A lot of focus is given to the riders seat and their upper body position but a foundation stone of a good seat is a stable lower leg position.
A benchmark for the lower leg position would be to remove the horse at any given moment to see how balanced the rider would stand on their own. Providing the horse to not rapidly accelerating (eg. jumping) or decelerating (eg. 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.
So firstly, the lower leg should be as stable as possible. 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 rider’s 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 again.
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 not, the rider is using the stability of the horse’s back to remain balanced. If the rider’s upper body tilts backwards, the rider puts unnecessary weight on the horse’s back. If the unstable rider tips slightly forward, they often stop themself with the lower leg, but in doing so often send the horse forward accidently.
The best exercise I know to align the lower leg is best practiced in halt before attempting it in 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 tight and groin to allow this to happen. Then, with the toes still pointing down, close the calf gently back against the horse’s rib cage by bending your knee. To do this, you need to relax your groin and open your knee a little. 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. Come back to this exercise from time to time to reestablish the lower leg position.
An established lower leg position is not only vital for staying on your horse and engaging the hindquarters, it is a prerequisite for a stable, independent seat and higher movements in all disciplines.