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 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.

High-intensity resistance exercise derives a substantial portion of its energy from the creatine phosphate system and aerobic fat oxidation, particularly during rest intervals.

When people lift weights until they can't do another rep, their muscles grow about the same amount whether they use heavy or light weights — and how much they grow depends more on their own body than on how heavy the weights are.

Doing heavy weightlifting with your legs squeezed to restrict blood flow doesn't make your muscles grow more than just doing heavy weightlifting normally — at least not for most people who haven't trained before.