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.

You can build muscle by working one arm or leg at a time, even if your body doesn’t show higher levels of muscle-building hormones in your blood—so maybe it’s the physical stress on the muscle itself, not the hormones, that makes it grow.

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.

Lifting weights helps people build bigger, stronger muscles, probably because it makes their muscles produce more protein and release helpful signaling molecules that help them grow and work better.

For small muscle groups, unilateral and bilateral resistance training produce equivalent hypertrophic adaptations when volume and effort are matched.

Getting stronger from lifting weights doesn’t always mean your muscles are getting bigger—and vice versa. Strength and size don’t go hand in hand.