MTB lower back fatigue under high intensity

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.

What core work do you do? @Jonathan talks about this in his latest podcast themtbpodcast.

Mtb is very full body intensive vs road. CX does not seem as full body as mtb does to me.

Build the core. Fit might be an issue but core IS a part.

I do everything on my MTB, to include all the TR workouts, all outside rides and did 10 Cat 2 XCO races in 2018. I also did an MTB 100 that took 9 hours. I can’t remember a single time when I ever noticed my lower back. I also have an aggressive cockpit with a -7 degree stem slammed with flat bars. At 6’1” I’m pretty leaned over/aggressive on my large bike frame.

I think it’s a bike fit expecially since you don’t get it on your cross bike. Analyze your cross bike, take measurements and see what the big differences are between your MTB. Try to mimic that position the best you can. To experiment, change one thing at a time and go for a few rides/workouts and see if you notice a difference. If not, switch it back to how it was and try another change. It may be an annoying process, but at least you’re working towards a solution.

You can start by making adjustments that don’t cost money (spacers under the stem, roll of the handlebar, move grips in as if the bars were shorter, saddle height and tilt).

You weren’t kidding, very helpful rec, thanks, sounds very familiar. Thinking I just need to do more.

Thats probably a good idea, especially setback and reach. I might just put both bikes on the trainer with a camera setup and take a look side by side to.