How Do Muscles Move You?

Step 1 of 5

A muscle can only shorten.

Contraction pulls its two ends together. There is no mechanism to lengthen forcefully, so a muscle can never push anything.

Step 2 of 5

So they work in opposing pairs.

Your biceps bends the elbow; your triceps straightens it. Each undoes the other, because neither can reverse itself.

Step 3 of 5

Bones are the levers.

Muscle attaches near a joint and pulls a small distance. The bone multiplies that into a large movement at the far end.

Step 4 of 5

Inside, filaments ratchet past each other.

Two sets of protein filaments overlap. Tiny heads grab, pull and release repeatedly — millions ratcheting together shorten the fibre.

Step 5 of 5

Different fibres for different jobs.

Fast fibres give power but tire quickly; slow fibres are weaker but keep going. Sprinters and marathon runners rely on different mixes.

Your jaw muscles are proportionally among the strongest you have — biting force can exceed 70 kilograms.