After really intense muscle workouts, healthy guys can have super high levels of a muscle enzyme in their blood—way above normal—peaking a few days later, which shows their muscles are breaking down a bit, but their kidneys are still fine.

After a tough leg workout, a blood marker of muscle damage (CK) spikes by more than 1.5 times and stays high for two days, showing the muscles are still healing.

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.

Guys who work out regularly might only start showing signs of muscle damage after the third month of a strength training plan, possibly because the workouts get too intense or they’re not recovering enough.

After running for 24 hours, elite runners have extremely high levels of a blood marker that signals their muscles are breaking down, and those levels stay high the next day.

If you slow down the lowering part of weightlifting, your muscles might get more damaged and your body might release more of the hormones that help build muscle—this is what the claim says about guys who lift weights.