If you suddenly do really intense stretching-type workouts that your muscles aren't used to, it can seriously damage them — so much that the muscles actually get smaller for a few weeks.

It’s not that slow lowering of weights damages muscles—it’s that your muscles aren’t used to it. Once they get used to it, no damage happens, even if you do the same move hard.

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.

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.

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.

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.