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How the Horse’s Back Works

An equine physiotherapist and a veterinarian discuss anatomic knowledge and current biomechanical concepts involving ‘deep and round’.

By Solange Schrijer and Paul Rene van Weeren

Whenever two or more people meet, there will be different opinions, and different topics sometimes lead to rather fervid debates. Equestrian sports are no exception. The latest discussion in Reiter Revue on the reputed advantages and disadvantages of the ‘deep and round’ training method, of which Anky van Grunsven and Sjef Janssen approve, is a good example.

In this article, we do not intend to take a stance on the ways to train a horse. Our intent is to supply the reader with information on anatomic knowledge and current biomechanical concepts of the horse so he can make his own judgment. We begin with concepts involving the function of the horse’s back, historically and today.”

The question of how the activity of the horse’s back can be described best can be traced back to the ancient world. Claudius Galenus (130-200AD) developed the concept of the ‘vault roof’, in which the horse’s back and the upper part of the thorax create a roof over the abdominal and thoracic cavity (Ill. 1) The spinal processes prevent the roof from collapsing, However, the fact that the spinal processes (the bony projections of the spine) normally do not touch each other, made this representation improbable.

A new concept was developed in 1947 by C. Bergmann and further developed by E. Zschokke in 1892. This concept saw the back as a grid bridge on four pillars – the horse’s legs. The upper fixed point is the nuchal/supraspinous ligament (Ill. 2) which withstands the tension forces. The loser fixed point – the vertebrae – are under pressure. The spinal processes and the ligaments in between constitute the smaller supports between both fixed points. This concept served as a foundation for many studies and was generally accepted until the 1940s. However, it contained one fundamental mistake. Such a bridge is stressed through tension from below and pressure from above – the opposite of how a horse’s anatomy works.

After thoroughly studying the anatomical shapes of the spinal processes of a variety of animals, zoologist E.J.Slijper created a model in 1946 that is still accepted today. His so-called ‘bow and string’ concept not only takes into account the horse’s spinal column and limbs but also his sternum and abdominal muscles. P.J. Barthez had presented such an idea 150 years earlier in 1798, but apparently he was ahead of his time.

This model, now agreed upon throughout the world, presents the spinal column as a bow that is held under tension by the string (the abdominal wall). In this concept, the nuchal/supraspinous ligament is stretched and loaded under tension, which is the only stress it is able to withstand.

There are many factors that determine the strain on this system: the simultaneous contraction of the abdominal muscles, the minor psoas and the iliopsoas muscles (Ill. 3) which both lie under the longitudinal axis of the spinal column. Contraction of musculature lying above this axis will lead to a hollow back (‘stretching’ of the back). Contraction of musculature under this axis will cause the back to curve upwards or bed (tense the bow, i.e., lift the back )

The minor psoas and the iliopsoas muscles also rotate the pelvis, which helps in tensing the bow and shifts the centre of gravity backward. The iliopsoas muscle has a special role in this mechanism as it runs from under the lumbar spine to the thighbone, and accordingly arches the back and pulls the hind legs forward when contracting. Therefore, these muscles are of the utmost importance for dressage horses, where the hind legs step far underneath the body and where the point of gravity is to be transferred backward.

Because these muscles are located within the abdominal and pelvic cavity, they’re not visible from the outside. It’s important to keep in mind that the back muscles visible from the outside lie above the longitudinal axis of the spinal columns and therefore stretch (hollow) the back when contracted.

The lifting of the back is also achieved indirectly when the horse’s front legs are moved back or when the hindquarters are moved forward. The string is brought under tension (i.e., the back is stretched or hollowed), not only through the moving forward of the front legs and moving back of the hind legs, but also through the considerable weight of the viscera (internal organs). This effect can be seen clearly when looking at older broodmares that have hollow backs.

A rider has a similar effect: as gravity always works in a downward direction, both a rider and a foal will act downward on the vertebral column and tend to make the back hollow.

The Influence of the Head-and-Neck Position
What influence does the horse’s head position have on this whole mechanism? It is not true that a young horse’s back is carried by the nuchal/supraspinous ligament and later by the ventral serrated muscle after it is developed through training, even though this muscle plays a big role in athletic performance.

First of all, this muscle consists of two parts: the cervical ventral serrated muscle and the thoracic ventral serrated muscle (Ill. 4). The cervical ventral serrated muscle is situated in the neck area in front of the saddle, but originates in the lateral processes of the third to seventh neck vertebrae and is connected with the inner side of the shoulder blade. Together with its chest counterpart, which originates in the middle third of the fifth to eight rib and is connected with the same shoulder blade, this muscle keeps the horse’s trunk between the forelegs. The horse doesn’t have a bony connection between the axial skeleton and the skeleton of his forehand. In other words, the horse doesn’t have a collar bone.

Even though the tension of the whole head-neck position keeps the horse’s body in balance and counteracts the effect of gravity on the abdomen’s content as well as of the rider’s weight, the horse’s head position certainly plays an important role. When the head is lowered, the nuchodorsal ligaments put tension on the wither and lift the spine, making the back arch. The lifting of the neck on the other hand, will have the opposite effect. The back will be stretched and the vertebral column will thus assume a hollow position,

Therefore, riding a horse with a low head position lifts the back, separates the spinal processes across the length of the thoracic vertebrae, and relaxes the back muscles. This is what we call ‘forward and downward’.

But what happens when we put the horse’s head intentionally onto his chest – for example, an extremely bent neck and, at the same time, activating his haunches. Through the simultaneous action of asking the horse to step forward with his hind legs and bending his neck to the fullest extent, we draw the bow to its maximum. This kind o training is useful for the musculature system since the abdominal muscles together with the above-mentioned sublumbar muscles (below the thoracic vertebrae and sacrum)must counteract the increased tension of the bow., The nuchal/supraspinous ligament is stretched through the forward pull on the nuchal ligaments and through the upward contracting of the abdominal muscles. This improves the elasticity of the spine. 1

Denoix and Pailloux express clearly, however, that this way of training a horse should be restricted, since excessive use – like every unbalanced from of training – can cause injury. In the beginning, these injuries will affect the muscles. However, as the stress increases, it will become more likely that ligaments and bones will be affected as well.

Abdominal Muscles
Through the explanation above, the role of the abdominal muscles should be clear. It is one of the mechanisms that, always in combination with the sublumbar musculature, draw the bow, and lift the back.

It would be too simplistic to assert that the size of the abdomen is a measure for its muscular development. This is neither the case with humans nor with horses. The visceral content of the abdomen and to a certain degree the makeup of the thorax are much more responsible for the size of the abdomen than the abdominal muscles.

It is true that the abdominal muscles play a role in the horse’s breathing. However, the horse, like most mammals, has a chest-abdominal breathing system. That means that breathing is done through a combination of abdominal muscles, diaphragm and intercostals (situated between the ribs). Consequently, it is possible that breathing is transferred from a predominantly abdominal type to a predominantly breast type of breathing during intense workout situations.

Furthermore, it should be stressed that, if the animal is healthy – doesn’t suffer from discomforts such as chronic bronchitis – the breathing process is active when inhaling, when chest and abdominal muscles increase the negative pressure in the thoracic cavity.

The breathing process is passive (i.e., with no muscular effort) when exhaling: when the air is pressed out through the elasticity of the tissue, which was stretched in the breathing – in phase. Therefore, the assertion that the horse unable to breathe when the abdominal muscles are in tension (when he is ridden with his head flexed to his chest, for example) is therefore not true.


Denoix, Jean-Marie and Pailloux, Jean-Pierre,, Physical Therapy and Massage for the Horse.

Reprinted from Dressage Today, November 2003.