while the larger deltoid muscle provides the power to raise the arm. Without this compressive force, the humeral head would migrate upward and jam against the bony arch above the joint.
Working With the Rest of the Rotator Cuff
The four rotator cuff muscles (supraspinatus, infraspinatus, teres minor, and subscapularis) function as a coordinated unit. The supraspinatus helps initiate arm elevation. The subscapularis, on the front of the shoulder blade, handles internal rotation. The infraspinatus and teres minor cover external rotation and posterior stabilization.
The infraspinatus and teres minor overlap significantly in function, but they aren’t redundant. The infraspinatus is more active in the mid-ranges of shoulder motion, while the teres minor contributes more when the arm is already elevated. If the infraspinatus tears, the teres minor can partially compensate, but external rotation strength drops noticeably.
Common Injuries and Problems
Infraspinatus injuries usually fall into two categories: tendon tears and nerve-related weakness.
Infraspinatus tendon tears rarely happen in isolation. When the rotator cuff tears, it almost always starts with the supraspinatus, and the tear can then extend backward into the infraspinatus tendon. MRI studies of shoulders with supraspinatus tears show that a high percentage also have infraspinatus involvement, particularly partial-thickness tears on the undersurface of the tendon. Full-thickness infraspinatus tears tend to occur in the setting of larger, multi-tendon rotator cuff injuries and are associated with greater loss of function.
The other major cause of infraspinatus dysfunction is compression or injury to the suprascapular nerve, which is the sole nerve supply to both the supraspinatus and infraspinatus. This nerve travels through two narrow notches on the scapula, and it can become trapped at either point. If the nerve is compressed at the first notch (near the top of the scapula), both the supraspinatus and infraspinatus weaken and eventually waste away. If the compression happens at the second notch, closer to the back of the shoulder socket, only the infraspinatus is affected. Visible wasting of the muscle on the back of the shoulder blade is a telltale sign of this condition, which is most common in overhead athletes like volleyball players and baseball pitchers.
Signs of Infraspinatus Weakness
The clearest sign of infraspinatus dysfunction is loss of external rotation strength. You might notice difficulty holding your arm in a rotated-out position, or your arm may drift inward when you try to hold it externally rotated against gravity.
Clinicians use a test called the external rotation lag sign to check for infraspinatus tears. Your arm is passively rotated outward, and then you’re asked to hold that position. If the arm drops back toward your body, it suggests the infraspinatus isn’t functioning. This test is highly specific: when it’s positive, there’s a 98% chance an infraspinatus tear is present. However, it’s not very sensitive, meaning it misses many tears, especially smaller ones. Only about 10% of people with confirmed infraspinatus tears test positive, which means a negative result doesn’t rule the injury out.
Best Exercises for the Infraspinatus
If you’re looking to strengthen or rehabilitate the infraspinatus, exercise selection and arm position matter significantly. Research using electromyography (muscle activity sensors) has identified which positions activate the infraspinatus most effectively while minimizing contribution from surrounding muscles like the posterior deltoid.
Side-lying external rotation, where you lie on your side and rotate your top arm outward against gravity or a light weight, is one of the most commonly prescribed exercises and produces strong infraspinatus activation (roughly 64% of maximum voluntary contraction). An exercise called modified abduction and external rotation, performed face-down with the arm hanging off the edge of a table, produces even higher activation at around 83% of maximum.
Arm position during external rotation exercises changes which muscles do the work. Performing external rotation with the arm at your side (0 degrees of abduction) produces the best infraspinatus isolation. As the arm moves further from the body, the posterior deltoid takes over progressively, and overall infraspinatus activity actually decreases. Some clinicians suggest placing a rolled towel between your arm and body during side-lying exercises, but research suggests this doesn’t meaningfully improve infraspinatus isolation, though it may be more comfortable.
For people recovering from injury, starting with the arm at your side and using light resistance allows the infraspinatus to work without excessive demand on the surrounding structures. Progressing to higher-activation positions like prone external rotation can build strength once the early healing phase is complete.