Stirrups aren't a necessity. But they do help us balance as we ride. They aren’t there to carry all of our weight, and especially when we are posting the trot, we need to learn to lift ourselves with our core, not by putting pressure on the stirrups. Doing so actually transfers more pressure to our horse’s back and our goal when riding should be to keep our horses as comfortable as possible and keep our aids clear.
When mounting either in an English or western saddle, it’s important to turn the stirrup so that it isn’t twisted against your leg once you’re seated. Because of the way the fender is made on a western saddle, you’re less likely to turn the stirrup the wrong way. However, in an English saddle, doing so can result in some nasty chaffing along the inside of your calves. The stirrups will hang from the saddle sideways to the horse. Before you mount, turn the stirrup towards you so that the stirrup is ‘flat’ against the horse and there is a half turn in the leather. The part of the stirrup that was against the horse should now be to the right or hindquarters of the horse. Turning it the other way will cause an extra twist in the leather when you get on and that will be very uncomfortable. The stirrup leather or fender should lay gently twisted, but flat against your leg.
The ball of your foot should rest firmly on the tread.
Be careful not to jam your foot too far in. This will be uncomfortable as any weight you put on your foot will be under the arch of the foot. This will also make it harder for your foot to slide out if you should fall. You also don’t want just your toes resting in the stirrup, as you’re foot will slide out too easily, your leg won’t be secure and it will make posting difficult and uncomfortable.
Always remember heels down. Don’t think of it as shoving your heels downward, but of letting your weight drop through your legs allowing your heels to drop naturally. This helps with your balance, and it prevents your foot from slipping forward through the stirrup. Again, if you should fall off, your foot will more naturally slip back out of the stirrup, rather than staying caught in it.
Losing Stirrups When You Ride
Losing one or both stirrups can be unsettling when you first begin to ride. If you lose one, you may feel as if you’re tipping to one side. Losing both may make you feel as if you’re going to pop or slip out of the saddle if you’re going any faster than a walk. If you find this happens often the solution is to work without stirrups. This helps you improve your seat, and develops your balance and strength.
It’s best to work with a coach who can put your horse on the lunge line and work with you at the walk, trot and canter/lope. Yes, it is possible to post without stirrups at the trot! It’s hard work, but posting without stirrups will help you learn to lift with your core muscles and develop a firmer seat, rather than just lifting with downward pressure in the stirrups.
As your seat improves, you will be less likely to lose stirrups, but it can still happen.
Because your seat is more secure, it will simply be a matter of picking the stirrup up again. This is done by turning your toe in towards the horse, and feeling where the stirrup is dangling. If you’re trotting or cantering/loping, you might want to come back to a walk. However, as you gain skill, you’ll find that you will be able to carry on without changing gaits. This is important if you’re showing especially. You don't want the judge to see you struggling to pick up the stirrup, so it's worth learning how to keep a secure seat and make picking up the stirrup unobtrusive. Even over jumps, good riders can lose a stirrup and stay securely in the saddle. Like any riding skill, it takes practice.