Just a shot in the dark here, but it could be caused by a hip flexibility issue.
If your hip joints are less flexible, you aren't able to stretch out your back to reach the longer stem position, so you compensate by bending (rounding) your back. That in turn could put more strain on your lower back and hamstrings.
Can you touch your toes while standing with straight knees?
__________________
Ride, Rest, Repeat. ROUVY:
terrymorse
