I’ve had this issue for a while, and thought I had figured it out this year by doing more trunk/core work, but yesterday I saw it come back doing my first intensity on the MTB. My lower back tends to fatigue very often under high intensity, after 1.5 hour races, I could be sore for a few days, and have trouble standing prior to finishing the race.
Yesterday I noticed on a hard 2 min climb I had the same feeling, and it made me wonder about my setup. It seems like it has to do with my back muscles holding my body down when seated.
I have a feeling part of this could come down to fit, but im not sure what direction to start pushing things to experiment. My setup is reasonably aggressive, with a bit of drop to the bars, but that seems pretty standard for a 6’4" rider. Maybe I just need to put even more focus on the strength work?
I dont tend to get this on my cross bike for what its worth.