You can see that as the seat leaves the saddle the heels drop slightly. This is what should happen as the weight of the rider transfers from the seat to the feet. This cannot happen if the stirrups are too long.
Many riders tend to ride with their stirrups too long rather than too short. This may be because they want their legs to look as long as possible.
In the second diagram the heels are rising as the seat is rising (in rising trot) which is incorrect. This is because the stirrups are not short enough to allow the heels to drop slightly at the same time that the seat leaves the saddle.
If your stirrups are too long you will also not be able to clear the pommel of the saddle (in rising trot), therefore you will either hit it each time you rise or you will not be able to swing your hips far forward enough.
Your lower legs (in rising trot, sitting trot and canter) will be 'disengaged' when your heel is higher than your toe because as soon as the heel comes higher than the toe the lower leg 'disengages'.
When your heel is higher than your toe you are standing on 'tip-toe' (or rather the balls of your feet) and you are in a very precarious position.
You will also lose your balance more easily and you will tend to rely on the reins for stability.
In sitting trot and canter the stirrups will either 'clatter' around on your feet or you will lose them altogether.
If the stirrups are too short several things tend to happen:
The rider will tend to sit further back in the saddle, this can put too much weight on the weaker part of the horse's back (the back gets weaker the further it goes from the withers).
The rider will tend to rise too high in rising trot, this means that their centre of gravity will be too high when they are at the top of the rise, making them less secure.
The rider will tend to tire more quickly, this means that they become more difficult for their horse to carry.
Stirrups that are too short are much less common because a rider tires more quickly when they are too short.
The muscles in the legs have to work a bit harder and the joints ache quite quickly due to being too constricted. So riders tend to self regulate stirrups that are too short.
When your stirrups are just right you will feel much more comfortable and secure. You should be able to rise to the trot for quite a period of time without feeling tired (once you are riding fit) and without pain occurring (especially in the outside of the ankle joint).
Your thigh should be at about 45 degrees to upright (for riding ‘on the flat’ i.e. not jumping).
Possibly slightly less (40 degrees) for a very experienced rider.
As a general rule of thumb the bottom of your stirrup irons should be level with your ankle bones when you take your feet out of the stirrups.
Another (even more) general rule of thumb that you can use before you mount is to put your knuckles to the stirrup bar (the metal bar that the stirrup leather threads on to) and with your other hand lift the stirrup iron itself and see if the base of it reaches to your arm pit.
Both of these methods give you a ‘ball park’ figure.
To get a more accurate result try standing in your stirrups while your horse is either stationary or walking, let your heels drop slightly and your seat should be able to clear the pommel (front) of the saddle.
You will need to revaluate when you start to trot.
If you suspect that your stirrups are too long (they often are) experiment with taking them up just one hole at a time and see what a difference it makes to the engagement of your lower leg in particular.
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