Examples of common positional mistakes - Stirrups too short
Your legs become too far back making it difficult to balance in your seat
It also makes it more difficult to give the correct leg aids and for your horse to work around your inside leg.
Short stirrups causes upper body instability due to your seat being unbalanced when riding flatwork/dressage.
In jumping they should be shorter than in dressage but not so short it is easy to be tipped out.
Short stirrups causes tension through your leg therefore this continues through your hips and into your back.
This tension then follows through to the horse preventing a relaxed rhythmic gait.
To correct this you should sit tall in the centre of the saddle, keep your weight even in your stirrups and relax through your shoulders.
Try taking your feet out of your stirrups and just relax your leg down
There should be an invisible line from your shoulders through to your hips and heels.
A rough guide to where your stirrups should be is at your ankle bone for flatwork.
Your stirrups should be long enough to be able to do this but not so long you can’t put the weight in your stirrups. They must be comfortable.
Don’t grip with your knees and keep yourself relaxed in your thigh and through your pelvis