Here's what Dr. Squat said:
"It was once believed that there was only 2 types of fast twitch fibers (IIA and IIB), recently however a more complete continuum has been established and it looks like this:
IIC -- IIAC -- IIA -- IIAb -- IIAB -- IIaB -- IIB
The more on the right of the spectrum a fiber is the higher is it's speed of contraction and contractile power and the lower it it's endurance.
The fibers on the left of the spectrum are less powerful but more enduring. Now, with strength training all the fibers have a tendency to "move to the right" (except the IIB, which are destroyed with prolonged training), that is they increase their speed and strength of contraction but become less enduring. With endurance training the opposite happens.
Of course these changes happen after prolonged systematic training, it is not an overnight thing.
There is a possibility that with a lot of aerobic training that IIC fibers can be converted into IC fibers (ST fibers spectrum IA -- IC), however IIC and IC fibers do seem to be very similar in their properties so it is not sure if that convertion happen.
POINT: In the weight room, go with the type II fibers! Always!"
I misspoke when I said "With proper training, the ratio of fast twitch should increase over time" but the effect is the same.
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