Are you sure...im not doubting you. But this would mean in theory that a distance runner (shot load of slow twich with little or inactive fast twicth) could turn into a sprinter (shit load of fast twitch with little slow twitch)?
Yes.
He would first have to train for conversion into the "middle group" (Type IIa).
BTW, sprinters often have lots of Type IIa fibers, weightlifters and shotputters are more explosive.
That's why you will see world class weightlifters outperforming world class sprinters on 10-25 meter dashes.
A lot can be said regarding muscle fibers, but yes you can convert. But no, I don't think you could practically turn someone with that amount of Type I fibers into a sprinter. But I may be wrong.
What I believe though, is that high-rep training (everything above 15-20 reps or so) is counter-productive in the long run.
You will no doubt see an initial growth, since the Type I fibers will be stimulated to grow to their maximum size.
But that training will also stimulate a conversion of Type IIa fibers into Type I.
-Hedge