Subscapularis

What it is

A large triangular muscle filling the entire front surface of the scapula — the side facing the ribs. It is the only cuff muscle there; the other three are all on the back.

Its tendon crosses the front of the shoulder joint to the lesser tubercle of the humerus, while the other three attach to the greater tubercle behind. Between the two tubercles runs the groove carrying the long head of biceps brachii.

Because it lies between the shoulder blade and the ribcage, it is essentially impossible to palpate.

What it does

Internally rotates the shoulder — turns the arm inward — the opposite of infraspinatus and teres minor.

It is also a powerful stabiliser, and specifically it resists the head of the humerus sliding forward. It forms the front wall of the cuff sleeve, opposing the three at the back. That front-and-back balance is what centres the joint.

It is the largest and strongest of the four cuff muscles.

What loads it

Internal rotation against resistance, and pushing or pressing movements, where it works alongside the much larger internal rotators.

What is commonly got wrong about it

That the cuff muscles all do the same thing. This one rotates the arm in the opposite direction to two of the others. Their common job is compression of the joint, not a common direction of pull.

That it is a small stabiliser. It is the biggest muscle of the cuff, and it produces substantial internal rotation force.

That the front of the shoulder needs no cuff attention. Subscapularis tears exist and are less often discussed than supraspinatus tears, partly because they are harder to see and test. That is a clinical matter and outside what this site covers.

That "internal rotation is bad". Internal rotation is a normal movement performed by a normal muscle. The idea that it is inherently harmful, or that a rounded shoulder posture is a pathology, is a much stronger claim than the evidence supports.