Whether you have more 'slow-twitch' or 'fast-twitch' muscle fibers before training doesn’t seem to affect how much muscle you can build with lifting weights.

When adults lift heavy weights, their fast-twitch muscle fibers grow much more than their slow-twitch ones, based on several studies.

For young people who have not trained before, lifting light weights or heavy weights to the point of muscle fatigue leads to similar amounts of growth in the major thigh muscle fibers, but the results vary widely between individuals, so it’s unclear which approach is better.

Lifting heavy weights might make your fast-twitch muscle fibers grow bigger than your slow-twitch ones, according to some new research.

Some people get much bigger and stronger from weight training than others—even when they do the exact same workouts—because everyone’s body responds differently to exercise.

In young, untrained people, lifting light weights and lifting heavy weights produce similar amounts of growth in both slow-twitch and fast-twitch muscle fibers in the quadriceps, based on measured changes in fiber size.