When some people lift weights, their bodies make more of a certain enzyme—but only if they’re the type who gains muscle easily. People who don’t gain much muscle don’t see this enzyme change. And even when the enzyme goes up, it doesn’t match up with how much muscle they gain, so it probably doesn’t cause the muscle growth—it just happens along with it.

Working out until you're exhausted might make your body release more growth hormone and testosterone, which could help your muscles grow — but we’re not sure if these hormones actually cause the growth.

People who build the most muscle from weight training show increased activity in their muscle cells’ systems that rebuild proteins, use energy during exercise, and fix damaged proteins.

You can build muscle by working one arm or leg at a time, even if your body doesn’t show higher levels of muscle-building hormones in your blood—so maybe it’s the physical stress on the muscle itself, not the hormones, that makes it grow.

Getting stronger from lifting weights doesn’t always mean your muscles are getting bigger—and vice versa. Strength and size don’t go hand in hand.

People grow muscle at very different rates — but each person tends to grow their arms and legs similarly, no matter what weight they use. This means your genes and biology matter more than how heavy your weights are.