Here's my story:
I injured my right shoulder in a catastrophic way in a 2002 bicycle crash. I had to baby it for more than a year, and it still hurts. I began using 7 1/2' 4wt rods, and that made my shoulder ache less. Around 2004, I bought a fast action 5wt, and went for distance fishing for smallmouth on a large river. The pain returned, but wasn't too bad. Fast foward a few more years, and I began using a fast action 9' 5wt for everything. I didn't notice any shoulder pain. A year later, I began using my 8 1/2' 4wt for nearly all of my fishing: lakes, rivers, streams; big fish, little fish; small flies and big flies. My longer fast action rods was little use for over a year, but then I decided to flinging a Clouser Minnow into the wind on the Delaware River was too much. My fast 9' 5wt returned came out of the closet, and I decided I was going to toss that sucker as far as possible. The result was almost immediate shoulder pain that would last well into the next couple of days. I went back to the smaller rod, but quickly realized that I was making 3-4 false casts to get close to what the fast 9 foot rod could do in 1-2 casts. I also realized that my shoulder was probably ripe for pain because of all the over-casting I had been doing with the 4wt (and CLOUSERS!!!), and it was time to start working smarter.
This year I've used a combination of both the fast 9' 5wt and the slower 8 1/2' 4wt, trying to match the rod to the distance, fly, and fish. I've had zero shoulder pain, but my elbows hurt a LOT lately. I could jump to the conclusion that casting down to the backing is putting too much pressure on my elbows (the left elbow gets used for hauling), but I sleep with my elbows bent bent over my head....think "Duck and Cover"...LOL! It's probably the sleeping position that's causing my elbows to hurt, and is exacerbated by trying to hit the other side of the lake with a 5wt rod. I was standing on top of the railing along a fishing pier last week, trying to gain an edge in keeping my back cast up.
Today's fishing was done at close distances, and with light flies, and my elbows still hurt.
I would conclude that when people get a fast rod, they're going to test the rod's limits. It's just as likely that they'll find their own limits, and that might anifest itself in some sort of pain or injury. I decided a couple years ago that I'd rather get closer to the fish, mostly because my 8 1/2 4wt was too much work to throw far. That all goes out the window when I'm on a lake, and I have to realize that I'm not getting any younger. My Google search for fly fishing and sore elbows revealed that I'm not alone, and many people say that distance casting gave them tennis elbow. More than one website had comments about hand grip and elbow pain, with an index finger on top grip causing the tendons to pinch the ulnar nerve (or something to the effect).
Guess who casts with his index finger fully extended.
When I first got my fast 9' 5wt, I was a little underwhelmed at its performance. After working with it for a while, I realized that I was putting too much effort into the cast, and the rod didn't like that. Once I realized how to work with the rod, I then decided that all that 70-90 foot casting stuff wasn't just wishful thinking, but be careful how you do it, and the suggestions about warming up make perfect sense. I never do, however...??? I'm back on my bicycle again, and that puts a lot of pressure on my elbows as well, so I don't know what to do. I'm alright with backing off a bit, but I don't do well with stopping. I had to stop riding my bike for close to 5 years because of the 2002 accident, and I gained an unbelievable amount of weight. That weight is 90% gone, and that a relief!
While googling elbows and fly fishing, the elbow supports that Rip mentioned were cited quite a bit. I'll have to look into that,
if only to keep my arm from bending so much when I sleep. There's supposed to be a sleeping brace for tennis elbow, but that sounds too uncomfortable. I'd much rather wait until I can't lift a coffee cup before doing something sensible.
P.S. I also don't like doctors, and never even had an x-ray for the 2002 bicycle crash. It was over a year before I could lift my right arm to turn the car radio on!
My wife loves to go see doctors, so I dunno....