For top powerlifters, two types of weight training—one that restricts blood flow and one that doesn't—both leave fast-twitch muscle fibers the same size after training.

Even though muscles got bigger, the actual muscle fibers didn’t get thicker or change type—so the growth might be due to other factors like fluid or connective tissue.

When adults do light weight training with restricted blood flow, their slow-twitch muscles grow as much or more than their fast-twitch muscles, which is different from heavy weight training where fast-twitch muscles usually grow more.

At first, your muscles might actually get a little smaller from this kind of training, but after doing it again later, they grow bigger than before — the growth just takes time to show up.

We just don’t know yet if light or heavy weights make muscles grow better — the results could swing either way in future studies, so we need more research.

Lifting light or heavy weights until failure doesn’t clearly make your fast-twitch muscle fibers bigger in your thighs — the data is too mixed to say one is better.