The last three days I have experienced tightness in my left thigh and left lower back about 6 miles into my daily lunch time 17+ mile ride. The leg tightness seems to go away after a few miles, but the back stays tight until I stop riding. It doesn't bother me later in the afternoon after my ride.
Nothing seems to have changed recently, though I did start riding this bike only 3 weeks ago. I rode a different one for the last 6 months. I believe the crank lengths are the same. and the pedals are, theoretically identical - Shimano 105's, though I am wearing different shoes with this bike.