@aregard are you actually “tipping or leaning forward” with your upper body (as in a hunter rider doing a half seat or even a light jumping seat), or are your shoulders going forward into a slouch where you collapse your midsection?
These are two separate issues.
(As an eventer, I feel ya! :lol:)
In the first case, this probably means that your hip is locked and has too much angle; leaning back is not the way to fix this since you need to soften and open your hip angles, “kneel down” on the horse, and allow your thighs to drape while keeping your belly button up - which will allow your pelvis to sit evenly so you can softly “plug in.”
Simply leaning back (in an exaggerated way) will result in you locking and compressing your lumbar spine/small of your back, and possibly also tightening your hip and pushing your seatbones down into the horse.
If you slouch (a problem that I struggle with), the fix is to open your ribcage, spread your collarbones and lift your sternum, allowing the elbows to drop back and down and the shoulder blades to softly slide down your back. You should imagine a force coming from back to front (like the wind in a sail), which will help straighten your posture without forcing you into rigidly “sitting back” which can result in everything locking up. Think of lifting your pubic bone while spreading your butt cheeks - but don’t force, this usually puts the pelvis in the correct position. Make sure your stirrups are the right length; too short and you will be forced into a chair seat, too long and you will be forced into a fork seat.
You don’t want to arch your low back in an effort to “sit up straight” in the saddle. Good posture should come from stretching the legs back and down and centering your pelvis, keeping the hips open and engaging your CORE without locking up your back. You should be absorbing the motion of the horse with your hips and knees, not your lower back.
Obviously the inside of the thigh and calf should be contacting the horse, not the back of the thigh and calf…
The upper body can then be lifted and carried (stack the blocks) with the contact originating from the elbows as a result of a supple, adhesive seat that is strong and balanced and centered.
The reason you sometimes see ULRs leaning back (to sit big trots, etc.) is because most of them (unlike us “mere mortals”) already have the above ^^^ qualities, and CORES OF STEEL. They are able to change their upper body posture without compromising their suppleness and seat connection, and of course they don’t go around leaning back constantly.
I have been working on my core strength daily for about 6 or 7 years (it not only helps me ride better but it protects my low back which is pretty arthritic at 62), and I am arguably riding better than I did 20 years ago as a result; it makes a HUGE difference.
I train and coach at the lower levels, and have been studying horse and rider biomechanics for awhile - working on a correct seat and developing a strong core helps riders tremendously. And anyone can do it! Core exercises don’t take much time and are truly transformational.
This is “lazy instruction” (IMHO), and overly simplistic. Your trainer should find better and more detailed ways of explaining and conveying postural corrections so that your entire position improves; it’s not all about the upper body being perpendicular to the ground.
I hope this makes sense, and studying rider biomechanics is incredibly beneficial (and will help your horse as well!)