There is no data anywhere on fiber types switching. A type I fiber will always be type I. Same with all of the type II’s. They can take on characteristics of one an other due to training and activity stimulis. Also, type I fiber characteristics and cross sectional area aren’t just a result of ‘years of endurance training.’ All recovery is aerobic. Even when you are lifting heavy, you are developing type I fibers (size and function in non-type I fibers). Yes innervation has some to do with it. Human beings are no where near 50/50 when it comes to distribution. The highest distributions are in olympic sprinters and sedentary people in wheelchairs and those barely hit 30% in RARE cases.[/quote]
I’m still not sure we’re on the same page here. IIx is the default fiber type and produces the most force, but it the last to be recruited. It shifts to IIa with training, so yes, I get what you’re saying with all training being aerobic. For clarification, you’re saying that sprinters and sedentary people are 30% type I? From the literature I’ve read, and this is multiple papers confirming the same data, elite sprinters are around 80/20 fast-twitch to slow-twitch. Marathoners are 30/70, and Olympic lifters and powerlifters alike are around 50/50. The discrepancy could be how you’re defining FT and ST, because new techniques allow us to see transitional fibers as I think you’ve described. But if you’re talking strictly FT vs ST, what I’m saying stands.
Again, from what I’ve read and have been taught, the motor nerve determines fiber type. I’ll dig through Blackboard and see if I can find the studies that back that up.[/quote]
There is way more too it than just innervation. Parapalegics (spell check) have the highest cross sectional area of type II fibers when compared to other populations… including elite athletes.
In terms of distribution, no one on earth has every been anywhere near 80% for fast twitch fibers. Maybe if you are talking about cross sectional area, 80% could be possible in extremely elite athletes, with perfect diets, and superior genetics.
Fiber types can’t change. They can take on characteristics of other fibers but no amount of training will actually shift a type I to be a type II. The only way the distribution of fiber types could change is if new fibers are made. This means hyperplasia would have to happen. Even thought, personally, I think it can happen, it’s never been shown to happen in humans.