After six months on the International Space Station, astronauts come home with weaker skeletons. In weightlessness, according to NASA, the hips and spine can lose around 1 to 1.5 percent of their bone mass each month, despite about two hours of daily exercise on special machines. Bone, it turns out, is not a dead framework like the beams of a house. It is living tissue that listens to how it is used, and that fact shapes the body from birth to old age.

Living bone

An adult skeleton has 206 bones. A newborn has around 270 to 300, many of which later fuse, as in the skull and the base of the spine. Bone combines collagen fibers, which give it some flexibility, with calcium phosphate minerals, which make it hard. It holds about 99 percent of the body's calcium, acting as a bank the blood can draw on, and the marrow inside many bones makes blood cells.

Bone is constantly rebuilt. Osteoclasts break down old bone, and osteoblasts build new bone, so that the adult skeleton is replaced over roughly a decade. In 1892 the German surgeon Julius Wolff described what became known as Wolff's law: bone strengthens where it is loaded and weakens where it is not. A tennis player's serving arm has measurably thicker bones than the other arm. Bones meet at joints, held together by ligaments, which join bone to bone, while tendons join muscle to bone.

How muscles move

The body has three kinds of muscle. Skeletal muscle moves the bones and is under conscious control. Smooth muscle lines the gut, the airways and the blood vessels and works without our attention. Cardiac muscle is found only in the heart.

Inside a skeletal muscle fiber lie overlapping filaments of two proteins, actin and myosin. In 1954 two pairs of researchers, working independently, proposed the sliding filament model: during contraction, myosin pulls on actin, sliding the filaments past each other so the fiber shortens. Each pull uses energy from a molecule called ATP. After death, ATP runs out and myosin stays locked onto actin, which causes the stiffness called rigor mortis.

A muscle shortens when it works and lengthens only when something else stretches it, so a muscle can pull but never push. That is why skeletal muscles work in opposing pairs. The biceps bends the elbow; the triceps on the back of the arm straightens it.

Growing up

Children's long bones lengthen at growth plates, discs of cartilage near each end where new bone is added. At puberty, surges of estrogen, testosterone and growth hormone drive a growth spurt. Then the growth plates harden and close, usually in the late teens or early twenties, and height growth ends. Bone keeps thickening for some years after that, and bone mass peaks at around age 30. Exercise and good nutrition in youth fill the bank that later life will draw on.

Growing old

From middle age, most people slowly lose bone and muscle. Age-related muscle loss is called sarcopenia. In women, falling estrogen after menopause speeds bone loss, which is why osteoporosis, a thinning of bone that makes fractures likely, is far more common in older women than in older men. A hip fracture in old age can end independence.

Aging is not one process but many. Damage gradually builds up in DNA, proteins and cells, and many cell types lose their ability to divide. The longest documented human life is that of Jeanne Calment of France, who died in 1997 at 122. Researchers disagree about goals. Some treat aging itself as a disease that could be slowed or cured. Others argue that the realistic aim is a longer span of healthy years, with less time spent frail.

Much decline can be resisted. Studies since around 1990 have found that strength training can rebuild muscle even in people in their nineties, improving strength, walking and balance. Weight-bearing exercise, such as walking and stair climbing, helps preserve bone. Frailty is common, but it is not simply fate.

Worked example: Maria at 68

Maria, a retired teacher, has a bone scan at 68 that shows thinning bones, a warning sign for osteoporosis. Her doctor explains the picture this course has built. Since menopause her osteoclasts have been outpacing her osteoblasts. Her thigh muscles are weaker than they were, which makes a fall more likely, and a fall on thin bone could break a hip. Maria starts brisk walking most days and lifts weights twice a week. By Wolff's law, the load tells her hips and spine to keep their bone. The training rebuilds muscle and sharpens her balance, so she is less likely to fall in the first place. She eats enough calcium and gets vitamin D, which her kidneys activate so her gut can absorb that calcium. A year later her bone density has held steady and she climbs stairs more easily. Bones, muscles, hormones, kidneys and gut have all been working together, the same partnership this course has traced from the first level.