Muscles

Every movement you have ever made, every step, blink, word and heartbeat, was produced by muscle. You have about 600 of them, and together they weigh more than any other tissue in your body. They work by one simple trick: a muscle can pull, and it cannot push. Everything else follows from that.

Three kinds

Skeletal muscle is the kind you can see and control. It attaches to bones through tough cords called tendons, and when it shortens, it moves the bone. There are more than 600 skeletal muscles, from the gluteus maximus in the buttock, the biggest, to the tiny muscle that steadies the stirrup bone in your ear. They are the only muscles under conscious command.

Smooth muscle lines the walls of hollow organs: the gut, the blood vessels, the bladder, the airways, the womb. It works slowly and automatically, squeezing food along the intestine, narrowing arteries to raise blood pressure, making the hairs on your arms stand up. You cannot control it and rarely feel it.

Cardiac muscle exists only in the heart. It contracts on its own rhythm, set by a cluster of pacemaker cells, about once a second, for a lifetime, without ever resting or tiring. It combines the strength of skeletal muscle with the automatic action of smooth muscle, and its cells are linked so that a contraction sweeps through the whole heart as one.

How a muscle pulls

A skeletal muscle is a bundle of long thin cells called fibres, some running the full length of the muscle. Inside each fibre are thousands of parallel threads, and inside each thread, repeated end to end, is the working unit: two kinds of protein filament, thick and thin, interleaved like the fingers of two hands. When a nerve signal arrives, the thick filaments grab the thin ones and ratchet along them, pulling the two sets together and shortening the unit by a tiny fraction. Millions of units shortening at once shorten the whole muscle by up to a third of its length.

The ratcheting is powered by the body's energy currency, a molecule called ATP, made by burning sugar and fat with oxygen. Muscles at full effort use energy so fast that oxygen cannot be delivered quickly enough, and they switch to a less efficient process that produces lactic acid, which is part of why a sprint burns. Heat is a by-product of all this, which is why exercise makes you hot, and why your body shivers, contracting muscles rapidly to warm itself, when you are cold.

Pairs and levers

Because muscle only pulls, every joint needs at least two muscles working in opposition. The biceps on the front of the upper arm bends the elbow; the triceps on the back straightens it. As one contracts, the other relaxes and is stretched, ready for the return. Nearly every movement is a conversation between such pairs, with other muscles steadying the joint as they work.

Muscles usually attach close to joints and pull on bones that act as levers, so a short, powerful contraction near the elbow swings the hand through a wide arc. The design trades force for speed and range; the forearm muscles must pull with far more force than the weight they lift.

Fast and slow

Muscle fibres come in two main types. Slow fibres contract gently, use oxygen efficiently and can work for hours; they are rich in blood vessels and the oxygen-storing protein that gives red meat its colour. Fast fibres contract hard and quickly but tire within seconds or minutes. Marathon runners' leg muscles are mostly slow fibres; sprinters' and weightlifters' are mostly fast. The mix is partly inherited and partly shaped by training.

Exercise does not add fibres; it enlarges the ones you have. Lifting heavy weights causes tiny tears in the fibres, which the body repairs thicker and stronger over the following days, which is why muscles ache after a hard session and why rest matters as much as work. Unused muscle shrinks; a leg in a plaster cast for six weeks loses a visible amount of bulk, and astronauts in weightlessness must exercise two hours a day to keep theirs.

Control

Each muscle fibre is commanded by a nerve fibre running from the spinal cord. One nerve may control a few fibres, in the fingers and eyes where fine control matters, or thousands, in the thigh where power matters. The brain does not think about individual muscles; it thinks about movements, and circuits in the spinal cord and cerebellum translate the intention into the precise pattern of contractions needed. Walking, which engages around 200 muscles in a sequence that changes dozens of times a second, happens with no conscious attention at all.

Why it matters

Muscle is what lets a body act on the world. It also burns most of the energy you take in, keeps you warm, stabilises your joints, and in the form of the heart keeps everything else alive. Keeping muscle strong into old age is one of the best predictors of staying independent, because the same tissue that lets a sprinter run lets an eighty-year-old stand up from a chair.