The Muscle Fibres
The tiny sliding engines inside you
You have over 600 muscles â and each one is packed with thousands of long, thin cells called muscle fibres. Inside every fibre are tiny engines that slide past each other to make the muscle shrink.
Muscle fibres close up
Real anatomy picture coming soon
Real anatomy photo spot: bundled muscle fibres with their stripey sarcomeres
The tiny engines
Under a microscope, muscle fibres look striped. Those stripes are sarcomeres â the smallest engines in your body. Each sarcomere is made of two kinds of filaments that grab and slide past each other, like rowers pulling an oar. When millions of sarcomeres slide at once, the whole muscle shortens. That's ALL muscle movement: slide, pull, release.
Slow and fast engines
There are two main kinds of fibres. Slow-twitch fibres are like diesel engines â they don't tire, great for standing and long walks. Fast-twitch fibres are like rocket engines â they fire with huge force but tire fast, perfect for sprints and jumps. Marathon runners grow mostly slow; sprinters pack extra fast!
Pull, never push
Here's a rule for your whole body: muscles can only PULL. They never push. That's why they come in pairs across every joint â one bends it, the other straightens it. Your biceps and triceps, your quads and hamstrings, all work as pull-partners.
Mr. Bones says: I'm a fictional skeleton, not a doctor! This is for learning and fun. If anything hurts, stop and tell a grown-up you trust.
