The Equine Thoracic sling forms the connection between the forelimbs and the body. Nestled between the forelimb and the rib cage, the brachial plexus lies protected. The integrity and function of the thoracic sling is essential to the function of the entire body. Restrictions or dysfunction of this area can restrict the nerves of the brachial plexus, restrict range of motion of the forelimbs as well as the hindlimbs, and cause imbalance throughout the body.
Let’s review the anatomy and some of the considerations of the thoracic sling.
The Shoulder Region
The range of motion and optimal function of the forelimb, and specifically the shoulder region, is essential to the success of the horse as an athlete, as the forelimb is attached to the body only through muscle, known as a synsarcosis. This muscular ‘joint’ affects the kinetic chains and fascia of the entire horse. A dysfunction in this area can be highly detrimental to the whole body.
As 60% of a horse’s weight is carried on the forelimbs, the wellbeing and correct functioning of these muscles are essential to the horse’s soundness and ability to function. Not only do the forelimb muscles support the weight of the horse, but they are also responsible for the movement of the forelimb in all planes.
Dorsal muscles of the shoulder
The first group of muscles attaching the scapula to the thorax lie on the lateral and medial aspects of the scapula. These include the Trapezius, Omotransversarius, Rhomboideus, Latissimus Dorsi and Serratus ventralis.
The trapezius is a beautiful, fan-shaped superficial muscle originating on the nuchal and supraspinatus ligaments from C2 to T10, having a cervical and thoracic portion. The cervical portion inserts on the entire spine of the scapula, while the thoracic portion inserts only in the dorsal third of the scapular spine. This superficial muscle is innervated by the dorsal branch of the accessory nerve and works to elevate the scapula and move it craniodorsally and caudodorsally.
Omotransversarius originates on the scapular spine and the fascia of the shoulder, and inserts on the transverse spinous processes of C2-4. The muscle is innervated by the medioventral branch of the local cranial nerve, and acts to protract the limb when the neck is in a fixed position, and flex the neck when the limb is in a weight-bearing position.
Rhomboideus originates from the nuchal and dorsoscapular ligaments from C2-T8 and inserts on the dorsal cartilage of the scapula. This muscle is innervated by the medioventral branch of the local thoracic and cervical nerve, and acts to move the scapula dorsally and cranially, as well as elevate the shoulder.
Latissimus Dorsi originates on the supraspinous ligament from T3 and caudally to the thoracolumbar fascia. It inserts on the teres major tuberosity of the humerus, along with the teres major muscle, and is innervated by the thoracodorsal nerve. It acts to retract the limb and flex the shoulder, and can draw the trunk cranially when the limb is weight-bearing.
Serratus Ventralis is an essential muscle of the thoracic sling, supporting the weight of the neck and thorax from its position on the medial aspect of the scapula. This muscle is again divided into a cervical and thoracic portion, with the cervical portion originating on the transverse processes of C4-7 and the thoracic portion originating on the 1st to 8th ribs. Both portions insert on the medial aspect of the scapular cartilage and are innervated by the ventral branch of the local spinal nerve and the long thoracic nerve.
The Ventral Muscles
The group of pectoral muscles situated between the forelimbs and under the chest form the next important aspect of the thoracic sling. These muscles help to support the weight of the thorax during movement. The muscles include the Subclavius, Deep Pectorals, Transverse Pectorals and Superficial Pectorals.
The Subclavius or Prescapularis originates on the sternum and costal cartilages 1-4 and inserts on the supraspinatus and shoulder fascia. It is innervated by the cranial pectoral nerve and acts to suspend the trunk and stabilise the shoulder joint.
The deep pectoral originates on the sternum and distally on ribs 4-9, as well as from the abdominal fascia. It inserts on the major and minor tubercles of the humerus and the tendon of origin of the coracobrachialis. It acts to suspend the trunk between the forelimbs and assists in limb retraction and shoulder stabilisation.
The transverse pectoral originates on costal cartilages 1-6 and the sternum, inserting on the fascia of the forearm. It acts to adduct and resist abduction of the forelimbs.
The superficial pectoral originates on the manubrium of the sternum and inserts on the deltoid tuberosity of the humerus. It acts to protract and retract the forelimb.
All three of the above muscles are innervated by the cranial and caudal pectoral nerves.
Taking it a little further
When we consider the function and integrity of this region, we have to consider the other muscles of this region, the brachial plexus, and the transition to the viscera that occurs here. In our interview with Celeste Leilani Lazarus, she shares multiple key observations from studying the anatomy of this area.
Recreation by Anna Lloyd, from the Atlas of Topographical Anatomy of the Horse
In the cross-section at the level of T2, we can see the brachial plexus as well as the start of the thoracic cavity. We can also see the muscular structures surrounding the shoulder joint. We can see how important an appropriate range of motion in these structures would be, as a restriction in the fascia or the muscular structures will result in restriction of the brachial plexus and mobility of the forelimb, which will translate to the rest of the body. We can also note from this view, that the supraspinatus has a slip on both the medial and lateral sides of the scapula.
Recreation by Anna Lloyd, from ABC of the Horse
In the cross-section at the level of T4, we can see more of the muscular structures that are incorporated into the thoracic sling region, as well as the diaphragm. The diaphragm is a structure we need to consider in relation to the function and mobility, as well as connection within, the entire body. The deep fascial lines connect through the spine and the diaphragm, and the diaphragm creates a connection between the deep structures of the forelimbs and the deep structures of the hindlimbs, including the iliopsoas.
The Function
Together all these structures form the thoracic sling, allowing the trunk not only to be suspended between the two forelimbs but endowing it with freedom of movement between two supporting pillars. The integrity of these structures allows the horse to lift the thorax between the forelimbs to shift the centre of weight backwards, as well as to draw the limb closer to the thorax on one side while opening the other. This movement shifts the centre of mass towards or away from a forelimb, allowing for balanced movement around a circle or through a bend.
The glide of the scapula is the next important functional aspect to consider. Through the phases of a stride, the scapula will glide cranially and caudally, move slightly dorsally and ventrally, as well as rotate around a point more or less in the centre of the scapula. The amount of scapular glide that occurs will depend on the speed of the gait and the length of the stride. It is the scapular glide and the freedom, suppleness and dynamic strength of the shoulder musculature that allows for greater limb protraction and retraction in athletic endeavours such as dressage movements and clearing a jump.
The close connection to the brachial plexus, together with the fascial connections throughout the body, are essential for us to consider when working with this region and with any horse. The tensegrity of the entire body
Resources
Atlas of Topographical Anatomy of the Horse, by Peter Popesko
Watch our interview with Celeste Leilani Lazaris on the Equine Thoracic Sling
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