Support and Movement in Humans and Animals, worked answers

Muscles can only **pull by contracting**, not push, so they work in **antagonistic pairs**. To **bend** the arm, the **biceps contracts** and pulls the forearm up while the **triceps relaxes**. To **straighten** the arm, the **triceps contracts** and pulls the forearm down while the **biceps relaxes**. Because one muscle reverses the action of the other, the **hinge joint** at the elbow can move in both directions.

muscle can only pullantagonistic pairbiceps contracts, triceps relaxestriceps contracts, biceps relaxeshinge joint

A muscle produces force only by **contracting**, which **shortens** it and **pulls** its two attachment points (the **tendons**) closer together; it has **no way to lengthen actively to push** a bone. Therefore one muscle can pull a bone in only **one direction**. A **second, antagonistic muscle** is needed to contract and **pull the joint back** the other way once the first muscle relaxes.

contraction only shortens and pullscannot pushone direction onlyantagonistic muscle neededpull back the other way

A fish has a **flexible vertebral column** and **blocks of muscle called myotomes** arranged along **both sides of the body**. The myotomes contract in **sequence**, bending the body into **side-to-side waves** that **push against the dense water** and drive the fish forward. The **tail fin** provides most of the **forward thrust**, while the other **fins steer, balance and brake**. A **streamlined body** reduces **drag** as the fish moves through the water.

flexible vertebral columnmyotomes on both sidescontract in sequencebody waves push against watertail fin thrustfins steer and balancestreamlined reduces drag

A bird bones are largely **hollow**, which makes the skeleton **light** so less mass has to be lifted, and extensively **fused**, which reduces the number of separate joints and makes the frame **rigid and stable**. The **keeled sternum**, a ridge of bone on the chest, gives a **large surface area** for attaching the **powerful flight muscles**. These muscles pull the **wings downward** to produce **lift and thrust**, then relax as smaller muscles raise the wings again.

hollow bones reduce massfused bones give rigiditykeeled sternumlarge surface for muscle attachmentflight muscles pull wings down for lift

A **hydrostatic skeleton** uses **fluid pressure** held inside the body, and muscles squeeze against the fluid to move, as in the **earthworm**. An **exoskeleton** is a **hard outer casing** that also **protects** the soft body, as in **insects and crabs**, but because it is rigid the animal must **moult** to grow. An **endoskeleton** is an **internal framework of bone and cartilage** that supports the body and **anchors the muscles**, as in **humans and other vertebrates**, and it grows steadily with the animal.

hydrostatic uses fluid pressure (earthworm)exoskeleton is outer casing (insect)must moult to growendoskeleton is internal (human)grows with the animal

**Cause:** low intake of **calcium or vitamin D** (also hormonal change or ageing), so bone is **broken down faster than it is rebuilt**. **Effect:** bone **density falls**, making bones **fragile** so they **fracture easily**, even from a minor fall. **Preventive measure:** regular **weight-bearing exercise** together with an **adequate calcium and vitamin D** intake helps keep bone dense.

low calcium or vitamin Dbreakdown faster than rebuildingbone density fallsfractures easilyweight-bearing exercise and calcium

A **tendon** joins a **muscle to a bone**, so contraction pulls the bone; a **ligament** joins **one bone to another** and holds a joint together. A **dislocation** is a **bone forced out of its joint**, often **tearing ligaments**; a **sprain** is the milder **overstretching or partial tearing of a ligament** with the **bone still in place**.

tendon joins muscle to boneligament joins bone to bonedislocation is bone out of jointsprain is torn ligament, bone in place