When you lift light weights until you're exhausted, your muscles might still grow because they get really tired and activate the same powerful muscle fibers as heavy lifting.

When your muscles get really tired during a workout, your body has to turn on more muscle fibers to keep going — and that might help them grow bigger.

You can build muscle just as well by lifting light weights with your blood partially squeezed off, as you can by lifting heavy weights—so maybe it’s not the weight itself that matters most, but how much your muscles burn and swell up.

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.

The total amount of work done (weight × reps × sets) seems to matter for muscle growth — more work generally leads to more muscle, even if the weights are light.

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.