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In our last few webinars, I’ve realised that I am missing something – that intense focus on anatomy that we went through in college, where we studied body parts and their functioning until anatomy was coming out of our ears! Do you remember? Those days when we had to know the innervation, insertion, origin, action and pathway of each and every muscle, ligament and tendon; we had to know every fossa and trochanter on every bone, every nerve root and pathway, and we had to know it in our sleep. I think that first year of college was the hardest academic year I have ever faced. But I miss it!

And so, to combat my nostalgia, I bought myself the kindle version of Horse Movement: Structure, Function and Rehabilitation by Gail Williams, illustrated by Alexa McKenna. It is a stunning book on functional anatomy, really reinforcing and deepening one’s understanding of this vital topic. Today I’m going to share a small nugget from the book: the thoracic sling.

Yes, I know it’s revision, but it’s only through repetition that something truly becomes cemented in our brains – as we learnt in that first year at college. It is energising to know a topic and know it well. Let’s go …!

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 are lie on the lateral and medial aspects of the scapula. These include the Trapezius, Omotransversarius, Rhomboideus, Latissimus Dorsi and Serratus ventralis.

Equine Trapezius muscle

Trapezius, Figure 4.19, illustrated by Alexa McKenna Page 75, Horse Movement: Structure, Function and Rehabilitation, Gail Williams PhD

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 Muscle equine

Omotransversarius, Figure 4.20, illustrated by Alexa McKenna Page 75, Horse Movement: Structure, Function and Rehabilitation, Gail Williams PhD

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 weightbearing position.

Rhomboideus equine muscle

Rhomboideus, Figure 4.21, illustrated by Alexa McKenna Page 76, Horse Movement: Structure, Function and Rehabilitation, Gail Williams PhD

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 Equine

Latissimus Dorsi, Figure 4.22, illustrated by Alexa McKenna Page 77, Horse Movement: Structure, Function and Rehabilitation, Gail Williams PhD

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 weightbearing.

Serratus Ventralis equine muscle

Serratus Ventralis, Fig 4.23, illustrated by Alexa McKenna Page 78, Horse Movement: Structure, Function and Rehabilitation, Gail Williams PhD

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.

Subclavius equine muscle

Subclavius, Figure 4.25, illustrated by Alexa McKenna Page 79, Horse Movement: Structure, Function and Rehabilitation, Gail Williams PhD

The Subclavius 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. 

 

Deep Pectoral muscle equine

Deep Pectoral, Figure 4.26, illustrated by Alexa McKenna Page 80, Horse Movement: Structure, Function and Rehabilitation, Gail Williams PhD

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.

 

Superficial and transverse equine pectoral muscle

Superficial and transverse Pectoral, Figure 4.27, illustrated by Alexa McKenna Page 81, Horse Movement: Structure, Function and Rehabilitation, Gail Williams PhD

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.

The Function

Together all these muscles form the thoracic sling, allowing the trunk not only to be suspended between the two forelimbs, but endowing it with a freedom of movement between these two supporting pillars. These muscles allow 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.

And so concludes my walk down anatomy lane, at least for today. For more on the equine shoulder and treating dysfunctions and restrictions, you can watch our webinar by Jenny Williams in the Equine Members library, Restrictions of the Shoulder.

Until next week!

 

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