This is an excerpt from Gentle Art of Horseback Riding, The by Gincy Self Bucklin.
Posting in the Stirrups
Different disciplines ride with stirrups of different lengths, which affect posting. With a longer stirrup your leg is straighter, which means you can't lean very far forward without losing your balance, nor can you go up very high. With a shorter stirrup your hips are farther back on the saddle, so you have to lean farther forward to get up, but you get more longitudinal support. You can also go up higher. Shorter stirrups are easier on a horse that is thin or has a big, bouncy trot; longer stirrups are easier on a wide horse or a horse with a smooth trot. There are other reasons for stirrups of various lengths, depending on the discipline.
In a Western saddle you can't lean very far forward without running into the horn. The same is true of a dressage saddle with a deep seat and high pommel. Both disciplines are ridden with a fairly long stirrup. Your center must be over your bubbling spring, so it's important that the stirrup not hang too far forward; the farther forward the stirrup hangs, the farther forward you have to lean to be centered over it (figure 16.4). If the stirrup is long or the pommel is high, it is impossible to post in balance.
Once you're back in the saddle or have your stirrups back, after warming up, start working in half-seat open position to get grounded and comfortable on the stirrups. Next, with your horse standing still, start posting, very slowly at first. Your hands should be holding the grounding strap but pressing down lightly with your knuckles on the withers or lower neck for balance (figure 16.5). Begin with your body in full-seat forward and feet in 'A' position, then bring your hips up and forward until you are in half-seat open or slightly closed position.
Focus on keeping your knee pressed forward as you did in the box exercise and your center over your feet so that your stirrups don't move forward and back at all. They may move toward and away from the horse's sides as you put more or less pressure on the inner edges of your feet each time you sit and stand. Avoid the tendency to curl your toes, which might occur as your body tries to adjust to the changes in position. It might help to have the instructor hold your lower leg in place while you find the correct moves to keep you in balance. You can also try with your arms folded or hanging down to make sure you are in good balance. Closing your eyes is another good test.
Next, work at the walk to check your timing and centering on a moving horse with your feet in the stirrups. Take up three-quarter seat at the walk and think about your following seat and feet. As your outside seat and foot drop, think down; as your inside seat and foot drop, think up. When you feel ready, move into the trot in half-seat closed position, with a light pressure with your knuckles on the horse's withers area, wherever you feel comfortable and balanced.
Again, pay attention to your following seat and feet. As you feel your outside seat and foot dropping, allow your hips to go back and down until your seat bones touch the saddle, then come immediately forward and up. Regain your balance if necessary, then repeat. Continue in this pattern, doing only one posting stride at a time, until you can maintain your center and balance through one stride. Then try two or more, always returning to the half seat if you lose your centering. Rest fairly frequently and go through the seven steps to relieve any tensions you might have developed.
When the rhythm feels steady, which will probably take a lesson or two, you might notice that you are hitting the saddle a little hard or double bouncing as you connect. This occurs because holding your body in forward position uses the buttock and low back muscles, making them a little tense. You need to add a little rest-and-relaxation movement, the final action that makes long-term posting possible and both you and the horse more comfortable.
Read more from The Gentle Art of Horseback Riding by Gincy Self Bucklin.