How much your biceps stretch during your first hard workout tells you how sore and weak you’ll feel the next day—but not how much enzyme leaks into your blood.

After the second tough arm workout, blood markers of muscle damage tend to be lower, but the difference isn't strong enough to be sure it's real.

After doing a hard arm workout twice, four weeks apart, people feel stronger sooner the second time—even though they did the same amount of work.

People push just as hard during the second workout as the first, even though they feel less sore and recover faster.

When people who don't normally lift weights do a tough arm exercise twice, four weeks apart, their biceps don't stretch as much the second time, even though they're trying just as hard.

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.