Muscle · Shoulders

Serratus anterior

Musculus serratus anterior

A broad fan gripping the side of the ribcage with a row of finger-like slips and wrapping around to the underside of the shoulder blade's inner edge. It does two things that matter enormously. It holds the shoulder blade flat against the ribs — when it fails, the inner edge lifts away and the blade wings — and its lower slips swing the bottom corner of the blade outward and forward, turning the socket to face upward. That upward turn is what lets the arm clear the head, which makes serratus the muscle behind almost every overhead reach. Momentum's clinical system names it as one of the commonly weak muscles of the upper body and puts it in the activation step of the shoulder flow for exactly that reason.