Your muscles don’t make more protein faster when you lower weights slowly vs. lifting them up—so growth must come from rearranging what’s already there, not just making more.

When you lift weights, your muscles grow bigger because your body makes more tiny protein-making machines inside muscle cells, which helps build more of the proteins that make muscles strong and bulky.

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.

Whether you push weights up or lower them slowly, if you do the same amount of total work, your muscles grow about the same size.

Your muscles don't grow while you're lifting weights—they grow later, while you rest, because your body uses that time to repair and build new muscle tissue after the workout stresses them.

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.