Working out one arm can make the other arm stronger and more steady, even if you didn't touch it—your brain is learning to control it better.

When your brain sends more coordinated signals to your muscles and less random noise, your movements become smoother and more controlled—even in muscles you didn’t train.

When people train one leg at a time, that leg gets much stronger than the other leg, even if the other leg didn’t do any training.

When you train one arm or leg with weights, the other arm or leg doesn't get stronger or bigger—even though your body is adapting in other ways. This suggests those body-wide changes aren't enough to build muscle in the side you didn't train.

If you're a young adult who doesn't lift weights or do strength training for 8 weeks, your muscles won't get noticeably bigger or stronger on their own — you need to actually train to see changes.

Whether you use machines that change resistance as you lift or keep the same weight all the way through, after 20 weeks of training, healthy men gain about the same strength and muscle size either way.