I've read something similar (with zero recall of where). Like other areas, it's likely a matter of degree, though with a pretty wide range. So, given how un-flexible my legs have always been (in spite of years of stretching in soccer and MA), I probably have some bad genetics in the mix, and would never be as flexible as some of my training partners were with little effort. Of course, a "me" that worked more on flexibility would be more flexible, and a "me" that did more bodybuilding back in the day would likely be less flexible.I can't recall where, and it's too early for me to dig around, but I could swear I've read articles/studies on flexibility that suggest it, like many things, is significantly impacted by genetics. Not to say that you can't improve it with effort, but some people are inherently more flexible, and will be able to maintain or increase their flexibility fairly easily. Some will lack flexibility and will need to work very hard for minimal gains.