Steve, nice review! Funny how the different camps have such debates over differences that are minimal (functionally at least). Recently I learned that type II x were closer to type II a than they were to II b because of the development of higher oxidative properties in IIx fibers, thus their intermediate nature. So in my post I simplified calling IIx fibers IIa fibers. But I guess to clarify, my impression was that the reality of this situation is that you have IIb…IIx…IIa…and type I. And that the shift from IIb over to IIx or IIa is what causes the loss of strength with cardio. Correct or no?
Yes, that is essentially it.
And you are right, most of the arguments are really over semantics. If you are talking IIb fiber typing by ATPase activity, IIb is pretty much IIb. If you are talking transcript sequence homology, that is where the argument gets heated. But hey, there are still arguments about whether or not the actin or myosin provide the locomotive force for muscular contraction.
The practical interpretation would be that the phenotypical characteristics most people attribute to IIb fibers will switch to IIa phenotype with training. IIa's are more oxidative than IIb/x (err here we go again), but produce less force probably because of the ATPase as opposed to the MHC phenotype switch.
With detraining, the muscle will lose the oxidative adaptation and switch to the IIb ATPase phenotype which will result in greater force production with the same PCSA.
Irondoc and Mike…thanks for the comments, fellas, I appreciate the input. Irondoc, I’m sure getting the ol’ bodyfat to those levels would be somewhat advantageous for putting on size, but it’s much easier said than done(mental barriers to overcome). I’ll have to look into Westside.
JB and Steve-O...Whoa! Keep talking, fellas, that's some good stuff, as always, of course. I just want to let you both know that I thoroughly enjoy and reap benefits from each and every post of yours, regardless of whether or not the topic at hand is of interest. That's an interesting bit about detraining and possible conversion to IIB...I wonder if all the strength coaches who incorporate detraining periods into their atheletes' programs are aware of that or if it's done to allow for delayed transformation:-)
Steve McGregor: Thanks for the great info.
This is the first time I read into this
discussion. Glad I didn’t miss it.