I've had some back problems, that were mostly taken away with consistent ab workouts. But to help even further and prepare for a coast to coast race, I decided to get a bike fit. The fitter noticed that my hips were rocking a little too much and recommended a shorter crank. I'm not the tallest at 5'9" and according to him, the crank length I had was probably too long anyway. I settled on 165's and the rocking lessened. He also cited various articles and research that concluded shortening the crank did not decrease performance. After almost 2 years since the change, I am happy with the results.