Men naturally have bigger fast-twitch muscle fibers than women, even if neither exercise, showing a clear biological difference between sexes.

One study with women showed a much bigger muscle growth difference than the others — all of which were only with men — suggesting sex might matter, but we need more studies to know for sure.

When men and women do strength training, their fast-twitch muscle fibers grow about the same amount—so sex doesn’t really make a difference in how much these muscles get bigger.

When men and women do strength training, their big, powerful muscle fibers grow about the same amount—neither sex consistently gains more muscle than the other.

In young men who don't exercise, fast-twitch muscle fibers are more tightly packed with muscle-building parts than slow-twitch fibers. When they start lifting weights, this difference stays the same, but all muscle fibers get bigger.

Young men tend to have faster and bigger type I muscle fibers than young women, but this difference disappears in older adults and isn't seen in other fiber types — so the muscle differences between men and women depend on age and fiber type.