i'll give a quick muscle fiber physiology lesson because i love this stuff.
there are actually more than just type I/type II or slow/fast twitch fibers. it is more like this:
fast twitch:L IIX-->IIAX-->IIA
slow twich: IC-->I
Type IIX is what is considered to be the type of type II fiber for an individual who does not activate this fiber (a sedentary individual). With training, these fibers an change to a high quality fast twitch fiber, which is type IIA. You cannot change between type II and type I (aka fast and slow) or vice verse. Whatever distribution you genetically have of slow and fast twich fibers is what you are stuck with
The reason why a person with proportionally more fast twitch fibers have a higher rep max even if the 3-5RM is the same is complicated but come down to a simple principle. High force/power exercises (such as a 1RM) require the recruitement of high motor units (fast twitch fibers). Exercises that aren't as demanded don't require their recruitment. In this example, a 1RM is more demanding than a 5RM, so it will recruite more fast twitch fibers to perform than the latter. A person who has more fast twitch fibers to recruit will therefor be able to have a high 1RM than somebody who doesn't.
|