Even though your muscles feel just as sore and your blood shows similar signs of damage after doing leg exercises with your knee bent far back or only slightly bent, your muscles are actually weaker...

Just because your muscles aren’t sore or your blood doesn’t show damage markers doesn’t mean you’re not getting stronger—your body can adapt without those signs.

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.

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 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 young guys do a tough arm workout a second time, their muscles aren't as sore, swollen, or damaged as they were the first time — their bodies seem to adapt after the first go-around.