The pectoral muscles are comprised of the anterior pectoral muscles and the posterior pectoral muscles.
Anterior pectoral muscle
Origin: The sternum
Insertion: into the humerus.
Function: This muscle draws the forelimb forwards, adducts the forelimb and cushions the limb on landing. Under concentric contraction, it aids in shoulder extension.
Soreness: Repetitive concussion can cause damage to this muscle due to responsibility of cushioning the limb on landing. On palpation, the horse may object to pressure as the area may be tender and trigger points can be felt as tight knots against the sternum. This muscle is often stressed more with green and juvenile horses and those ridden on the forehand.
Posterior pectoral muscle
Origin: The rear aspect of the sternum.
Insertion: Into the medial side of the humerus, subclavius muscle, 4th to 9th ribs and the abdominal tunic.
Function: This muscle draws the forelimbs backwards and forms part of the thoracic sling, which attaches the forelimb to the ribcage with the subclavius muscles.
Soreness: If a horse has tension in this muscle, they may be sensitive to palpation between the limbs or they may resist the thoracic lift. During exercise, the horse may struggle to reach during lateral exercises (leg yielding, half pass etc.) on the affected side as the adduction ability of the muscle will be reduced.
Origin: The spine of the scapula.
Insertion: into the deltoid tuberosity of the humerus.
Function: The deltoid is mainly responsible for flexion of the shoulder joint and can also aid in abduction of the shoulder. This makes it an important contributor to stride length.
Soreness: If there is tightness or discomfort in this area, the horse may have a shortened stride. The horse may also move away from palpation over the shoulder or stomp the forelimb.
Massage can relieve tension and discomfort in this muscle, whilst stretching exercises such as lifting the forelimb forward and extending the shoulder can help to stretch the deltoid muscle as well as other shoulder and forelimb muscles on a more regular basis. Good flexibility at the shoulder is important for jumpers to achieve good scope over the fence.
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.