The rhomboid is a deep muscle located along the topline of the horses neck.
Origin: The occiput, nuchal ligament and the supraspinous ligament from C2 to approximately T10
Inserts: Into the medial surface of the scapula cartilage.
Function: This muscle works with the trapezius muscle to elevate the shoulder and is responsible for protraction and retraction. It also helps to stabilise the shoulder and when the stance phase it aids with neck extension. Under concentric contraction, the rhomboid incline the base of the neck downwards and assume a more U shaped position, resulting in a poor head and neck carriage. Hypertrophy of the rhomboid muscle can appear in some horses if the horse is ridden in a hyperflexed position and if the horse is heavy on the contact.
Soreness: If this muscle is tight, the horse may struggle to stretch and extend the neck fully, which may be noticeable when practicing free walk and long and low exercises. As the rhomboideus muscle lies under the scapula, if the saddle is fitted too tightly or too forward, this may result in contraction of the rhomboid muscle to alleviate the discomfort by pulling the scapula closer to the body.
Lateral neck extension and chin to chest exercises help to stretch this muscle.