This belief came from a couple of older studies that showed a slightly greater proportion of type II muscle fibers in the hamstrings compared to other muscles. (strengthandconditioningresearch.com)
I have ordered it and will update this post after my review, but I can also add older studies from my own literature review that demonstrate the same. (believemidwiferyservices.com)
Older studies in humans have also demonstrated caffeine's effect on fertility. (fertilityauthority.com)