I know there's gotta be a such thing as genetics determining which type of fiber predominates in any given individual. I've seen the evidence in my sport of inline speed skating. I've been observing that the really fast, powerful sprinters tend to have thicker, more muscular legs (myself included) and the skaters who seem to go forever and ever, with tons of endurance, yet do not have a lot of explosive power for sprinting, tend to have thinner legs, less muscle mass on their legs in general. And I've had at least one experienced trainer tell me I have a genetic propensity to develop short twitch muscle fibers, which are the thicker ones. Definitely an interesting subject. I'd be interested to see any studies done.