Is it possible to change muscle fiber? For example, if a person has mostly slow-twitch is it possible for them to change their muscle fiber to fast-twich by training with heavy weight and low reps?
I dont believe its possible to change the amount of fibres you have, however, you can make the ones you have more effective. For example, an endurance training has the effect of increasing the capacity of all the muscle fibre types for oxidative metabolism, and this may lead to the type 2 fibers of the well trained individual having a higher oxidative capacity than that of the type 1 fibers of a sedentary individual.
The short answer is no. You can’t change from slow twitch to fast twitch. There is some evidence to suggest you MIGHT be able to go the other way…but much of this research has been on animals.
Echoing what everyone else has just said, no, you can’t grow more fibers. They can increase in density, and through training specific to more fast twitch fibers you could increase the fiber types utilized from slow to fast.
Start doing more low rep, heavy weights. Also keeping the sets short and explosive (i.e. olympic lifts) will stimulate more of the type II b/a types.
It is like asking how you can make your penis bigger…you are born with what you have, so make the most of it.