You don’t need to be sore after a workout to build muscle, but being sore might help you build a little more muscle—scientists aren’t sure how much, if at all.

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.

Your muscles don't grow while you're lifting weights—they grow later, while you rest, because that's when your body repairs tiny tears and builds more muscle tissue using the energy and nutrients you've eaten.

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 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 move your muscles through their full stretch and contraction during exercise—like fully lowering and raising a dumbbell—you build more muscle than if you only move partway, because the muscle gets stretched under load, which seems to trigger better growth.