Horses do not have a collar bone and so the subclavius muscle, as well as the pectoral and serratus ventralis muscle, make up the thoracic sling.
Origin: The manubrium of the sternum
Inserts: into the fascia over the supraspinatus muscle.
Function: This muscle is involved in suspension and stabilisation of the trunk.
Soreness: Palpation along the front aspect of the scapula will elicit a stomping response from the horse or they will move away from you if there is tension or discomfort in this area. Tension in this muscle can result in shortening of the muscle, where the horse will struggle advancing the limb (shortened stride). The shoulder may become restricted, stiff and may appear more upright on the affected side. Exercises and disciplines which involve a lot of loading onto the forelimb will exacerbate the discomfort in this area and issues will become more noticeable.