How Muscles Gain Strength: When you lift weights, your muscles are working so hard to lift that they actually develop microscopic tears. The body quickly heals these microscopic muscle tears, usually within 24-48 hours. When the muscle heals, the body puts extra muscle tissue on the tears to help reduce the chance of it tearing again. Think of an envelope you want to make sure stays closed in the mail. First you seal the envelope with your spit or water. Then you add tape on top of the seal to be extra careful it doesn't open. Your body is adding extra tape to your muscles with more muscle fiber. This extra muscle fiber makes the muscle stronger, like the tape makes the envelope seal stronger. And when your muscle becomes stronger, you become stronger. This process is repeated every time you lift weights, creating stronger and stronger muscles and a stronger and stronger you.