The skeleton: what it does, where it moves, and the words for how

The syllabus asks for structure and function, and students reliably give one function — movement — because that is the module's title. There are five, and an answer that gives all five, then says which one this subject is about, is doing more work than an answer that gives one.

• Support. The skeleton gives the body its shape and holds the soft tissues up against gravity. Without it you would be a puddle with opinions.
• Protection. The cranium encloses the brain, the vertebrae enclose the spinal cord, the ribs and sternum enclose the heart and lungs, and the pelvis shields the lower abdominal organs. This is why helmets and mouthguards protect what the skeleton cannot.
• Movement. Bones act as levers, joints act as fulcrums, and muscles supply the force. The skeleton does not generate movement; it makes movement mechanically possible and decides its direction. Lesson 3 returns to the lever idea properly.
• Blood cell production. Red bone marrow, found in the flat bones and the ends of long bones, manufactures red blood cells, white blood cells and platelets. Note this one: it is the link to Lessons 4 and 5, because the oxygen-carrying capacity of blood begins here.
• Mineral storage. Bone is the body's reservoir of calcium and phosphorus, released into the blood when needed. This is why bone density is a nutrition question as much as a training one.

The fourth and fifth are the ones that make the point about the skeleton being alive. A frame does not make blood cells. Bone is living, vascular, dynamic tissue that remodels itself continuously in response to the loads placed on it — a principle known as Wolff's law. Load a bone regularly and it lays down more tissue and becomes denser; unload it, through bed rest, immobilisation or spaceflight, and it demineralises. That is the single most important reason weight-bearing exercise matters for lifelong health, and it is a direct bridge from Module 1: the skeletal benefit of physical activity is not a bonus, it is a structural adaptation.

There are 206 bones in the adult skeleton and you are not expected to know them all. You are expected to know the majors, and to place them. Learn them in the two divisions rather than as a list, because the division itself carries meaning: the axial skeleton mostly protects, and the appendicular skeleton mostly moves.

Two orientation rules that save marks. In the forearm, the radius is on the thumb side and the ulna on the little-finger side — and the radius is the one that rotates, which is why the whole forearm can turn over. In the lower leg, the tibia is the large weight-bearing shin bone and the fibula is the slender bone beside it, which bears almost no weight and is largely a muscle attachment site.

Bones are also classified by shape, and the shape follows the job:

• Long — femur, humerus, tibia. Levers for large movements.
• Short — carpals, tarsals. Stability with small multidirectional movement.
• Flat — cranium, sternum, ribs, scapula. Protection and broad muscle attachment.
• Irregular — vertebrae, pelvis. Specialised shapes for specialised jobs.
• Sesamoid — the patella. Sits within a tendon and improves the angle of pull, which Lesson 3 will show is a mechanical advantage.

Every joint in the body sits somewhere on a single trade-off: mobility against stability. You cannot maximise both at one joint, and every joint in the body is a settled compromise between them. Once you see that, the three classes stop being a list to memorise and become three points on a scale.

• Fibrous (immovable). Bones joined directly by dense fibrous tissue, with no joint cavity. The sutures of the cranium are the example. Maximum stability, zero mobility — which is exactly what you want around a brain.
• Cartilaginous (slightly movable). Bones separated by a pad of cartilage. The intervertebral discs and the pubic symphysis are the examples. Each individual joint moves only a little; the twenty-four of them in the vertebral column together produce a spine that bends substantially while still protecting the spinal cord. That is the compromise working.
• Synovial (freely movable). Bone ends separated by a fluid-filled cavity and held by a capsule. Every joint you think of as a joint — shoulder, hip, knee, elbow, ankle — is synovial. Maximum mobility, and consequently the joints that dislocate.

Every synovial joint, whatever its shape, contains the same six components. Learn them as a set, because a question about any synovial joint is answerable from this list.

• Articular cartilage — hyaline cartilage capping each bone end. Reduces friction to almost nothing and absorbs compressive shock. It has no blood supply, which is why cartilage damage heals so poorly.
• Joint capsule — a tough fibrous sleeve enclosing the whole joint and holding the bones together.
• Synovial membrane — the capsule's inner lining, which secretes synovial fluid.
• Synovial fluid — a viscous fluid that lubricates the cavity and delivers nutrients to the avascular cartilage. It thins as the joint moves and warms, which is the physiological basis of a warm-up.
• Ligaments — bone-to-bone bands reinforcing the capsule and limiting movement to safe directions. The anterior cruciate ligament of the knee is the one you have heard of.
• Bursae — small fluid-filled sacs cushioning the points where tendons or skin cross bone. Inflamed, they produce bursitis.

Some joints add menisci — wedges of fibrocartilage that deepen the articulation and spread load. The knee has two, and they are the structure most often torn in a twisting injury.

Now go back to the squat from Card 01. Your hip is ball-and-socket, so it can rotate with the foot off the ground. Your knee is a hinge, so essentially it cannot. Nothing about training changes that; it is settled by the shape of the bone ends. The joint decides which movements exist, and the muscle decides whether they happen.

The syllabus names flexion and extension as its example, and those two carry most of the work. But the whole set is worth holding, because every one of these words replaces a sentence of vague description, and precision is what is being marked.

All joint actions are described from the anatomical position — standing upright, facing forward, arms at the sides with palms forward. Every direction term assumes that starting point, which is why "the palm turns up" is ambiguous and "supination" is not.

One refinement worth having. Actions happen in planes: flexion and extension occur in the sagittal plane (dividing left from right), abduction and adduction in the frontal plane (dividing front from back), and rotation in the transverse plane (dividing top from bottom). Naming the plane is the fastest way to check yourself — if you have claimed rotation at a hinge joint, the plane will tell you that you are wrong.

Choose one skill from a sport you know well — a netball shot, a rugby tackle, a swimming stroke, a bowling action, a tennis serve. Break it into three phases: preparation, execution, follow-through.

This activity works the interrelationship in the dot point rather than the anatomy alone. Take three joints: the shoulder, the knee and the ankle.

For each one, write down: its type; the actions it permits; the actions it does not permit; and then — the analytical step — what injury you would predict if a force pushed it into an action it does not permit.

• The knee is a hinge. What happens when a player is tackled from the side and the knee is forced into abduction? Which structure resists that, and what is the injury called?
• The ankle permits inversion, but only so far. What limits it, and what is the most common sporting injury in Australia?
• The shoulder permits almost everything. So what is the structure that fails, and why is it the most dislocated joint in the body?

Then write one paragraph generalising the pattern: what is the relationship between a joint's mobility and its injury risk, and what does that imply for how an athlete should prepare? Lesson 3 will give you the force vocabulary to sharpen this, and Lesson 2 will add the muscular half — but the structural reasoning is available to you now.