On big guitars and (past) shoulder pain

Since high school I've had on-and-off trouble with my right shoulder. I attribute it to playing too much whiffle ball as a kid. Trying to throw a plastic sphere that weighs about an ounce as hard as you can for hours on end can't be good for one's rotator cuff. Baseball as a teenager, volleyball in college, and softball in grad school didn't help things. I remember being tested by a doctor when I was in my early 20s, and he showed me that my left (non-dominant) shoulder was significantly stronger than my right.

One place this began to manifest itself was in playing guitar; I found I'd get an achy shoulder if I played a big guitar (e.g., a dreadnought or jumbo) for any prolonged period of time. This led me down the path of a few smaller bodied guitars, like the Taylor koa 414 grand auditorium, Larrivée LS-05, Larrivée parlour, and Collings OM2HA (all now departed) that were part of that phase. 

The other day I realized that, amazingly, this problem seems to have worked itself out. I've been playing bigger guitars almost exclusively for the last few years without any shoulder pain or issues at all. How could that be?

I think there are two factors: 

1. A few years ago I retired from playing softball (where I had always been a left-side infielder); I haven't thrown a ball since, and also haven't had shoulder pain. A coincidence?

2. I've playing guitar standing up (with a strap) much more since I started participating in bluegrass jams. No more being slumped over the guitar while seated, which puts stress on one's shoulder. I can play for hours at a jam with no shoulder trouble at all. It's amazing what a little bit of good posture will do! I'll get a sore back from standing in one place that long, but that's another problem (that can alleviated with stretching and movement).

Although I recently have jumped back into the small guitar world with a Martin 00-DB, that was not a reaction to shoulder pain (like with past small guitar purchases). I'm happy to learn that a 30-something year old body can still heal (my experience with going over the handlebars of my bicycle at 40mph a few years ago, with hardly a bump or bruise to show for it, also was encouraging as a sign of prolonged youthfulness, but that's another story).