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.

Even though lifting weights makes your body release growth hormone and testosterone, studies show that these short-term hormone surges don’t actually make your muscles grow bigger over time.

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.

Just because your testosterone, growth hormone, and IGF-1 levels spike after a workout doesn't mean those spikes are what make your muscles grow.

When you lift weights until your muscles burn and feel fatigued, the buildup of waste products might help your muscles grow bigger, but the main reason they grow is still from the physical force you're using.

Lifting heavy weights puts stress on your muscles, which is thought to be a main reason they get bigger and stronger.