I used to think my lower back pain was caused by my bike fit. I started stretching my hamstrings, doing ALOT of crunches, added deadlifts to my workout, and started doing a medicine ball workout twice per week. I no longer get that sore lower back and I can ride for a much longer amount of time before needing to stop to stretch. I also started using my massage chair every night before I go to bed...gotta keep those muscles lose.