The trapezius functions to move the scapula (shoulder blade). The cervical portion of the trapezius moves the scapula forward, and the thoracic portion of the trapezius moves the scapula backwards.
This muscle is heavily involved in the wither area, which means it’s impacted by saddle fit and girth or cinch tightening. It’s location also means it’s involved with the neck, front legs, and back.
Human trapezius for comparison 🥰
This is one muscle that plays a part in fluid movements and the horses ability to perform when compromised.