If you're a young adult who hasn't trained before and you do leg extensions slowly—either taking 2 seconds or 4 seconds to lower the weight—your thigh muscles will grow about the same amount, and you'll get just as strong either way.

If you take four seconds to lower the weight during leg extensions instead of two seconds, your muscles are under strain for about 95% longer—without doing more reps—over eight weeks of training.

If you're a young adult who doesn't lift regularly and you do leg extensions slowly (4 seconds down), your inner thigh muscle gets a little thicker than if you do it faster (2 seconds down)—but the outer and front thigh muscles don't change much.

When guys who’ve never lifted weights before start training their legs for 15 weeks, the more their thigh muscles grow, the stronger they get—so muscle growth is probably why they’re getting stronger, not just better at lifting.

If you're a young adult who hasn't trained much before, slowing down the lowering part of a leg extension from 2 seconds to 4 seconds won't make you significantly stronger, even though your muscle is under tension longer.

If you're a young adult who hasn't trained much before, doing squats slowly (4 seconds) or quickly (1 second) on the way down for 7 weeks will make your thigh muscles grow about the same amount — so how slow or fast you lower yourself doesn't really matter as long as you do the same total work.