When you lift weights with your blood partly blocked or using light weights, your muscles might produce chemicals that help them grow—but scientists aren’t sure exactly which chemicals or how much they really contribute yet.

When you lift weights with your blood partially restricted, your muscles might produce certain chemicals that could help them grow—but scientists aren’t sure which chemicals or if they really cause growth yet.

When you lift weights, your muscles grow because of the physical pulling and stretching forces they feel—this is thought to be the main reason they get bigger.

When you lift weights until your muscles burn and feel fatigued, the buildup of waste products might help your muscles grow bigger, but the main reason they grow is still from the physical force you're using.

When you lift weights until your muscles burn, the buildup of waste products might help your muscles grow bigger, but scientists aren't sure yet if it's actually causing the growth or just happening at the same time.

When healthy young men lift weights until they're too tired to continue, their muscles grow about the same amount in both their arms and legs, no matter how heavy the weights are.