Boxers use a lot of muscles, but the quintissential boxing move is a simple forward punch.  Contraction of the triceps brachii straightens (extends) the elbow.  At the same, contraction of the pectoralis major and anterior deltoid muscles bring the arm forward at the shoulder joint, which is considered flexion.

But only one muscle can enjoy the “title” of The Boxer’s Muscle — serratus anterior.  Depicted here, the serratus anterior runs between the ribs and the shoulder blade (scapula).  When it contracts, it pulls the scapula forward (protraction), in the direction of the opponent’s jaw.  It also rotates the scapula upward.  In the illustration, the bottom of the scapula moves left, and the top moves to the right.  This raises the shoulder joint and orients it toward the opponent’s face.  This combination of movements provides a firm base for a forward punch, hence the epithet “boxer’s muscle”.

The name serratus comes from the “saw-like” appearance of the muscle (as in the word serrated), resulting from its several attachments along the ribs.