I am almost 70 years old and I have MS (with almost no proprioceptive sense.)
I have been with my riding instructor for almost 10 years. She repeatedly told me about my shoulders and she told me she sort of gave up because it looked like I just could not keep my shoulders back, though she usually mentioned it EVERY RIDE.
Then I bought the “Horse Brain, Human Brain” book by Janet L. Jones PhD. In it (page 121) she has a picture of three human muscles that are necessary for good riding. One of these muscles is the teres major muscle “a small muscle at the back of each shoulder just below the armpit” at the BOTTOM of the human shoulder blade. “It opens and steadies your shoulders and upper back as you ride…Isolate your teres major, and you will be able to lift and stabilize your upper back while remaining relaxed.”
I told my riding teacher, showed her where the muscle is, and I started using it while I ride.
My riding teacher now repeatedly PRAISES my back, and she tells me that now I look erect in the saddle.
Today she had me show another of her students where this muscle is and my story about it. The lady tried it and understanding dawned on her face. I told her that I found that using this muscle moves my shoulder blades down and against my back so that my shoulder blades sort of act like a splint that keeps my upper back straight instead of hunched over like I used to be for decades.
And I did not have to pay a single penny for something to wear that really does not straighten my back.
I am not used to hearing my riding teacher praise my back, but she has stopped complaining about me being hunched over and now points me out as an example that it is possible for a little old lady who used to be hunched over to ride a horse with a straight back.