The serratus anterior is a fan-shaped muscle located on the side of the thorax, lying deep to the scapula (shoulder blade) and pectoral muscles.

Anatomy, Thorax, Serratus Anterior Muscles - StatPearls - NCBI

The "serratus" muscles are a group of chest and back muscles characterized by their saw-toothed (serrated) appearance. The most prominent is the , often called the "boxer's muscle" because it powers the forward motion of a punch. 1. Serratus Anterior