The trapezius muscle covers a vast area around the withers of the neck and over the top portion of the shoulder.
Origin: The nuchal ligament, supraspinous ligament of the 2-10 cervical vertebrae and thoracic vertebrae
Inserts: into the scapula spine.
Function: This muscle is responsible for elevation of the shoulder and cranial-caudal motion (moving the shoulder forward and backwards). Under bilateral concentric contraction the muscle causes slight neck extension. The trapezius muscle works as an antagonist to the brachiocephalic muscle.
Soreness: The thoracic section of the trapezius muscle lies directly under the gullet of the saddle, leaving it vulnerable to increased pressure and damage to the muscle with poorly fitting saddles. Atrophy of this section of the muscle is much more common than hypertrophy due to the restriction from the saddle. If the muscle is tight, the horse will lack flexibility in the neck and the shoulder, leading to a shortened stride.