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 heavy weights, your muscles get stretched and stressed just enough to tell your body to build more muscle fibers—this happens because a special molecular switch (mTORC1) turns on, making your muscles produce more proteins and the machinery needed to make them.

Lifting weights makes your muscles grow mainly because the force you create when you contract your muscles sends a signal inside them to build more protein—other things like muscle pump or hormones don’t really matter for growth.

When you lift weights, your muscles grow bigger not just through the usual pathway scientists know about, but also through other hidden backup systems in your cells that turn on genes and make more protein factories—all without needing the main growth switch.

Lifting weights makes your muscles grow mainly because of the physical force you create when you contract your muscles—not because of hormones, burning sensations, or muscle pumps.

When rats lift weights with different numbers of reps, their muscles show the same early biological changes no matter if they do few or many reps—and those early changes don’t tell us which workout will make muscles grow bigger over time.