The abdominal muscles used in conjunction
with key back muscles provide stability to
the entire torso.
The postural muscles work in two
opposing groups. Broadly speaking, the
abdominal ones flex the torso at the waist
(useful for petting that small dog) and the
spinal ones extend (arch) the back. The
key to upper-body stability is to get the
two opposing groups to work together creating a sort of bodice around the
abdomen, which prevents slouching but is
not so stiff as to create rigidity.
You can practice this anytime, anywhere, standing or sitting: Put your hands
on your stomach and blow out an imaginary candle. Feel the tension in the
abdomen. This is good tension. It is not
enough force to create a contraction of the
abdominal group, which would result in
full flexion (pet the dog) but it is enough
to stabilize the soft tummy in front and
prevent too much lateral wiggle. Practice
holding that kind of tension while walking, driving, doing dishes, etc.
As for the spinal muscles, they get a
break when the abdominals are doing part
of the postural work, and they can relax a
little to allow movement to flow up the
spine from the pelvis all the way to the
head. You are now "through your back" in
exactly the same way that a horse must be
"through" his back.
The most important thing to remember (although counter-intuitive) is that the
more you work at creating tension in the
abdominal group, the easier movement
can flow in a wave-like motion up your
spine. If you never think to engage the
abdominals in this way, then you are forcing your spinal group of muscles to try to
do all the work of holding your torso
upright, balancing your head and absorbing motion from below. This is a recipe for
back strain for both you and your horse,
because your pelvis will not be able t