How the Horse's Back Works
By Solange Schrijer and Paul Rene van Weeren
An equine physiotherapist and a veterinarian discuss anatomic knowledge.
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. 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. 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 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 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. 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'.
___________________________
Judy
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