Originally Posted by
stilltooslow
Could be a lot of things:
Tilt of the seat
Fore/aft setting of seat
Stem length
Stem height
You're still a beginner and certain muscles have yet to be acclimated
There's no definite formula for success, so it's best to make small changes one at a time. Be patient and persistent....you'll eventually dial it in.
This would be my vote. When I first started my lower back would do same thing then as time wnet by I no longer have lower back pain. You have to build and train those muscles to get use to the riding position.