It's actually the ring of muscles which include the abdominal, the scalenus, and the semiteninosus.
The keep this ring of muscles active, the horse needs to be ridden in a way that will preserve the health of the horse's back. The easy gaits of gaited horses tightens or contracts the back muscles. To keep the back muscles healthy, they need to be stretched.
Proper use of the ring of muscles allows a horse the opportunity to reach
his full athletic potential and helps to keep a horse sound for the
long-term.
A TTEAM practitioner said, when talking about the balance rein: "The light
pressure on the base of the neck helps to trigger the "seeking reflex" which
is the third part to the "ring of muscles" that is necessary for proper
engagement. It encourages the withers to rise and the neck to telescope from
the horse's shoulder: the opposite of tightening the poll and jamming the
neck."
The horse's back is held up by a series of "cables" which includes ligaments
and muscles, some of which are shown on the image, such as the dorsal
ligaments, the scalenus, the semiteninosus, and the abs.
The longissimus dorsi (the muscle that runs along the length of the back) is
not responsible for holding up the back. It's job is to stabilize lateral
movement and serve as a medium for sending movement from the hindquarters
forward.
To keep the back of the horse healthy, the other muscles in the "ring of
muscles" are exercised and strengthened. This can include any number of
exercises such as head-down, long and low, lateral flexion, disengagement of
the hind, neck telescoping, belly lifts, etc.
Function of the Ring of Muscles begins with the loins, so let's take a look
at that part first. Here's a picture of my horse's loins (the width happens
to be marked in this picture).
The loins (also known as the "coupling") are between the lumbo-sacral joint
(LS) and the thoracic lumbar junction (TL). They are marked on the following
image.
You can find these areas on your horse by palpating for the LS joint. Feel
down the spine and when you get to the hip area, feel for a spot that
doesn't have bone--sort of a small trampoline-feeling area.
To find the TL junction, put your fingers on the last rib, and follow it up
to the spine.
Once you've found the loin area, you can define the loin triangle and
"evaluate" it.
In this picture, the loin triangle is outlined. It shows the line up
of the LS (lumbo sacral) joint in relation to the hips.
This horse has a wide loin--from side to side-- (good for carrying a rider),
but also a long loin--from front to back-- (not so good for back health). A
wide, but short (from LS to TL) is stronger for carrying a rider.
For this horse and his particular loin area, we try to keep it healthy with
suppling exercises.
Strength of top line and loin muscles over the kidneys are factors that
influence soundness and athletic ability.
Horses that have excessively long backs are unbalanced and weaker in their
top lines than short-backed horses. The loin should be well-muscled and
strong as opposed to being long, weak and poorly muscled.
Some things to look for are: pain, tight muscles in the loin area, atrophy,
"stringiness" feeling under the skin, bumps, lesions, obvious pathology of the
bones (for example hunters bump).
Good loins should be smooth and broad.
Problems in the loin area: Outward manifestation of symptoms might include
some of the following: a wry tail, turning away from approaching people,
movement while mounting, unsettled attitude while ridden.
We get to the tricky part now! In the gait of tolt (rack), the ring of muscles is not really working the way it should!
