The Horse
From The
Inside
written by Eiddwen Fletcher
The Importance of Ligaments
The horse’s skeleton provides stability and protection to the
body, connecting the bones and cartilage of the body are
bands of fibrous tissue known as ligaments, ligaments are
essential to provide support and strength to the joints of
the body, they are positioned around the joints to prevent
movement outside the natural range of motion, preventing
over stretching of the tendons and damage to the bones
and cartilage. At moveable joints the ligaments form a
capsular sac which contains synovial fluid and prevents the
articulating bones from rubbing upon one another.
Ligaments are made up of dense bundles of collagenous
fibres which are extremely strong and relatively inelastic. An
extracellular matrix of elastin fibres provides the ligaments
with some flexibility, but this is often limited and if put
under extreme stress a ligament will rupture. Blood flow to
the internal ligament cells is limited; resulting in slow healing
and repair functions, the scar tissue that forms is made up of
much smaller collagen fibres with altered cell connections
and flaws in the elastin matrix, retaining only 10-20% of the
normal viscoelastic properties of the original ligament fibres.
Ultimately the ligament becomes less able to maintain a load
and less able to absorb energy before failure.
As riders we all appreciate the joy we get from
horses, yet we rarely stop to wonder how they
achieve the amazing variety of movements we
ask them to perform. Whether you’re a happy
hacker, showjumper, jockey or dressage rider;
you are constantly reliant on your horse’s Anatomy and Physiology. In this series of articles you
will learn how the horse functions and how this
influences his ability to carry you as a rider and
perform as a competition animal.
In the ridden horse there are a number of ligaments that are
of vital importance to performance:
The main ligament of the neck is known as the nuchal
ligament, this elastic ligament attaches at the occipital bone
at the back of the horse’s skull and across the ridge of the
first two cervical vertebrae of the neck, it spans the crest of
the neck and attaches at the beginning of the withers where
it merges with the supraspinous ligament of the back. The
remaining cervical vertebrae of the neck are attached to the
nuchal ligament via two fan-like sheets of elastic ligament
which functions to restrain the movement of the cervical
vertebrae and support the weight of the head – similar to
a suspension bridge. While keeping the vertebrae in place
the nuchal ligament also assists the upper neck muscles by
storing energy during movement. When the horse’s head is
raised the nuchal ligament becomes slack and unstrained,
when the horse’s head is stretched down and forwards the
nuchal ligament is put in traction which loads the elastic
matrix with energy, creating elastic recall to assist the muscle
to lift the head back to its resting position. This energy
storage reduces the strain placed upon the muscles during
locomotion by up to 55%. The nuchal ligament also has a
huge influence on the support of the horse’s back. When the
horse is ridden in a ‘correct outline’ (on the bit) the cervical
vertebrae are flexed and the nuchal ligament is placed
under tension which passes to the supraspinous ligament
that runs along the midline of the horse’s
back and covers the tips of the spinous
processes. When under tension the
supraspinous ligament provides support to
the spine allowing the muscles of the back
to contribute more energy to propulsion
rather than support
Continuing down the back to the hind
quarters we reach the sacroiliac joint which
is of huge importance to the equine athlete.
The end portion of the spine is known
as the sacrum and consists of 5 fused
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Everything Horse UK Magazine • February 2014 • Issue 5
vertebrae, on either side of this lies the front most wing of
the pelvis (ilium) which is held in place by ligaments. The
joining of these areas is known as the sacroiliac joint which
holds the pelvis tightly to the spine allowing little movement
to occur and providing stability to the hind quarters. Two
sets of ligaments hold the sacroiliac joint together and these
ligaments are often subject to strain injury, it is suspected
that 50% of horses with back injuries present with sacroiliac
pain, unfortunately this joint is positioned deep within
the gluteal muscles of the hind quarters making diagnosis
difficult. Signs of sacroiliac strain include: asymmetry of
the hindquarters, poor performance (more
evident at slower paces), intermittent hind
limb lameness, toe-drag and tendency to
plait with hind limbs, lack of impulsion,
disunited canter, difficulty to collect, refusal
to jump, sensitivity at top of hind quarters
and loins and general signs of discomfort
during ridden exercise.
Causes of sacroiliac strain can range
from traumatic injury to conformational
weakness but modern training techniques
may also play a role. Commonly in
performance horses we require that the
canter is sustained for long periods, this asymmetric gait
combined with repetitive movements such as jumping,
will ultimately place uneven strain on the body resulting in
weakness and injury.
In performance horses the limbs are the most common
area for ligament injury to occur and the worst offender has
to be the suspensory liga ment! The primary function of
the suspensory ligament is to prevent excessive extension
of the fetlock joint during weight bearing. This ligament
lies along the back of the cannon bone where it splits into
two branches which pass either side of the fetlock and pass
to the front of the pastern where they join the extensor
tendon. The suspensory ligament is part of the suspensory
Issue 5 • February 2014 • Everything Horse UK Magazine
apparatus which prevent the fetlock joint collapsing during
standing and locomotion and acts as a spring which stores
energy when the fetlock is extended and returns energy to
the limb when the fetlock flexes. In addition the suspensory
apparatus is a critical part of the ‘stay apparatus’ which allows
the horse to stand at rest for long periods with barely any
muscular effort. So if the suspensory ligament is so vital to
the horse both during locomotion and inactivity, why is it so
commonly subject to injury?
As previously discussed ligaments are relatively inelastic,
when a ligament becomes over loaded it
is unable to stretch, resulting in tearing or
rupture of the fibres. During strenuous
exercise such as galloping or jumping;
excessive strain is placed on the flexor
muscles of the limb causing them to fatigue,
combined with extreme forces, the fetlock
is forced to over extend resulting in the
supportive structures being overloaded and
subject to injury. The signs of suspensory
ligament injury can vary depending on
the area of the ligament damaged; slight or
intermittent lameness may present but often
in the early stages a horse can continue to function in spite
of the injury resulting in further damage and progressive
thickening of the ligament. In more acute cases heat and
swelling may be evident and the area may become sore
to touch. It is essential that any intermittent lameness or
loss of performance be seen by your vet immediately to
rule out possible ligament damage, if left untreated the
ligament fibres will continue to tear creating further injury
and reducing the chance of recovery. Although ligaments
are able to produce new connective tissue, the scar tissue
is significantly weaker and it is important to limit further
damage as much as possible. In severe cases thickening of
the suspensory ligament can result in pressure upon the
structures of the limb and may result in splint bone fractures.
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