I want to talk about legs, an element of the riding position that is commonly misunderstood and overlooked. There are many riders that let their feet cramp up inside their boots or let the stirrup dangle underneath their feet when they are really trying hard to get a horse forward by kicking and pushing, usually seen in sitting trot or canter. But just think for a second, if there is no weight pushing down in the stirrups, where is the rider getting their stability and balance from?
The answer would be from gripping with their knees and thighs and tensing their seat, or balancing on their hands either leaning forward and blocking the horse with their hands drawn back to their body or sitting behind the movement and "water-skiing" off the horses mouth. Nobody wants to do any of that when they ride, so I'm just going to give a few tips to help understand what it really means to "push down into your stirrups" and be truly balanced on a horse.
Let's start by simply standing up straight with feet together, or what we call "mountain pose" (tadasana) in yoga. Most people would wobble and fall with a little poking and prodding, or with a strong gust of wind, because we are more than likely keeping our balance by holding tension somewhere in the body that isn't resistant to instability.
It sounds like the easiest thing in the world but actually it is quite difficult to stay still and balanced standing with feet together! But this is why it's called the mountain pose - mountains are tall, strong, with a super strong wide base that makes them solid and resistant to anything that mother nature throws at them, except earthquakes, because that disrupts their strong base and they become unstable and collapse.
Getting the picture? So when you stand, really push all four corners of your feet down into the floor and feel yourself being grounded. Feel your body become free of tension, relax your shoulders, and just push everything down into your feet. Feels stronger right?
Now think of that old training tip where if the horse were to be removed from underneath us, we should be able to stay upright and not fall forwards or backwards. Thinking of what we had to do in mountain pose, where would this balance come from? The feet!!
Now you might be thinking "yes but I'm not standing up straight on a horse, my legs are bent and I have to move my body with it", and this is where leg muscles come in. Now, the anatomy of the hip-knee-ankle alignment is quite complicated but the main thing you need to know about is muscular balance of the inner and outer thighs for correct "knee over toe" alignment.
To feel the balls of your feet pressing into the stirrup with a correct leg position, you need to find out where your weaknesses in your legs are. This can be done by single-leg balancing poses like warrior III, half moon pose and tree pose (google it if you don't know them!!). While balancing on one leg, pull your kneecap up and activate your quad. Then you will be able to feel where you wobble or feel most pressure, whether it be your in your feet with struggling to keep a neutral arch, or in your hip which would move left and right trying to catch your balance.
If you struggle to keep stability in your feet then you need to strength your lower leg. Try holding a squat position or be in chair pose (squat with legs and feet together and arms overhead... again, google it if you don't know!) and come up onto your toes and keep your balance there. That should help you feel through your feet more and strengthen all the tiny muscles inside your ankles and feet that help create a stable base.
If your hips wobble, or your knee collapses in or splays out (this can certainly be applied to the leg position in the saddle, whether your grip in with your knees or let your knees open and flap) then you need to work on strengthening your inner and outer quad muscles. When you ride if your toes stick out and knees are a bit off the saddle, you probably have stronger outer thighs than inner thighs. You could be using your outer thigh strength and glute muscles to push the horse forward, yet that would take you back off your seat bones and result in driving into the horse's back. More rarely for dressage riders is knees gripping with weak glutes and strong inner quads, which results in perching in the saddle, restricting the horse's forward movement.
In the single leg poses you need to focus your mind on which ever muscle group is weaker for you and try and create stability by activating it with your kneecap lifted. However this must be done with correct alignment with your knee over your second toe. This can be worked on by doing poses involving a lunge like warrior II. In the lunge the front leg that is bent shouldn't collapse in or point out, your knee should be at a 90 degree angle and if you were to look in a mirror front-on the knee should be pointing in the same direction as the feet and level with the second toe. Again, to find this spot you may need to activate your inner or outer thigh as necessary.
Translating all this into being on a horse (finally!), when our knee is bent it needs to be pointing in the same direction as your feet, which should be feeling grounded into the stirrup with the ball of your foot spreading across all four corners. Forget about pushing your heels down, that happens when your hips are relaxed and the leg drops down naturally in alignment. If you force the heel down it creates tension everywhere else. DONT DO IT! For me, I naturally ride on the outside of my foot which deactivates my inner quad, so I have to focus on pressing through my big toe and the inside of the ball of my foot a bit more to make the muscles in my leg work evenly.
When your leg is in a correct alignment over your stirrups, you can feel an entire, full contact with the saddle that makes you so much more effective with your legs. There is no driving into the horse's back because you're balanced on your seat bones, a result of using your feet as a foundation that keeps you rooted in position. Your entire upper body now just has to stack straight on top of your pelvis with a loose lower back to allow movement and a strong core to keep upper body stability; feel your chest lift, shoulders relax down, and follow the horse's forward movement with hands out in front of you holding an even, consistent and elastic contact, which again stems from the balance you have in the saddle.
It's that simple ;)