After training one arm, the other arm gets stronger because its motor neurons become easier to activate and receive more synchronized signals from the brain.

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.

After training one arm, both arms become easier to turn on—your brain can activate muscles with less effort, even in the arm you didn’t train.

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.