how the horse’s back functions

The horse’s back : anatomical and biomechanical considerations

Different anatomical structures intervene in the biomechanics of the horse’s back. How do they function ?

The skin, a structure suited to flight

One should always keep in mind that in the wild, the horse is a prey animal. His survival is therefore directly linked to his capacity to flee. The skin (particularly the dermis) is tightly attached to the subcutaneous tissues (fascia, muscles, skeleton). This anatomical particularity is a real advantage when he needs to escape rapidly at a gallop, the tensing of the skin having an immediate repercussion on the deeper tissues.

Several layers of muscle with very precise functions

There is not just one but several layers of muscle, which all intervene on different levels, to get the horse’s back moving.

Large superficial muscles for scopey movement

Figure1 : The most superficial muscles of the neck, withers and back © M. Oussedik (according to J.M. Denoix)

The superficial muscles correspond to several layers of large, long, powerful muscles. The most superficial layer is directly located under the skin. These are the gymnastic muscles, in charge of ample movements, like the brachiocephalicus muscle which links the temporal bone (below the ear) to the arm, or the trapezius, or the latissimus dorsi located in part under the saddle (see figure 1).

Figure 2 : Back muscles below the most superficial layer © M. Oussedik (according to J.M. Denoix)

Two large muscle chains are slightly deeper, these are composed of the spinalis dorsi, the longissimus, and the iliocostal muscles (which together are known as the « erector spinae group »- see figure 2) they are located on either side of the spinous processes of the backbone (shown in red in figure 3 below), and compose the main part of the back musculature. It is interesting to note that these muscles join the lower cervical muscles at the front end, and the pelvic muscles at the rear, allowing the transmission of forces from the hindquarters to the forehand.

Two main muscle chains : the flexor chain and the extensor chain

Position of the extensor chain and flexor chain relative to the spine — Figure 3 : Position of the extensor chain (topline) and flexor chain (bottom line) relative to the spine © I. Burgaud

No single muscle ever works alone. Apart from the local action with the antagonist muscle, muscles are often associated in groups or muscle chains which serve to establish action patterns and movement coordination. In horses, the back will function harmoniously as a result of the complementarity of the work between the two main muscle chains : the extensor chain (topline) and the flexor chain (bottom line) (see figure 3) .

muscles of the extensor and flexor chains — Figure 4 : muscles of the extensor and flexor chaines © M. Oussedik (d’après J.M. Denoix)

The extensor chain (topline muscles) is composed of the extensor muscles in the neck, further towards the tail, the erector spinae group (see figure 2), and prolonged by the femoral and caudal muscles (gluteal muscle group) which end, as their name indicates at the rear of the hindquarters. These are pairs of muscles located above the spine (in red in figure 4). When they contract, these muscles provoke an extension of the neck, back and hip joint, in brief, their contraction causes the horses back to hollow.

The flexor chain (or bottom line) is composed of flexor pairs of muscles located beneath the spine (yellow in figures 4 and 5). With regard to the horse’s back, these are mainly :

Muscles pectoraux, dentelés et subclaviers — Figure 5 : Pectoral, serrated and subclavian muscles © I. Burgaud (according to J.M. Denoix)

Muscles which suspend the thorax between the shoulders a bit like a hamac : pectoral, serrated and subclavian muscles – the horse does not have a collarbone (see figure 5).
Abdominal muscles (rectus abdominae, oblique internal and external, and transverse abdominal muscles) : by linking the sternum to the pelvis, their role is to maintain the abdominal cavity and allow the horse to round his back, flex his lumbar-sacral joint and engage his hindquarters (see figure 4).
Iliopsoas muscle group located deeply beneath the spine, within the abdominal cavity : these are the main muscles involved in the engagement of the hindquarters as they link the thoracic caudal and the lumbar vertebrae to the pelvis and femur (figures 4 and 6)

The flexor chain ensures the stability of the horse’s core, and are essential to maintain correct posture of the back. When they contract, these muscles cause extension of the neck, back and hip joint. In brief, their contraction lowers the neck and raises the back.

Smaller deep muscles to stabilise the spine

Figure 6 : Deep back muscles © M. Oussedik (according to J.M. Denoix)

The deep spinal musculature corresponds to a group of small muscles, located in depth, whose role is to maintain and stabilise intervetebral joints. Examples of these muscles are the multifidus cervicis and multifidus dorsi muscles. They are located all along the spine, in depth, and they link the veterbrae together, in groups of 2-5 vertebrae (see figure 6). They are equipped with many proprioceptive receptors which make them very sensitive to postural variations, which allows them to permanently readjust the vertebrae to protect the spine from any improper movement.

The ligaments : essential to allow tension in the topline

Ligaments are fibrous strips of collagen and they play a part in maintaining the spine. In horses the ligament system of the back is divided into two parts (see figure 7) :

The nuchal ligament where the ligament strips insert onto the spinous processes* of the cervical vertebrae, and the vertebrae in the withers;
And the supraspinous ligament which is inserted onto the spinous processes along the backbone from the withers to the sacrum (thoracic, lumbar and sacral vertebrae).

Figure 7 : The ligament system in the horse’s back © M. Oussedik

This ligament system plays an essential part in the way the horse’s back functions. To get a better understanding, we need to dwell on the features of first 9-10 thoracic vertebrae which form the base of the withers. Thses vertebrae have very long spinous processes. When the horse lowers his head, the nuchal ligament stretches and exerts a traction force on the supraspinous ligament. This widens the spaces between the spinous processes, like a fan opening. On the contrary, when the horse raises his head and hollows his back, the ligament system becomes slack. The spinous processes move towards one another : in the long term this can cause « overriding spinous processes », which can lead to back pain and be detrimental to the horse’s health and to the quality of his locomotion.

Radiographie du dos d’un cheval sain — X-ray of a healthy horse’s back © I. Burgaud

Radiographie du dos d’un cheval avec conflits de processus épineux — X-ray of a horse presenting overriding of spinous processes © I. Burgaud

*Spinous processes = bony projection off the back of each vertebrae (visible in figure 3)

The spine : a more or less mobile axis

Region of the spine	Number of vertebrae	Mobility
Cervical region	7 cervical vertebrae	Extremely mobile, from right to left and up and down
Thoracic region	18 dorsal and thoracic vertebrae	Reduced mobility, especially from D1 to D8 (area where the ribs are attached to the sternum, under the saddle)
Thoracic region	18 dorsal and thoracic vertebrae	More mobility as from D9, i.e. behind the the base of the withers, but still limited, which explains that bend around the leg is restricted
Lumbar region	6 Lumbar vertebrae	Very restricted mobility (the least mobile) due to the transversal processes of the lumbar vertebrae which are wery wide and flat (they cannot overrride)
Sacrum	5 Sacral vertebrae bonded together (= a single bone item)	The lumbar-sacral joint (between the last lumbar vertebra and the sacrum) is very mobile, but only for flexion-extension. This allows for strong engagement of the hindquarters and therefore impulsion
Tail	15 to 21 coccygeal vertebrae form the base of the tail	Extremely mobile

The spine, a more or less mobile axis © A. Laurioux

The digestive system and its impact on how the horse’s back works

Figure 8 : The horse’s digestive system © M. Oussedik

With the exception of the stomach and the liver which are attached behind the diaphragm, the intestines (colon, caecum, small intestine) are suspended beneath the lumbar region of the spine (see figure 8). Even if the intestines are maintained by the wall of the abdomen, and occupy the whole space, the fact that they are not attached to the side walls leads to some mobility which can lead to torsions or to serious colic. To picture this, think of an enormous ball weighing several tens of kilos hanging under the horse’s back, and which swings in rhythm to the pace, and has a strong impact on locomotion, especially at a trot.

How the back functions at the different gaits

At a walk

The walk involves the most complex movements of the vertebrae. This is what gives the rider the impression of a movement shaped like an 8, felt in one’s seat when the horse is walking. By solliciting the deep muscles nearest to the vertebrae on either side of the spine, this gait will help in preventing the appearance of disorders since the back will thus be educated. The walk is a very beneficial aid to proprioception, especially for young horses in training. Do not therefore spare yourself the pleasure !

At a trot

At a trot the flexion and extension movements of the spine are passive ; they are due to the inertia of the intestinal mass. It is the limbs striking the ground which are involved in the movements of the back.

When the diagonal is on the ground, the back hollows since the intestinal mass pulls the spine downwards. The back becomes rigid thanks to the abdominal muscles which resist the acceleration force of the intestines
When the feet are above the ground, the back rises, since the intestinal mass is pushing the spine upwards. The back becomes rigid this time thanks to the muscles of the dorsal chain in order to avoid the back over flexing.

fonctionnement du dos du cheval au trot — At a trot the back dips when the diagonal strikes the ground © A. Laurioux

The movement of the intestinal mass is controlled by the muscle tone of the superficial back and abdominal wall muscles. The extensor and flexor chains work together to build up strength in the back.

At a canter

Unlike the trot, where the back is submitted to the movements of the intestinal mass, the back muscles are much more active at a canter. It is in fact these muscles which will mobilise the horse’s back. The intestinal mass is passive in this gait, and follows the movements of the back. It is a much more natural gait. Take the example of a canter on left lead, where the alternating phases are as follows :

Contraction of the flexor chain (bottom line) during suspension to engage the hindquarters beneath the horse and when the right hind leg strikes the ground (see photos a and b- torso flexion).
Contraction of the extensor chain (topline muscles) when the right diagonal is on the ground (see photo c), until the left hind finishes propulsion (extension of the torso).
And once again contraction of the flexor chain as the horse’s limbs are once more above the ground to prepare the engagement of the hind leg for a new cycle.

temps de projection au galop à gauche — Photo a – Contraction of the flexor chain (bottom line muscles) during the propulsion phase of a canter on a left lead © A. Laurioux

poser du postérieur droit galop à gauche — Photo b – Contraction of the flexor chain when the right hind limb strikes the ground during a left lead canter © A. Laurioux

appui du diagonal droit galop à gauche cheval peint — Photo c – Contraction of the extensor chain (topline muscles) when the right diagonal is on the ground © A. Laurioux

The walk and the canter are both excellent gaits for horses with dorsal disorders, unlike the trot which exerts strong mechanical constraints on the horse’s back due to the inertia of the intestinal mass.

Putting this into practice in the horse’s work : « no abs, no back ! »

These few basic facts regarding the anatomy and biomechanics show us that it is impossible to build up the horse’s topline without stengthening the bottom line. As the saying goes : « no abs, no back ! ». Lack of stregth in the abdominal area is synonymous with a hollow back. In this posture, the horse cannot carry the rider’s weight without constantly contracting his extensor muscles (topline). One should remember that a muscle should not be contracted for too long. Muscular tiredness sets in, cramp can appear (accumulation of lactic acid) and a vicious circle begins. Work becomes more and more difficult for the horse and becomes painful… The aim is therefore to build up muscle strength in the bottom line through appropriate exercises, so as to promote correct functioning of the back, and allow the horse to carry the rider’s weight, rather than suffering it.

Exercising with a low neck : essential prerequisite to the back functioning correctly

By stretching the nuchal and supraspinous ligaments, exercising with a low neck will create a tension in the topline and space out the spinous processes, especially in the thorcic region. With regard to the muscular system, this low attitude, when associated to correct engagement of the hindquarters, reinforces the work of the abdominal muscles and the psoas, which need to counteract the tension in the ligament system of the back. It also promotes stretching, and consequently suppling of the extensor chain muscles. When repeated regularly, this exercise allows the horse’s back to carry the rider’s weight and retain freedom of movement, preventing back pain.

It cannot be repeated enough : it is not the back muscles which serve to carry the rider’s weight, but the tension of the bottom line.

fonctionnement du dos du cheval — Before asking the horse to collect...© A. Laurioux

And even when the horse has been collected, always request that he stretches out again afterwards….

For this reason it is important to maintain activity, even when asking the horse to lower his neck ! Without impulsion, there can be no engagement, without engagement, there can be no mobilisation of the bottom line, and no mobilisation of the bottom line means no back ! Otherwise the horse seems to be constantly chasing his balance, is unbalanced, and overloads the forehand, risking the development of disorders in the front limbs...

In practical terms, how should one go about it ?

Here are a few basic exercises to build up strength in the flexor chain

Favour exercise time in a low rounded frame engaging the hindquarters beneath the horse.
When in the stable, ask the horse to bend his head between his front legs with a carrot, and maintain the position for around ten seconds, without moving backwards, or bending at the knees- repeat the exercise towards the stifle on either side
Rein back (even more effective on a slight slope)
Downward transitions (even more effective on a slight downward slope, as the horse needs to work on his balance)
Ground poles (even more effective if they are slightly raised or on a downward slope).

assouplissement dos du cheval — Fetching a carrot between the front legs without moving backwards or bending at the knees © A. Laurioux

travail sur barres au sol — Ground poles © A. Laurioux

travail en terrain varié — Work on transitions over varied terrain © N. Genoux

Reinforcing the abdominal muscles will gradually lead the horse to become more balanced, i.e. to lighten the forehand, and lower his haunches, and all the while maintain a correct posture with his back. In other words, the horse appears to « grow ». The withers appear to push up between the shoulders, the back is rounded, and the hindquarters engaged, promoting flexion of the poll and cervical vertebrae. This then leads to the horse being collected. This learning process takes a long time, months or even years… Take care to remain gradual in your demands, depending on the horse’s age and experience.

None of this can occur if the horse’s back is not relaxed

Never forget that muscles can only work efficiently if they are relaxed. General L’Hotte explained this well : « when one flexes a joint, it is not the muscles which determine the degree of flexion that are being suppled, but the opposing muscles, as they must relax and yield ». Therefore, reinforcing the flexor muscles, necessary for the back to function harmoniously, can only occur if the antagonist muscles of the extensor chain are sufficiently relaxed. This shows the importance of a good warm-up, in a rounded low attitude, before any more stringent work.

Because of the mechanical constraints on the back at a trot, one should favour a warm up at a walk (at least 20 minutes) then at a canter, preferably balanced over the stirrups, especially for horses with back conditions.

To remember

First of all, seek a low rounded frame to tense the topline, and to promote muscular relaxation while warming up.

Then, mobilise the flexor chain (abdominal muscles). It is impossible to have the back function correctly with no abs !

Take care not to favour only the work of the superficial muscles ; the deep muscles are equally important and essential !

All this takes time, and does not happen overnight. The learning process should be gradual to be effective and to work in the respect of the animal’s physical integrity.

The horse’s back : understanding how it works so as to better train him