When I first started to strength train in high school, one of the first things I remember reading in the weight room was this giant chart that highlighted areas of so-called fast-twitch and slow-twitch muscle fibers in the body. Lots of "the boys," that is, the friends that I trained with, took this apparent knowledge to heart, and for different looks each of us wanted to achieve, we designed different workouts based on whether they would fatigue our "slow-twitch" or "fast-twitch" muscles. For example, those of us who wanted a lithe look of the runner did low-weight, high repetition workouts so that we might fatigue the "slow-twitch muscles", and those who wanted a more bulky, cut look of a sprinter performed more high-weight, short-repetition workouts to fatigue the "fast-twitch" muscles.* This information was particularly relevant for those few of my friends who wanted to try out for various positions on the football team, which as everyone knows weight on stamina, endurance, and power differently.
Unfortunately, the study of fitness and nutrition as a scientific discipline is replete with misinformation and pseudoscience, to the point that one can't really Google for even basic information without being afraid that one is being marketed to or upsold in some fashion. Still, I distinctly remember reading later somewhere that either "fast-twitch" and "slow-twitch" muscles were pseudoscience or, at the very least, outdated terms, much in the same way "alpha dog" is an outdated term for those who study dominance hierarchies among wolves, but a term which still persists in the common parlance. Alas, I can't remember exactly, and lots of different websites in my search results seem to take "fast-twitch" and "slow-twitch" seriously.
So here are my canonical questions:
- Are "fast-twitch" and "slow-twitch" actual scientific terms? If they were so, are they so now?
If they are indeed scientific, does knowing about them help one design better workouts? E.g.,
- Does one have a natural endowment or ratio of "fast-twitch" to "slow-twitch"? Can it be determined practically?
- If not the above, is it possible to convert one type of muscle to another in some way?
- Do certain muscles contain more of one muscle fiber than other muscles? Is that relevant to designing workouts?
- Do members of certain athletic professions (sprinters, swimmers) possess different ratios of fast-twitch to slow-twitch than others?
(*) I am well aware that many strength trainers recommend high-weight, low repetition workouts regardless of which look one wants to achieve. That is not what I'm asking. I'm asking whether the rationale that we came up with was sound.