When the whole leg got bigger from training, it was because the fast-twitch muscle fibers got larger—but only if the men did the upward lifting motion. If they did the slow lowering motion, the leg got bigger for other reasons, not because the fibers grew.

This study found that lifting weights by pushing up (concentric) makes the big muscle fibers grow bigger, which is why your whole muscle gets larger. But lowering weights slowly (eccentric) doesn’t make those fibers grow bigger—even though your muscle still gets stronger, maybe by adding more tiny parts inside the fibers instead.