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.

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.

Even if you lift light weights, if you go until you can’t do another rep, your muscles end up using almost all the same fibers as if you lifted heavy — which might be why both ways make muscles grow similarly.

When you lift weights, what really makes your muscles grow is how much tension they feel, how long they're under that tension, and the total work done — not how tired or 'burning' they feel during the workout.

Scientists aren't sure if the burning feeling you get during a tough workout (called metabolic stress) actually helps your muscles grow, because it's always happening at the same time as the physical pulling on muscles—and we can't separate the two in people.

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.