Muscle

body · movement · biology

Three kinds. Your body has three types of muscle: skeletal muscle that moves your bones, cardiac muscle in your heart, and smooth muscle inside organs like your stomach.

Tiny tug-of-war. Muscles work using two proteins called actin and myosin that slide past each other to pull, making the muscle contract.

Never resting. Your cardiac muscle squeezes your heart over and over, all day and night, without ever taking a break.

Built before birth. Muscles start forming while you're still a tiny embryo, in a process scientists call myogenesis.

Stretch and squeeze. Muscle tissue can both contract (tighten) and relax (loosen), which is what lets you bend, jump, and smile.