Do You Run Better in the Evening?

This study (Diurnal Variation of Hamstring and Lumbar Flexibility) published in the Journal of Strength & Conditioning Research, found that hamstring and low back flexibility increased throughout the day in men and women who were tested every 2 hours for 12 hours.

MY COMMENT: I think they should have tested only once per day at the different times. If you test every 2 hours on one day, how can you be sure that previous testing sessions didn't influence the outcome of subsequent tests?

I don't know that improved static hamstring flexibility has ever been shown conclusively to improve run performance, but it makes sense that a few degrees of added range of motion (active flexibility) would help.