Originally posted by Snake_Eyes
Of course. No one was arguing to the contrary.
However, there is a certain subtlety in regards to the fast-twitch fibers-- there's a rather wide spectrum of activation thresholds, which is related to the force-producing and fatiguability of the fibers.
The highest-threshold fibers (really motor units or MU's) are those that produce the most force for the briefest intervals-- these are highly glycolytic in nature. They tire very easily, and thusly are most subject to growth.
A step down from these are the MU's with a slightly lower threshold-- these are hybrid fibers (glycolytic-oxidative) and oxidative fibers with moderate force-producing capacity.
Interestingly, the fiber classification, which is based on Myosin Heavy Chain (MHC) content, is entirely independent of the fiber's metabolic capacity and its neural recruitment. When you hear of fiber type conversion, this is most commonly what is being referred to.
There's also some controversy over the FT fiber classifications. You'll hear IIa and IIb tossed around pretty regularly; however, in most trained athletes, the IIb (IIx depending on who you ask) are all but gone. Its speculated that these fibers convert to IIa based on the fact that they contract *too* quickly to be of use in any loaded movement.
When training, I really avoid any attempt to train any single "fiber type" because its ultimately hopeless due to all the conflicting factors. I train to increase either A) force output (maximal strength, the high-end MU's) B) rate of force development (how many fibers are recruited and how quickly), or C) metabolic endurance (which is as it sounds- training to improve endurance in the appropriate bioenergetic pathway). Of course, I'm also a strength athlete.
Still, if you're concerned with maximal hypertrophy, these factors still apply, just in different ratios. The bodybuilder should focus on A and C; B is more of a "sport-specific" thing.