The importance of the rider’s seat cannot be underestimated but an independent seat is impossible without an established lower leg position so I would like to take a moment to examine the importance of the position of the rider’s leg.
A benchmark for the position of the leg would be to remove the horse and stand the rider on the ground at any given moment. Providing the horse to not accelerating (e.g. jumping) or decelerating (e.g. stopping in front of a ditch), the rider should be balanced with their centre of gravity over the middle of their foot. This should be the case whether the rider is in an upright dressage position or a forward racing position so as the rider is not using the horse to maintain their balance.
Now I am really only qualified to comment on the position of the lower in disciplines such as dressage, jumping and eventing – the purpose of the lower leg and its influence on the balance and seat of the rider in other disciplines such as campdrafting, cutting , barrel racing etc. could be quite different.
In my area of expertise, the most common fault with the leg is that riders grip too much with the knee and thighs, allowing the lower leg to come off the rib cage and often float forward to the girth. For the base of the foot to be under the rider’s centre of gravity, the riders toe generally needs to be in line with the girth. This will often bring the mid-calf into contact with the horse’s ribs. When the horse is not used to the rider’s leg being in this position, they will often rush forward prompting a nervous rider to take the leg off or push it forward again but in doing so, losing the opportunity to engage the horse`s hindquarters. Engagement and impulsion require that the rider’s lower leg be on the rib cage and acceptance means that the horse doesn’t rush forward. So the position of the rider’s leg is absolutely vital for the effectiveness of engaging the horse’s hindquarters.
Secondly, for the rider to be most effective and in an independent seat, the rider’s upper body must be balanced over a stable lower leg. If the lower leg is not stable with the weight of the rider pushing down through the heel, the upper body tends to move more to maintain the rider’s balance, particularly in rising trot where many riders have to lean forward.
I always remember an old jumping instructor of mine forcing me to “post” (stand in the stirrups) in walk, trot and canter until I found balance of my upper body through a stable lower leg. I didn`t appreciate the pain at the time and only under stood the importance of the lesson much later in my riding career but it did the job.
Other than “posting”, the best exercise I know to align the lower leg is best practiced in halt before the walk, trot and eventually canter. Take both feet out of your stirrups and make your legs as long as possible by stretching your toes towards the ground. Make sure to relax your thigh and groin. Then, with the toes still pointing down, close the calf gently against the horse’s rib cage. To do this, you knee may need to roll out a little, relaxing your groin a little further. Finally bring the toe up so that the heel is the lowest point. If safe to do so, have a walk around to get the feel for this new leg position.
When riders put their feet back in their stirrups following this exercise, they often comment that the stirrups feel too short. This is an ideal way to get more weight in your heel and a prerequisite to sitting straight and tall.
If you notice the knees griping again when you move up into trot and canter, come back to walk and repeat the leg lengthening exercise. Get help to notice when your leg starts to slip forward or your knee draws up again then give yourself time to properly establish this new position. The benefits of the exercise will generally be better balance, better engagement and more softness from your horse as you relax your seat.