How do your muscles move you?

Every move you make, from a blink to a sprint, is a muscle pulling. You have more than 600 skeletal muscles, the ones fixed to your bones, and together they make up about a third of your body mass: about 38% in men and 31% in women, on average.

We are facing the person, so their right side is on your left. Across the shoulder sits the deltoid; in front of the upper arm the biceps, behind it the triceps. The chest has the pectorals, the belly the abdominals (the "six-pack", split by bands of tendon) and the obliques at the sides. Turn the body round to see the trapezius, the wide latissimus dorsi, and the gluteus maximus, your biggest muscle. The thigh has the quadriceps in front and the hamstrings behind; the calf pulls on the heel through the Achilles tendon, the strongest tendon in the body.

Each muscle is joined to bone by a tendon, a tough white cord. Your smallest muscle, the stapedius in the ear, is about 1 mm long (see EarClear). And one of your hardest-working muscles is hidden inside: the diaphragm, a dome under the lungs that pulls down to suck air in, about 15 times a minute, all your life (see LungsClear).

The heart and the walls of your gut are muscle too, of different kinds. The next chapter compares them, and HeartClear shows the heart at work.

Your body makes three kinds of muscle, and under a microscope they look different.

Skeletal muscle moves your bones. Its cells, called fibres, are long cylinders with many nuclei, and they are striated: striped with light and dark bands. It is voluntary: it contracts when your brain sends a signal down a nerve (see NervousClear). One signal gives a quick twitch of about a tenth of a second; a stream of 20 to 50 signals a second fuses the twitches into a strong, smooth pull.

Cardiac muscle is found only in the heart. It is striped too, but its cells are short, branched and joined end to end by intercalated discs that pass the signal straight from cell to cell, so the whole heart squeezes together. It is involuntary: it has its own pacemaker. Each squeeze lasts about 0.3 s, and it can never lock into one long cramp. With a third of each cell packed with energy-making mitochondria, it beats about 1 lakh times a day and never rests (see HeartClear).

Smooth muscle lines your gut, blood vessels, bladder and airways, and even sets the size of your pupils. It has no stripes, and its cells are small spindles with one nucleus. It is slow, taking seconds to squeeze, but it can hold a squeeze for hours on very little energy. Waves of it push food along your gut (see DigestionClear).

A muscle is bundles inside bundles. The muscle is made of fascicles; each fascicle is a bundle of fibres, and each fibre is one long cell. Inside a fibre run hundreds of myofibrils, and each myofibril is a chain of tiny units about 2.5 µm long, called sarcomeres. They make the stripes.

A sarcomere runs from one Z-disc to the next. Thin filaments of actin reach in from each Z-disc, and thick filaments of myosin sit in the middle, bristling with little heads. When a nerve signal arrives (see NervousClear), the sarcoplasmic reticulum around each myofibril releases calcium. Calcium moves a guard protein, tropomyosin, off actin, and the myosin heads grab on.

Each head then pulls in a cross-bridge cycle: grab, swing (the power stroke, about 8 nm), let go, re-cock. Letting go needs one molecule of ATP, the cell's energy coin. Millions of heads tugging out of step slide the actin towards the middle, so the sarcomere shortens from about 2.5 to 2.0 µm. Nothing gets shorter itself: the filaments slide past each other. That idea, the sliding filament theory, was published in 1954 in two papers side by side.

A sarcomere pulls hardest at about 2.0–2.25 µm, where every head can reach actin. Stretch it too far and the filaments barely overlap; squash it and they crowd each other. With no ATP at all, the heads cannot let go and the muscle locks: that is rigor mortis. WorkClear's tired-muscle scene shows why even holding still costs energy.

A muscle can only pull. It shortens and tugs on its tendon, but it can never push. So to move a joint both ways you need two muscles working against each other: an antagonist pair.

At the elbow, the biceps in front bends the arm and the triceps behind straightens it. While one works, the other relaxes and is stretched. At the knee, the quadriceps straighten the leg and the hamstrings bend it. The pull reaches the bone through a tendon; the joints and bones themselves are SkeletonClear's story.

Your forearm is a lever with the elbow as its pivot (see ForceClear for levers and forces). The biceps tendon grips the forearm only about 5 cm from the elbow, but your hand is about 33 cm away. To balance the turning effect, the biceps must pull roughly 7 times harder than the weight in your hand: holding a 5 kg bag with your forearm level takes a pull of about 400 N, like holding up 40 kg. You pay in force, but you win in speed: a small shortening of the muscle swings your hand a long way, fast.

The biceps also works when you lower a weight: it pulls while it gets longer, braking the fall (an eccentric contraction). Lifting is concentric (shortening) and holding still is isometric.

Every myosin stroke spends one ATP, yet a muscle holds only enough ATP for about 2–3 seconds of hard work. So it keeps remaking it, three ways.

First, phosphocreatine (PCr) hands its phosphate straight to ADP. It is instant but runs out in about 10 seconds: a 100 m sprint. Next, anaerobic glycolysis splits glucose without oxygen. It is fast and powers efforts of up to about 1–2 minutes, like a 400 m, and makes lactate on the way. Last, aerobic respiration burns sugar and fat with oxygen in the mitochondria. It is slow to get going but can run for hours. By about 75 seconds of an all-out effort, half the energy has come from oxygen.

Muscles have two main kinds of fibre. Slow-twitch (type I) fibres are red with blood vessels and mitochondria and hardly tire. Fast-twitch (type II) fibres are paler, contract several times faster and are more powerful, but tire fast. Most people are about half and half; top marathoners can be three quarters slow, sprinters three quarters fast. Genes set much of the mix; training tunes it.

Some myths. Lactate does not cause next-day soreness: it is cleared within about an hour and even used as fuel. DOMS, the ache 1–3 days later, comes from tiny damage after unfamiliar exercise, especially lowering weights or running downhill, and it fades as muscles adapt. Cramps are sudden locked contractions; the cause isn't fully known, but tired, over-excited nerve endings are the leading idea, and gentle stretching helps. Training makes muscle bigger by making each fibre thicker (hypertrophy), not by adding many new fibres.

This chapter explains common muscle problems in general; it cannot tell you what is happening in your own body. For pain, weakness or an injury that worries you, see a doctor.

A strain, or pulled muscle, is fibres overstretched or torn, often in the hamstrings, calf or thigh during a sprint or a sudden stretch. Most mild strains get better in a few weeks with rest and then gentle movement. See a doctor if you heard a pop, can't walk on it, or the pain and swelling are bad or don't improve.

From about age 30, muscle slowly shrinks: about 3–8% each decade, faster after 60. Severe loss that weakens a person is called sarcopenia, and it raises the risk of falls. The good news: strength training helps at any age. In one famous study, people in their 90s nearly tripled their strength in 8 weeks.

A warm-up of 10–20 minutes of easy movement raises muscle temperature by a degree or two, and warm muscle is more elastic and a few per cent more powerful per degree. The WHO suggests strength activities for all the big muscle groups on at least 2 days a week for adults and 3 days for ages 5 to 17: push-ups, squats, climbing, carrying, yoga, dancing or sport all count. Muscles are built from protein: Indian guidelines suggest about 0.8 g per kg of body weight a day for adults, from dal, milk, curd, paneer, eggs, fish or nuts. Protein only helps if the muscle is also used.

Some conditions are genetic. In Duchenne muscular dystrophy, which affects about 1 boy in every 3,500–5,000, a missing protein called dystrophin lets muscle fibres break down over time.