If new guys do leg exercises to failure using medium weights, slowing down the movement doesn’t make their muscles grow more than doing faster reps — both ways work about the same.

If new guys do leg exercises with slow reps (6 seconds) instead of faster ones (2 seconds), they might get stronger in a specific knee position—even if they do the same number of sets and their muscles grow the same amount.

When people who’ve never lifted weights before do leg exercises with either 20-second or 2-minute breaks between sets—while doing the same total amount of work—their thigh muscles grow about the same amount.

After 10 weeks of lifting weights, whether you rest 20 seconds or 2 minutes between sets, your thigh muscle gets about 15% bigger and you can lift about 40% more weight.

If untrained guys do leg exercises to failure using medium-heavy weights, going slow or going a bit faster on each rep gives about the same strength gains.

When untrained guys do leg extensions with a medium weight until they can't do any more, moving faster (2 seconds per rep) makes their quads work harder for most of the movement — but moving slower (6 seconds per rep) makes them work harder right at the end. So, how fast you lift changes which part of the move your muscles feel most.