T Nation

fiber switching

I’ve heard type IIb(strongest) fast twitch fibers can be converted to type IIa fast twitch fibers, but can it go the other way, IIa to IIb(strongest)? thanks for any info

Yes they can but there’s probably really not much training related you can do to enhance that conversion besides taking a rest and doing absolutely nothing. Even athletes such as sprinters have fiber conversion from IIB to IIA when in intense training cycles. Type IIB’s are the default fiber. When the body isn’t subjected to much stress the % of IIB’s increases. Fred Hatfield even likes to call them “couch potato fibers” as couch potatoes typically have more of them then athletes.

I believe that this is the premise behind the detraining portion of HST.

With resistance training, you’ll see a subtype conversion of IIb to IIa (with several subtypes in between). After a period of detraining, the percentage of IIb fibers increases back to normal.

I imagine that if IIb fibers increase to greater than normal levels after a brief period of detraining (i.e. one week), then you’d theoretically increase your potential for hypertrophy of IIa fibers. Thus the detraining period would hold some water. I don’t think the literature points in this direction, however; I may be mistaken.