I believe it is the type IIb muscle fibers that are the fast twitch and have to largest capacity for mass. These fibers are generally built with heavy weight such as the 1-8 rep range and with heavy compound lifts.
Type IIa muscle fibers are the fast twitch muscle fibers that are built more from the 8-12 rep range and respond well to isolation exersises. they have a less capacity for hypertrophy.
So if I was an ectomorph and not on steroids would I want to be training the type IIb fibers mainly or the type IIa fibers?
Or if I was an ectomorph on steroids would I want to be training the type IIb fibers or the type IIa fibers primarily?