When you bend your arm, a muscle gets shorter and tugs the bone. But how does it shorten? Look very close and a muscle is made of long rows of tiny threads, two kinds slotted between each other like fingers from two hands almost touching. When your brain sends a signal down a nerve, the threads grab hold and ratchet — each one pulls the other a tiny step closer, lets go, grabs again, and pulls once more. None of the threads actually shrink; they just slide past each other, and because millions do it together, the whole muscle gets shorter and pulls. It takes energy every single step, which is why your arm gets tired and warm when you hold something heavy. Fire a signal in the simulator and watch the filaments slide past each other while the muscle squeezes shorter.
Most people think a contracting muscle's parts physically shorten, like a balloon bunching up. In fact nothing inside shrinks at all: the actin and myosin filaments stay the same length and simply slide past each other so the overlap grows.
What's actually happening
Most people picture a muscle as something that inflates, like a balloon bunching up. The bulge is real enough, but it is a side effect, not the mechanism. The genuinely surprising thing is that nothing inside a contracting muscle actually gets shorter. Not a single one of its working parts shrinks. The shortening is an illusion of sliding, and once you see how it works you can never unsee it.
Zoom into a muscle fibre and you find it packed with two kinds of filament laid in neat, repeating rows: thin ones called actin and thick ones called myosin, slotted between each other like the teeth of two combs pushed partway together. The thick filaments are studded with tiny molecular arms — myosin heads. When a nerve signal arrives, it floods the fibre with calcium, which is the 'go' command. Each myosin head reaches out, grabs the neighbouring actin filament, and swings, dragging the thin filament a few nanometres past the thick one. Then it lets go, cocks back, and grabs again, further along — a rowing stroke repeated billions of times a second across the whole muscle. The filaments stay exactly as long as they always were; they simply overlap more, and the muscle as a whole pulls itself shorter. This is the sliding-filament idea, and it is the same in your eyelid, your heart, and a sprinter's thigh.
The consequence you can feel is the cost. Every one of those grab-pull-release strokes is powered by a molecule of cellular fuel, ATP, and the muscle is doing them constantly — even to hold still. Standing holding a heavy bag, nothing visibly moves, yet myosin heads are grabbing and slipping and re-grabbing thousands of times a second just to maintain the pull, burning fuel and shedding the waste as heat. That is why holding a weight tires you and warms you, why shivering (rapid useless contractions) heats your body, and why rigor mortis stiffens the dead: with no fresh ATP, the myosin heads grab the actin and cannot let go, locking every muscle in place.
Muscles only ever pull by ratcheting filaments past each other, burning ATP every stroke, which is why they come in opposing pairs and warm you even when nothing moves.
- 1Rest your forearm on a table, palm up. Put your other hand over the front of your upper arm (the biceps) and slowly curl your hand toward your shoulder.
- 2Feel the biceps bunch and harden as it shortens — those are millions of filaments sliding past each other and pulling the bone.
- 3Now straighten the arm slowly while feeling the back of your upper arm: the triceps takes over. One muscle bent it, a second one straightens it, because a muscle can only ever pull.
Common questions
Every grab-pull-release stroke is powered by one ATP molecule, and the heads keep working thousands of times a second even to hold still. That constant fuel burn tires you and sheds its waste as heat.
A muscle can only shorten and pull, never push. To undo a pull you need a different muscle pulling the other way, which is why your biceps bends the arm and your triceps straightens it.
After death the fuel runs out, so the myosin heads grab the actin filaments and can never release. Every muscle locks rigid for a day or two until the proteins themselves break down.