The horse has no collarbone. Hence the entire torso is attached to the shoulders by powerful muscles, tendons, and ligaments. The spine of a horse's back is supported by muscles three ligaments , and abdominal muscles.Horses' back shape can vary greatly from horse to horse. The upper curvature of a horse's withers, back, and loin is called the topline.The line of the belly from elbow to flank is the under line or bottom line. In terms of the back, both are important; a long underine with a relatively short topline is ideal.