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.