The main muscle sitting directly underneath your shoulder blade is the subscapularis, a large triangular muscle that fills the entire front (bone-facing) surface of the scapula. It’s one of the four rotator cuff muscles, and it’s the only one you can’t see or easily touch because it’s sandwiched between your shoulder blade and your rib cage. A second muscle, the serratus anterior, lies even deeper, filling the space between the subscapularis and your ribs. Together, these two muscles form the soft-tissue layers of the scapulothoracic joint, the gliding interface that lets your shoulder blade slide smoothly across your back.
The Subscapularis
The subscapularis is the largest and most powerful of the four rotator cuff muscles. It originates from the subscapular fossa, the shallow bowl on the front surface of your shoulder blade, specifically from the middle and lower two-thirds of a groove along the blade’s outer edge. From there, it fans outward and converges into a tendon that attaches to the lesser tubercle of the humerus, a small bony bump near the top of your upper arm bone, right in front of the shoulder joint capsule. Some fibers also extend onto the greater tubercle and the groove where the biceps tendon runs.
Its primary job is internal rotation of the shoulder. That’s the motion you use when you reach behind your back, throw a ball, or pull a seatbelt across your chest. Beyond generating that rotation, the subscapularis acts as a dynamic stabilizer: it holds the head of the humerus snugly in the shoulder socket during almost every arm movement. Without it, the ball of the joint would shift forward, making the shoulder unstable.
The Serratus Anterior
Directly beneath the subscapularis, separated by a thin fluid-filled sac called the subscapularis bursa, sits the serratus anterior. This fan-shaped muscle originates from the outer surfaces of your first through eighth (sometimes ninth) ribs along the side of your chest wall. It wraps around the rib cage and attaches along the entire inner border of the shoulder blade, from the top corner down to the bottom tip.
The serratus anterior does something no other muscle can do as effectively: it pulls your shoulder blade forward and flat against your rib cage. Every time you push something, throw a punch, or reach overhead, the serratus anterior protracts and rotates the scapula so the shoulder socket points upward and your arm can move freely. It’s sometimes called the “boxer’s muscle” because of how critical it is to any punching or pushing motion.
Why These Muscles Cause Pain
Because the subscapularis is hidden on the front surface of the scapula, tightness or trigger points in the muscle can create pain that feels like it’s coming from deep under or behind the shoulder blade. Trigger points in the subscapularis can also sensitize nearby muscles in the shoulder girdle, creating secondary tender spots that spread discomfort across a wider area. People often describe the sensation as a deep ache behind the shoulder that’s hard to pinpoint, sometimes radiating toward the back of the armpit or down the inside of the upper arm.
The serratus anterior can produce its own version of under-the-blade pain. When it’s weak or overworked, the shoulder blade doesn’t glide smoothly over the ribs. Instead, the bone’s edges can catch on the underlying tissue, producing grinding, clicking, or a dull ache along the inner border of the scapula. This is sometimes called snapping scapula syndrome, and it specifically involves the soft-tissue layers (subscapularis over serratus anterior) that define the scapulothoracic joint.
Scapular Winging
One visible sign that the muscles under your shoulder blade aren’t working properly is scapular winging, where the inner edge or bottom corner of the blade lifts away from the rib cage. Medial winging, the more common type, results from weakness of the serratus anterior, often caused by injury to the long thoracic nerve. When this happens, the shoulder blade shifts upward and its bottom tip rotates inward. You might notice it most when pushing against a wall or doing push-ups: the blade pokes out more with each repetition as the muscle fatigues.
Lateral winging looks different. It’s caused by weakness of the trapezius or rhomboid muscles (on the back surface of the scapula, not underneath it), usually from injury to the spinal accessory nerve or dorsal scapular nerve. In this case, the shoulder blade drops downward and its bottom tip rotates outward. Both types of winging limit how high you can raise your arm and can be painful during overhead activities.
How to Tell Which Muscle Is Involved
Pain location and movement patterns offer useful clues. Subscapularis problems tend to show up as deep shoulder pain that worsens with internal rotation, like reaching behind your back to tuck in a shirt or fasten a bra. You may also notice weakness when trying to press your hand into your stomach against resistance. Serratus anterior issues are more likely to produce pain along the side of your rib cage or the inner border of the scapula, with difficulty reaching forward or overhead. Wall push-ups are a simple self-check: if your shoulder blade lifts away from your back during the motion, the serratus anterior is likely weak.
Both muscles respond well to targeted stretching and strengthening. The subscapularis can be stretched with gentle external rotation holds (arm out to the side, forearm rotating backward like a door opening). The serratus anterior strengthens through scapular protraction exercises, such as push-up plus movements where you round your upper back at the top of a push-up to push the shoulder blades apart. For persistent pain, a physical therapist can assess whether the issue is muscular tightness, nerve involvement, or a problem with the bursa that separates these two layers.