Doing push-ups with light weights until you can't do more doesn't make your biceps bigger, but doing the same kind of light-weight bench presses does — so your arms respond differently to these two exercises.

If you're a young guy who's never lifted much before, doing push-ups and light bench presses until you can't do any more, twice a week for two months, will make you just as strong on the bench press as if you were lifting heavier weights — and you don't even need dumbbells or barbells.

If you're a regular gym-goer, doing workouts that group similar muscle moves together or alternate them doesn't seem to make a difference in how much stronger or bigger your muscles get after 8 weeks.

Whether you lift your arm weights slowly or with a little swing, your biceps and triceps both get noticeably bigger after eight weeks of training—your triceps grow even more than your biceps.

If you do push-ups or bench presses with light weights until you can't do another rep, twice a week for two months, your arms and chest will grow just as much — so push-ups can be just as good as gym machines for building muscle, if you push yourself just as hard.

If you're a young adult and you lift weights for 8 weeks—whether you use free weights like dumbbells or machines at the gym—you'll end up with about the same muscle growth and strength gains in your arms and legs, no matter which one you pick.