Even if middle-aged guys have been lifting weights for years, their muscles recover just as slowly after a tough workout as guys who don’t train—so being trained doesn’t seem to protect against muscle soreness or damage.

Both heavy lifting with few reps and light lifting with many reps cause muscle damage, but neither does it more than the other.

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.

For young active men, doing lighter weights with more reps three times a week for a month might cause less muscle damage than lifting heavy weights with fewer reps, based on a blood test that measures muscle stress.

When rats do low-frequency weightlifting, their blood doesn't show signs of muscle soreness or energy stress, meaning this kind of exercise doesn't hurt or tire their muscles.

When rats do resistance exercises less often, their blood doesn't show signs of muscle damage or stress—so this kind of workout seems gentle on their bodies.