Stimuli and sensors that initiate skeletal muscle hypertrophy following resistance exercise.

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 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 or do resistance training, your muscles get bigger — and one big reason is that your body turns on a molecular switch (mTORC1) that tells your muscles to make more protein, though other things are also helping out.

When you lift weights, your muscles grow bigger mainly because of the physical tugging and stretching they feel — and scientists think a special protein team (filamin-C and BAG3) might be the muscle’s internal detector that tells the cell to start growing, but we still don’t fully understand how it works.

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.