Interconversion between Fast and Slow Twitch

GIH

I think I've read conflicting things about this. I know I read somewhere that (and I know its implied in Dr. Coggan and Allen's book) that it is possible to convert fast twitch muscle fibers into slow twitch muscle fibers.

I also read somewhere else (perhaps on this forum or perhaps in a book) that in fact there is no interconversion, but actually the fast twitch cells become more like slow twitch cells.

I was hoping someone could clarify this issue. I mainly interested just because its interesting although perhaps it could affect my training in some way I don't yet forsee. Also how about the reverse direction, can slow twitch cells become fast twitch ones? And one about muscle building, when you build muscle do existing fibers just grow or are new ones synthesized, and is the ratio fixed by genetics or does it vary depending on the training stimulus? Or are these open problems?

Thanks as always for your insights.

john979

I have been thinking about this too recently, given the importance of % Type I muscle fiber to aerobic performance, specifically FTP. I have been looking for a conclusive longitudinal study, but I have not been able to find one.

Dwayne Barry

"Also how about the reverse direction, can slow twitch cells become fast twitch ones?"

Yes fibers can interconvert. In humans there are three main types of fibers Type I (i.e. slow twitch) and Types IIa and IIx (both fast-twitch). In most respects Type IIa are intermediate between Type I and IIx. In general, any type of activity tends to covert fibers toward Type I and anykind of detraining converts toward IIx. Interconversions appear to be more sensitive within the Type II fibers, most well-trained endurance athletes probably have very few IIx fibers. I believe it's a fairly open question as to how much Type II to Type I conversion in well-trained endurance athletes.

"And one about muscle building, when you build muscle do existing fibers just grow or are new ones synthesized, and is the ratio fixed by genetics or does it vary depending on the training stimulus?"

Most evidence indicates that existing fibers get bigger (hypertrophy). I believe that there are a few studies with bodybuilders that have suggested new fibers were generated (hyperplasia) but some of the subjects were known anabolic steroid users. I think it can be said fairly confidently that under most normal situations of increased muscle mass it is due to hypertrophy and if any hyperplasia is occurring it is not significant.

robkit

i recall there is an old article here which ponders whether a specific combination of intervals is able to increase the endurance of fast twitch muscle fibers

http://www.abcc.co.uk/Articles/wright1.html

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