Yeah, psych, if you can land a job doing that since its dominated by NPs, especially in NP independent practice states. But I don't imagine it's impossible by any means. I don't know any psych PAs, but I'm certain they are out there, and can probably do quite well. I would set your sights on that if you have mobility issues, because apart from that role, any work that a physician would have you doing would probably involve not only some occasional leaning and crouching at least, but definately quite a bit of walking is a given. The walking around part between exam rooms and rushing from patient to patient might be more difficult in the long run than actions such as leaning in to listen with your stethoscope, or crouching to look at an appendage. In psyche, it's not unheard of for the patients are brought to you, but that's not a given if you want to be as efficient as you can. The key will be finding the right physician to work for that will accomodate you. When folks are paying you lots of money they make the rules, and can be pretty critical of even little stuff.
The folks who trained for over ten years get to look at films and pathology slides all day, and even in the case of radiologists, any of the interventional radiology folks are doing procedures that involve some contortions. And IF there was a role for a PA, you can be certain it would be to do work that the physicians don't want to do.
So I kind of feel it's psyche or nothing, with even psyche requiring you to move. I shadowed a derm PA, and they do a lot of crouching, leaning, walking, procedures, etc, maybe more than many other medical PAs.