Type I fibers, also called slow-twitch, are used mainly for endurance activities. Type II, or fast-twitch, begin to work when a task utilizes more than 25 percent of your maximum strength. A movement doesn’t have to be “slow” for the slow-twitch fibers to take over; it just has to be an action that doesn’t require much of your fast-twitch strength. And an effort doesn’t have to be “fast” to call your fast-twitch fibers into play.
A personal-record bench press is going to use every possible fast-twitch fiber, even though the bar probably isn’t moving very fast.
Most people are thought to have a more or less equal mix of slow- and fast-twitch fibers. (Elite athletes are obvious exceptions—a gifted marathoner was probably born with more slow- than fast-twitch fibers, just as an Olympic-champion sprinter or NFL running back probably started life with more fast-twitch fibers.) However, the fast-twitch fibers are twice as big as the slow ones, with the potential to get even bigger. Slow-twitch fibers can get bigger, too, although not to the same extent.