For good impulsion, a horse must either be balanced or have most of its weight tilted back toward its hindquarters.
Good riding aims to help transfer some of the animal's body weight back, getting the horse "off the forehand," but some riding disciplines require a greater amount of this transfer of weight (or "collection") than others.
The Brachiocephalicus is a wide strap-like muscle that begins at the base of the skull behind the jaw and ends below the point of the shoulder to the humerus.
The Trapezius is a flat, sheet-like muscle that begins at the Occipital bone and ends at the spines of the 7th cervical and all the thoracic vertebrae.
It lifts the shoulder and forehand, and if this muscle is well developed, the horse will work in a good outline.
The Nuchal ligament begins at the poll and ends at the withers, and helps the muscles in the neck support the head.