The skeletal and muscular systems

What the skeleton does

An adult human skeleton has 206 bones. Babies are born with nearly 300, but many fuse together as they grow, for example in the skull and at the base of the spine. More than half of all your bones are in your hands and feet. The skeleton has five main jobs:

• Support: it holds your body up and gives it shape.
• Protection: the skull protects the brain, the ribs protect the heart and lungs, and the vertebrae protect the spinal cord.
• Movement: muscles pull on bones, which act as levers.
• Making blood cells: red bone marrow, inside bones such as the hip bones, ribs and breastbone, makes red and white blood cells and platelets.
• Storing minerals: bones hold calcium and phosphate that can be released into the blood when needed.

Bone is alive

A dry skeleton in a classroom looks dead, but bone in your body is living tissue with its own blood vessels and nerves. Bone cells are surrounded by a hard material made mostly of calcium phosphate, which makes bone hard and strong when squashed, mixed with fibres of the protein collagen, which makes it slightly flexible so it doesn't shatter. Long bones such as the thigh bone are hollow tubes filled with marrow, which makes them light but still strong, the same reason bike frames are made of tubes.

Because bone is alive, it can repair itself. When a bone breaks, blood clots in the gap, then cells lay down new bone that slowly fills it in. A plaster cast or splint holds the ends in line while this happens, which takes around 6 to 8 weeks for a typical break. Bone also responds to use: weight-bearing exercise like running and jumping makes bones denser, while astronauts in zero gravity lose about 1% of their bone mass per month.

Joints

A joint is where two bones meet. Some joints don't move at all (the joints between the plates of your skull), some move a little (between the vertebrae), and most move freely.

Freely moving joints are called synovial joints. They are built to move smoothly for decades without wearing out.

• Cartilage, a smooth, slightly springy tissue, covers the ends of the bones. It reduces friction and absorbs shocks.
• The synovial membrane makes synovial fluid, which fills the joint and lubricates it like oil in an engine.
• Ligaments, tough, slightly stretchy bands, hold the bones of the joint together. A sprained ankle is a stretched or torn ligament.

Three types of muscle

Your body has about 640 named skeletal muscles. All muscle tissue can do one thing: contract (get shorter). Muscle fibres contain protein filaments that slide over each other, using energy from respiration. There are three types:

Muscles work in pairs

A muscle can only pull, never push. So to move a bone back and forth you need two muscles pulling in opposite directions. These are called antagonistic pairs. Muscles are attached to bones by tendons, tough cords that don't stretch, so all the muscle's pull goes into the bone.

• To bend your arm, the biceps contracts and the triceps relaxes.
• To straighten it, the triceps contracts and the biceps relaxes.

Other pairs work the same way: the hamstrings and quadriceps bend and straighten the knee, and the calf and shin muscles point and lift your foot.

Your forearm is a lever. The elbow is the pivot, the biceps pulls about 4 cm from the elbow, and the load in your hand is about 35 cm away. Because the muscle pulls so close to the pivot, it needs a much bigger force than the weight it lifts.

When things go wrong

• Osteoarthritis is the most common type of arthritis. The cartilage in a joint wears away, so bone rubs on bone, making the joint painful, stiff and swollen. It's more common with age, after joint injuries, and in people carrying extra weight. Badly worn hips and knees can be replaced with metal and plastic artificial joints; over 100,000 hip and knee replacements are done in Australia every year.
• Rheumatoid arthritis is an autoimmune disease: the immune system attacks the synovial membranes, often in the small joints of the hands, which can become badly deformed.
• Osteoporosis means "porous bone". Bones lose calcium and become fragile, breaking easily, especially in older women. Plenty of calcium, vitamin D (from sunlight) and weight-bearing exercise while you're young help build strong bones for life.
• Fractures, sprains (ligament injuries) and strains (torn muscle fibres) are common sports injuries. The usual first aid for sprains and strains is rest, ice, compression and elevation.