Doing a new, intense workout that focuses on lengthening muscles under tension can cause temporary muscle damage and soreness. This shows up as a noticeable drop in strength, higher pain levels, and a spike in a specific muscle enzyme in the blood for up to four days.

If you do a tough, unfamiliar full-body workout and then repeat the exact same workout two weeks later, your body will experience much less muscle soreness, strength loss, and tissue damage. This shows that your muscles quickly adapt to the same type of stress, protecting you from injury the second time around.

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.

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.

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.

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.