The brachiocephalic muscle runs along the entirety of the neck
Origin: the mastoid processes of the temporal bone and the wing of the atlas
Insertion: the humerus at the deltoid tuberosity.
Function: The function of this muscle is to flex the neck and to move the shoulder forward. It is also involved in adduction of the limb, which essentially means that it allows the limb to move towards the other. This muscle is actively engaged when the horse is asked to work in a contact.
Soreness: Soreness in the muscle can result in restriction in the shoulder and the horse may struggle to flex on a circle, inconsistencies in the contact and the horses’ ability to leg yield may be reduced. Long-term under and overworking of this muscle can lead to a shortened stride cycle. Horses with poorly fitting saddles can develop soreness in this muscle at the base of the neck due to weakness developed as the shoulder is restricted. Rider asymmetry can also lead to tension and tenderness of the muscle.