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.

Scientists think that two proteins, Filamin-C and BAG3, might act like a sensor in muscle cells that detects when the muscle is being stretched or worked hard, and then turns on or off other signals that help the muscle grow — but they’re not sure exactly how this works yet.

When you lift weights, your muscles grow because of the physical pulling and stretching forces they feel—this is thought to be the main reason they get bigger.

When you lift weights, your muscles sense the tension and turn on internal signals that help them grow bigger—and they might even become better at using leucine, a protein-building block, to help with that growth.

When you lift weights, it's the pulling force on your muscles—not the tears or burn—that makes them grow bigger, and scientists think a specific molecular signal inside the muscle cells is behind this growth; other things like soreness or muscle burn probably don't cause the growth on their own.

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.