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 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.