I believe ignorance of the use of these core muscles is the cause of most of the frustrations for the rider and a lot of suffering for the horse.

If you don’t use your deep postural ‘control centre’ muscles located within your torso (between your diaphragm and pelvic floor), the outer muscles in the shoulders, arms, hands, hip, thigh, knee, calf, ankle WILL take over the job. You cannot then move your arms and legs independently and freely. Stiffness and tenseness are then the result. (see ‘Want to ride better - read this”)

TO RECAP - WHY YOU NEED TO USE THESE CORE  MUSCLES

Consider for a moment the force that a horse, no matter what size, exerts when moving forward. Add in gravity and the up, down, side to side movement of a horse and you can see the rider’s body needs to use some muscles somewhere to avoid being flung off! What tends to happen is that, when we first learn to ride, we naturally use every other muscle but the correct ones (because we don't use them much in day to day life) to avoid falling off - its simply an instinct for survival.  These habits then tend to stay with us forever unless we are specifically taught how to locate and use core muscles or we discover them by chance.

FINDING AND DEVELOPING YOUR CONTROL CENTRE CORE MUSCLES

A good image to use for visualising the important core muscles is to think of your torso as a box sitting on end on the saddle.

The top of the box is your diaphragm (starts just below your last ribs) and this needs to feel like it is held down whilst still breathing in and out (will cover breathing later!).

The sides of the box are from your underarm to your hip both sides and your psoas muscle* needs to be switched on or 'in tone' to support the sides of the box.

*(the deep core muscle that runs either side of your spine - see http://en.wikipedia.org/wiki/Psoas_major)

The front of the box is your abdominals that run down the front of your torso and these need to be switched on or 'in tone' to support the front of your torso.

The bottom of the box is the pelvic floor which runs from the front of your ‘seat’ to the back and needs to be in tone or switched on to keep yourself level..

GET ALL YOUR GUY ROPES WORKING

Now, when all these muscles are brought ‘up’ into tone (you may need to be shown how to do this) they work in opposition to each other and hold you stable. However, beware, its very like the guy ropes that hold a tent up - if one part of the system is weaker, the stronger side will pull the weaker side over (more on asymmetry later).