Q: There are different seat positions, from classical seat, to 2-point, to light seat, to the dreaded chair seat... Can you explain which is best for form and function?
A: First of all, there is not one ‘correct seat’, just different seats appropriate for different situations.
Think of a seat appropriate for riding a thoroughbred race horse, a seat for doing a slide stop and a seat for being able to go from an extended trot to piaffe in 2 strides. These are not the same seats.
Can you picture a jockey racing in a classical dressage seat, or jumping a 5 foot fence from the position you do a slide stop in? Or doing a piaffe in 2-point? Each seat is totally correct... or totally wrong... depending on when you are using it!
What they all have in common is that they are a position of perfect human balance in relationship to the horse's balance. The jockey is in balance... on the galloping race horse, but would likely go flying off the front of the horse if it did a slide stop!
So the question becomes not 'what is the correct seat?' but instead: 'What will you be doing?' Then find the place of human balance on top of that movement, and do it well! Any of the above positions, if they are not in balance, will be a hinderance to the horse.
In my book, on pages 82-89 I talk about characteristics of the athletic balance for a seat that will serve you through the basics of dressage, and some exercises to help you find it. This can be fine tuned and developed to serve you through the highest levels. But the range of seats I may use on a regular basis are: the stretching posture: I exaggerate the softening of my lower back to help the horse do the same as he stretches. 2-point: I do this if I am out for a hand gallop and want to get out of the horse's way and stay with his forward center of gravity (horse's natural center of gravity is basically between his shoulder blades). 'Balance point' seat if I am doing a passenger lesson (especially if I am bareback) to keep from gripping which will make it harder for me to be loose enough to stay with my horse. And of course, the seat for riding a horse engaged and with a round back.
I save my finished collected posture for when I am riding that. I definitely have a ... 'softer' posture (for lack of a better word) on the young horses who are not able to collect and engage yet. I am balanced on them, for sure, but not in my same position as I am on the advanced horse when I am schooling collected transitions. I want that posture to be meaningful.
I do my best to mirror and be the embodiment of the best version of what my horse is capable of. For example, until my horse can sit and carry himself, I don't make him carry me sitting heavy on him. There are plenty of horses that I find difficult to sit before they are balanced and have learned to carry their backs up under my weight... and I would rather do an excellent posting trot, or 'half seat' than struggle through a difficult attempt at a classical sitting dressage seat in this moment. I do make sure I am balanced, athletic and moving with them, no matter what.
The ultimate seat for dressage has a supple lower back, engaged abdominal muscles, open hips, a long leg that hangs under the center of the rider, allowing the hips to swing with the horse, an upper body that has the ability to balance dynamically over the pelvis, and a shoulder joint that allows the body to move, while keeping the hands floating still relative to the horse's mouth.
As far as how much weight in the seat vs legs? I have heard lots of different ratios for percentage of weight, but that doesn't usually help me when I am actually riding (my brain just doesn't work like that). I think about what I want from my horse... But in a finished dressage seat in an ideal circumstance, I would say that the amount of weight in my stirrups is about the same amount of weight that my leg would register on a scale if I cut it off and weighed it (Sorry for the gruesome image). I want my leg to hang down from an open relaxed hip. If I push more weight into my legs, my seat will pop up. If I grip or squeeze with my legs, my legs will float up and by seat will squish down on the horse's back. When I am in the best moment it feels like gravity is taking care of the weight in my legs. If you aren't getting that sensation, there is something missing in how your horse is moving, or your balance, or you have tensions somewhere.
The best dressage seat is the one where the horse has the best possibility to engage, round their back and to feel like the human stays in the middle, so he can move equally to lengthen, shorten move sideways, etc as well as swing his gaits freely.
The key is to have the athletic dynamic, and not the static picture of it. The 'photo' doesn't matter if the 'video' doesn't look good! In other words: 'pretty is as pretty does'!
Achieving this athletic position is a result of athletic freedom, range of motion and consistent discipline. Balance is the key. When you are riding, ask yourself: 'If my horse disappeared right now, would I land on my butt, nose or feet?" You want to always be able to land on your feet.
Balance will always look, feel and function beautifully!