Start on the Joint & skeleton tab. On the right is a synovial joint — the freely movable kind, like your knee or elbow. Find the smooth articular cartilage capping each bone, the slippery synovial fluid in the gap, the capsule enclosing it, and the ligaments strapping bone to bone. ▶ Play to label each part in turn. These features all cut friction and absorb shock.
Stay on this tab. ⏭ Step to compare the two synovial joints: the ball-and-socket (shoulder, hip) lets the limb swing in all directions and rotate, while the hinge (elbow, knee) bends in one plane only, like a door. Recall the skeleton is axial (skull, spine, ribs) plus appendicular (girdles & limbs). 🔀 New to flip between the joint types.
Switch to the Antagonistic muscles tab — an arm at the elbow. ▶ Play and watch the front biceps contract (it swells and shortens) while the back triceps relaxes: the forearm rises — the elbow flexes. Then the reverse — triceps contracts, biceps relaxes — and the arm extends. A muscle can only pull, so it needs an antagonist to move the bone back.
Keep watching the arm. The biceps is the flexor — it bends the elbow; the triceps is the extensor — it straightens it. Each muscle pulls on bone through a tendon. ⏭ Step to freeze the flexed and extended positions and read which muscle is shortened (red) and which is relaxed (pale) at each moment.
Now the Sliding filament tab. We have zoomed right down: muscle → fibre → myofibril → one sarcomere, the unit between two Z-lines. The thin red actin filaments are anchored to the Z-lines; the thick blue myosin sits in the centre with little projecting heads. ↻ Reset to see the sarcomere relaxed and wide before contraction.
▶ Play the contraction. Ca²⁺ uncovers the binding sites on actin; the myosin heads grab on as cross-bridges and tilt — the power stroke — dragging the actin inward. ATP then detaches and re-cocks each head to repeat the cycle. Watch the Z-lines move in so the whole sarcomere shortens — the filaments slide, they don't shrink. ⏭ Step through one full cross-bridge cycle.