The first time you do hard eccentric exercises—like slowly lowering a heavy weight—your muscles get way more sore and damaged than if you just push the weight up.

When you do a new, super tough workout that stretches your muscles (like lowering weights slowly), your body gets sore and inflamed—but if you do it again every week for 10 weeks, your muscles adapt and don’t get as damaged. The key isn’t the type of movement, it’s that your body wasn’t used to it.

If you do a tough workout that stretches your muscles hard (like lowering weights slowly), then wait four weeks and do it again, your muscles won’t hurt as much or get as damaged the second time—they adapt and get tougher.

If you do hard eccentric exercises like lowering weights slowly once a week for 10 weeks, your muscles stop getting sore and damaged—even though the exercise is tough—because they get used to it.

It’s not the slow lowering that hurts—it’s that your muscles have never done it before. Once they’ve done it a few times, even doing it hard doesn’t hurt anymore.

When healthy young men do a tough new workout that involves lowering weights slowly (like lowering a dumbbell), their muscles get sore and some blood markers go up, but within five days, everything goes back to normal—even if they’ve never done that workout before.