You may ask - what is the chair seat? Well, the name is sort of self-explanatory. In horse riding, a chair seat is when your leg slides too far forward and you sit too far backwards in the saddle, until you look like you're sitting on a chair instead of riding a horse.
The chair seat disrupts the ear-shoulder-hip-heel line of balance that a rider should aim for by placing too much weight in the seat and not enough in the heels, which is why people with a chair seat also have the tendency for their heels to slip up. I know I definitely struggle with this problem, except in a different way from most - most chair seats will be accompanied with a tendency to lean too far backwards in the saddle, but for me it's the opposite. Too much jumping has caused me to "perch" and lean forwards, and I really struggle with tucking my hips underneath me and sitting deep. This "perching" position has thrown my centre of gravity forwards, making my leg slip forwards as well to compensate.
So, how do we fix the chair seat?
I believe that the problem stems from not the leg, but the seat - if you sit correctly in the saddle, then your leg should find a more correct position by itself. If you're sitting correctly, you should be able to feel your seat bones on the horse, so experiment with shifting your seat forwards and backwards until you can feel your seat bones sort of "connect" with the horse's back. I'm still working on my seat, and I constantly have to remind myself to tuck my hips underneath me and lean backwards to create a more balanced position. I have found that by doing this, it is easier for my leg to stay underneath me and for my heels to stay down.
If you don't think your problem stems from your seat, it may be that your knees are the problem - if you close your knees too much around the saddle, they create a block in your flow of balance and cause your lower leg to swing around. Think about opening your hips and your knees - this should help move your leg backwards so it is touching the horse's side.
Sometimes, you just need to put your leg into the correct position and focus with all your might on making it stay there. Fixing your seat and leg position isn't a quick process - it takes a lot of hours in the saddle to change your muscle memory and adjust your position. It helps a lot if you can have someone watch you ride and tell you every time your lower leg moves forwards, as sometimes you can be unaware that you've slipped into a chair seat yet again.
As I've stated, I'm still working on fixing my chair seat, so here are a lot of articles that have helped me a lot with my leg position and my position in general:
- Correcting Riding Problems- Legs: http://www.themanestreet.com/authors/lauramartlock/training/legs.htm
(DEFINITELY recommend reading this one!)
- 15 Exercises to Correct Common Riding Problems: http://practicalhorsemanmag.com/article/15-exercises-to-correct-common-riding-problems-30090
(the section on improving your leg position is quite helpful)
- 10 Time-Tested Rider Position Fixes: http://practicalhorsemanmag.com/article/10-time-tested-rider-position-fixes
- The Importance of the Lower Leg: http://www.tanjamitton.com/the-importance-of-the-lower-leg/
And some stretches to help you open your hips if (like me) your closed hips cause your knees to grip the saddle:
Hope this helps! Happy riding!
