Lifting light weights and lifting heavy weights both temporarily turn up the activity of muscle-building genes right after you work out, but after 12 weeks of training, your body’s baseline gene activity returns to normal no matter which weight you use.

People’s muscles before they start lifting weights already have different gene activity patterns, and those patterns seem to predict how much their muscles will grow after training.

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.

Lifting heavy weights (70% of your max) makes your muscles turn up the volume on certain genes that help them grow and turn down a gene that holds them back, more than lifting light weights (16% of your max) does after a single workout.

Your muscles don't grow while you're lifting weights—they grow later, while you rest, because your body uses that time to repair and build new muscle tissue after the workout stresses them.

Your muscles don't grow while you're lifting weights—they grow later, while you rest, because that's when your body repairs tiny tears and builds more muscle tissue using the energy and nutrients you've eaten.