Training Based On Muscle Fiber Type: Are You Missing Out?

Ever since I’ve been into lifting, I’ve seen the idea that muscles should be trained differently due to the predominant muscle fiber type in that muscle.

For example, I’ve seen a lot of people say that it’s best to train hamstrings or triceps with low reps because they’re 70% fast-twitch, or that it’s best to train your delts with super high reps because they’re overwhelmingly slow-twitch.

Your nervous system activates muscle fibers based on how much force you need to produce.  It starts with Type 1 muscle fibers, activating more and more until it needs to call upon Type 2 muscle fibers, activating more and more until you eventually can’t produce any more force (this is called Henneman’s Size Principle or the Principle of Orderly Recruitment).  Things can get a little more complicated, especially under fatigue where motor unit cycling1 comes into play, but that’s the basic gist of where this idea comes from:  Type 1 muscle fibers are recruited first and take a long time to fatigue, leading you to think they’d grow the most when exposed to lighter weights for high reps.  Type 2 muscle fibers are recruited more when the muscles are loaded heavier, at least for the first few reps, leading you to think they’d grow the most when exposed to heavier loads for lower reps.2

However, there are three basic problems with this idea, for both practical (1&2) and scientific (3) reasons:

1) Most muscles have a pretty even split of Type 1 and Type 2 muscle fibers on average.