I know, there are way too many of these! But there is a twist here that I couldn't find with a search.
I have lower back pain while climbing, but only when I am doing a race-pace effort on a long climb. Usually around mile eight or so is when it starts. It's a dull ache in my lower back that saps all of my power, and it won't go away unless I get off the bike, stretch, and back off to an easy pace — obviously not an option in a race!
I do core workouts, have been professionally fitted, have been riding for many years and racing for several, and climb quite a bit, so I'm not expecting my body to do something new. I just did the Death Ride with over 15,000 feet of climbing and had no back pain at all, so none of the usual suspects seem to apply. Something's happening when I'm going all out, but I can't figure out what. My SO said that my right leg was more knee-in when this was happening, but that my legs are normally symmetrical. Whether that's a cause or symptom, I just don't know. Anyone have any suggestions?