ELI5: Explain Like I'm 5

how are muscles formed

Muscles are made up of small, thin filaments of protein called myofibrils. Myofibrils are made up of bundles of even smaller parts called myofilaments, which are made up of two different kinds of proteins- actin and myosin. When your body tells your muscles to move, your nerves send electrical signals and your muscles contract- meaning the actin and myosin filaments bunch up together very tightly. This causes the muscle to tighten and shorten, giving you the ability to move your body parts.