Choose a movement under What moves? and follow it step by step. Contracting muscles glow red and get shorter and fatter; relaxing muscles turn blue. The close-up in the corner shows the tissue at work.
An adult has 206 bones and more than 600 skeletal muscles. Bones meet at joints; muscles are joined to bones by tendons and can only pull. Click any part of the model, or choose one from the list.
3D anatomy: Z-Anatomy (CC BY-SA 4.0), based on BodyParts3D © The Database Center for Life Science (CC BY-SA 2.1 Japan); simplified for the web by SciSim. The simplified model files are shared under CC BY-SA 4.0.
This is the same 3D skeleton seen from the front, the view you label in an exam. Switch between the labeled diagram, a blank one to test your memory, and a quiz where you place each label yourself. You can also download both versions or print a worksheet.
Tap a word, then tap the numbered box it belongs to (on the diagram or in the list below). Tap a filled box to take the word back. On a phone, swipe the diagram sideways to see every number.
Tap a card, then tap the box it belongs in. Press Check when all 10 are placed.
A printable worksheet: label the skeleton, sort the joints, match muscles to their actions, multiple choice, a lever calculation and explain questions, with an answer key.
The skeleton has four main jobs: it supports the body and gives it shape, protects soft organs (the skull protects the brain, the ribs the heart and lungs), provides a framework for movement, and makes blood cells in the red bone marrow. Bones also store calcium. Bone is living tissue with blood vessels and cells.
Bones meet at joints. Some are fixed (the sutures of the skull), but most are moving synovial joints: hinge joints such as the elbow and knee bend in one direction, ball-and-socket joints such as the shoulder and hip move in all directions, and the pivot joint between the first two neck vertebrae lets the head turn. Ligaments hold bones together; tendons join muscles to bones.
A muscle can only contract (get shorter and pull); it cannot push. So muscles work in antagonistic pairs: when the biceps contracts the elbow bends and the triceps relaxes; to straighten the arm the triceps contracts and the biceps relaxes. Inside each muscle fibre, protein filaments (actin and myosin) slide past each other to shorten it, using energy from respiration.
Press Bend your arm and watch the biceps glow red and bulge while the triceps turns blue. Step 4 turns the forearm into a lever: the biceps pulls with about 400 N to hold a 5 kg weight. Then try Kick a ball and Stand on tiptoe to find two more antagonistic pairs, and use the Skeleton layer to see which bones move.
| Function | Example |
|---|---|
| Support | The backbone and leg bones hold the body upright and give it shape. |
| Protection | The skull protects the brain; the ribs and sternum protect the heart and lungs; the vertebrae protect the spinal cord. |
| Movement | Muscles pull on bones, which move at joints. |
| Making blood cells | Red bone marrow in flat bones and the ends of long bones makes red and white blood cells and platelets. |
| Storing minerals | Bones store calcium and phosphate and release them into the blood when needed. |
| Type | Shape | Examples |
|---|---|---|
| Long bones | Longer than they are wide; act as levers | Femur, humerus, radius, tibia, phalanges |
| Short bones | Roughly cube-shaped; give stability | Carpals (wrist), tarsals (ankle) |
| Flat bones | Thin plates; protect organs and anchor muscles | Cranium, sternum, ribs, scapula |
| Irregular bones | Complex shapes | Vertebrae, facial bones, hip bone |
| Sesamoid bones | Grow inside a tendon | Patella (kneecap) |
| Joint | Movement | Examples |
|---|---|---|
| Fixed (immovable) | None | Sutures between the bones of the cranium |
| Hinge | Bends in one plane, like a door | Elbow, knee, ankle, finger joints |
| Ball-and-socket | In all directions and rotation | Shoulder, hip |
| Pivot | Rotation around one axis | Atlas and axis (turning the head); top ends of the radius and ulna (turning the palm) |
| Gliding | Small sliding movements | Between the carpals of the wrist |
| Joint | One muscle (movement) | Its antagonist (opposite movement) |
|---|---|---|
| Elbow | Biceps and brachialis (bend the elbow) | Triceps (straightens the elbow) |
| Knee | Hamstrings (bend the knee) | Quadriceps (straighten the knee) |
| Ankle | Tibialis anterior (lifts the foot and toes) | Gastrocnemius and soleus (point the toes, lift the heel) |
| Hip | Iliopsoas (lifts the thigh forwards) | Gluteus maximus and hamstrings (pull the thigh back) |
| Skeletal muscle | Smooth muscle | Cardiac muscle | |
|---|---|---|---|
| Where | Attached to bones | Walls of the gut, blood vessels, bladder, airways | Heart only |
| Control | Voluntary (you decide) | Involuntary | Involuntary |
| Look under a microscope | Long striped fibres with many nuclei | Spindle-shaped cells, no stripes | Striped, branched cells joined together |
| Tires? | Yes | Slowly | Does not tire |
Each skeletal muscle fibre is packed with myofibrils divided into sarcomeres. A nerve impulse at the neuromuscular junction releases acetylcholine, the fibre releases calcium ions, and myosin heads pull actin filaments towards the middle of the sarcomere using ATP (the sliding filament model). The filaments themselves do not get shorter; they overlap more. Most limbs work as third-class levers: the muscle pulls between the joint (pivot) and the load, so a large force produces a fast, wide movement of the hand or foot. Bone is constantly remodelled by osteoblasts (which build bone) and osteoclasts (which break it down); weight-bearing exercise makes bones denser, and in osteoporosis bone is lost faster than it is made.
An adult has 206 bones. A newborn baby has about 300 bones and cartilages that will become bone; many of them fuse together during childhood, for example in the skull, the sacrum and the hip bones. Hands and feet contain more than half of all the bones.
Key takeaway: adults have 206 bones; babies have about 300 that later fuse.The skeleton supports the body and gives it shape, protects organs (the skull protects the brain, the ribcage the heart and lungs, the vertebrae the spinal cord), allows movement because muscles pull on bones at joints, makes blood cells in the red bone marrow, and stores minerals such as calcium.
Key takeaway: support, protection, movement, making blood cells and storing minerals.Fixed joints, such as the sutures of the skull, do not move. Moving (synovial) joints include hinge joints (elbow, knee, ankle, fingers), which bend in one direction; ball-and-socket joints (shoulder, hip), which move in every direction; pivot joints (between the first two neck vertebrae), which allow rotation; and gliding joints between the small bones of the wrist and ankle.
Key takeaway: fixed, hinge, ball-and-socket, pivot and gliding.Muscles are joined to bones by tendons. When a muscle contracts it gets shorter and pulls on the bone, which moves at a joint. Muscles can only pull, not push, so a different muscle is needed to move the bone back.
Key takeaway: a contracting muscle shortens and pulls the bone by its tendon.Two muscles that move a joint in opposite directions. When one contracts, the other relaxes. The biceps bends the elbow and the triceps straightens it; the hamstrings bend the knee and the quadriceps straighten it.
Key takeaway: two muscles that pull a joint in opposite directions, one contracting while the other relaxes.A tendon joins a muscle to a bone and passes the pull of the muscle to the bone; it is very strong and hardly stretches. A ligament joins a bone to another bone at a joint and holds the joint together while letting it move. Both are made mostly of collagen.
Key takeaway: tendon: muscle to bone; ligament: bone to bone.Skeletal muscle is attached to bones, is under voluntary control and looks striped under a microscope. Smooth muscle is found in the walls of the gut, blood vessels and other organs and works involuntarily. Cardiac muscle is found only in the heart; it is striped and branched, works involuntarily and does not tire.
Key takeaway: skeletal (voluntary), smooth and cardiac (both involuntary).The forearm is a lever with the elbow as the pivot. The biceps is attached only about 4 cm from the elbow, while a weight in the hand is about 33 cm away. For the moments to balance, the muscle force must be about 33 ÷ 4 ≈ 8 times the weight: about 400 N for a 5 kg weight. In return, a small shortening of the muscle moves the hand a long way, quickly.
Key takeaway: a short lever arm for the muscle means a large force but a fast, wide movement.A nerve impulse reaches the muscle fibre at the neuromuscular junction and triggers the release of calcium inside the fibre. Myosin heads then bind to actin filaments and pull them towards the centre of each sarcomere, using energy from ATP. The filaments slide past each other, so every sarcomere and the whole fibre get shorter (the sliding filament model).
Key takeaway: actin and myosin filaments slide past each other, powered by ATP.Bone marrow is the soft tissue inside bones. Red bone marrow, found in flat bones such as the hip bones, sternum, ribs and vertebrae and at the ends of long bones, makes red blood cells, white blood cells and platelets. Yellow bone marrow, mostly fat, fills the middle of long bones in adults.
Key takeaway: red marrow makes blood cells; yellow marrow stores fat.