Subscapularis

The subscapularis lies on the rib-facing surface of the shoulder blade (scapula). In other words, it is hidden beneath the shoulder blade when viewed from behind. It begins in the broad, concave area on the front of the scapula (the subscapular fossa), extends forward, and attaches to a small prominence near the top of the upper arm bone (the lesser tubercle of the humerus). Of the four rotator cuff muscles—the infraspinatus muscle, teres minor, supraspinatus, and subscapularis—this is the only one located on the front of the scapula.

It has two main functions.

• Rotating the arm inward (internal rotation) — It is the main muscle involved in turning the arm so that the palm faces toward the body. The latissimus dorsi, teres major, and pectoralis major also assist with internal rotation, but the subscapularis differs in being part of the rotator cuff.
• Stabilizing the shoulder — When you raise your arm overhead or move it backward, it provides support from the front to keep the head of the humerus properly positioned in the shoulder socket (the glenoid). In effect, it acts as a front wall during large arm movements.

Because the subscapularis lies between the scapula and the ribs, it is difficult to feel directly from the outside. Aching is often felt deep in the armpit or deep at the front of the shoulder. Common movements that use this muscle heavily include:

• Reaching into a back pocket or taking out a wallet
• Pulling up a zipper on the back of your clothing or fastening a bra hook
• Scratching your back or reaching behind your lower back
• Push-ups, bench presses, swimming, and throwing movements that draw the arm toward the body

If you have trouble reaching your hand up behind your back or feel aching at the front of your shoulder during these movements, the muscles involved in internal rotation and the surrounding shoulder structures may have become stiff together. Shoulder discomfort often involves not only the subscapularis but also other rotator cuff muscles, the joint capsule, and scapular movement. It is therefore helpful to look at the movement of the shoulder as a whole rather than considering one muscle in isolation. You can learn about overall shoulder coordination in Scapulohumeral rhythm.

• Work on both internal and external rotation. Because internal rotation is used frequently in daily life, strengthening external rotators such as the infraspinatus muscle and teres minor as well can help balance the shoulder. A typical exercise is to hold a resistance band and rotate your arm outward while keeping your elbow against your side.
• Doorway stretch — Bend your elbow and place it against a doorframe slightly below shoulder height, then slowly move your body forward to stretch your chest and the front of your shoulder. Stop at a comfortable stretching sensation, without pain.
• Practice reaching behind your back — Holding a towel behind your back with one hand reaching from above and the other from below, then gently pulling, can also help loosen the front of the shoulder.
• Check your posture. Working for long periods with your shoulders rounded inward keeps the muscles at the front in a shortened position. Getting into the habit of opening your chest and gently drawing your shoulder blades back together can help.

If you suddenly lose strength when raising your arm forward or out to the side, shoulder throbbing when lying down at night disrupts your sleep, or pain at the front of your shoulder does not ease after several days, have your shoulder assessed by a shoulder specialist.