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.

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.

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.

If you do the same tough muscle workout (like lowering weights slowly) once a week for 10 weeks, your muscles stop getting sore and stop showing signs of damage—even compared to easier workouts—because your body gets used to it.

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.

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 and harder to activate for longer after the deep bend version.