Form 4 · Common mistakes

Support and Movement in Humans and Animals, common mistakes

The mistakes SPM students make on Support and Movement in Humans and Animals, why each one loses marks, and the correct version.

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The mistakes, why they lose marks, and the fix

Common mistake	Why it loses marks	What earns the mark
Saying a muscle can push a bone.	Students picture muscles acting like rods that extend.	Muscles only pull by contracting, so they act in antagonistic pairs where one pulls the bone back.
Writing that the biceps and triceps contract together.	Both are active in an arm movement, so they seem to work at once.	They are antagonistic: when one contracts the other relaxes, otherwise the joint could not move.
Confusing a tendon with a ligament.	Both are tough connective bands near joints.	A tendon joins muscle to bone; a ligament joins bone to bone.
Calling an exoskeleton an internal skeleton.	The prefix is overlooked.	An exoskeleton is on the outside, as in insects; an endoskeleton is inside, as in humans.
Saying cartilage and bone are the same tissue.	Both are skeletal, so they are treated as identical.	Bone is hard and mineralised for support and protection; cartilage is softer and flexible, cushioning joints and forming the outer ear.
Describing osteoporosis as a disease of the joints.	It is grouped loosely with arthritis as a bone-and-joint problem.	Osteoporosis is a loss of bone density; arthritis is the separate condition that inflames a joint.
Assuming every animal moves with the same kind of muscle action.	The arm example is applied to all animals.	Locomotion matches the habitat: a fish bends its body against water with myotomes, a bird beats hollow wings against air, a mammal swings limbs against ground.
Thinking a keeled sternum is found in all vertebrates.	It is assumed to be an ordinary chest bone.	A keeled sternum is mainly a flight adaptation in birds, giving extra surface area for the large flight muscles.
Writing that one strong muscle can move a joint both ways.	Strength is confused with direction of pull.	One muscle pulls a bone in only one direction; a second antagonistic muscle is needed to pull it back.
Using dislocation and sprain as if they mean the same injury.	Both involve pain around a joint.	A dislocation is a bone forced out of a joint; a sprain is an overstretched or torn ligament with the bone still in place.
Thinking an exoskeleton grows continuously with the animal.	It is assumed to stretch like skin.	An exoskeleton is rigid and cannot expand, so an insect must moult, shedding the old casing and hardening a larger one.
Confusing the periosteum with the marrow.	Both are parts of a long bone.	The periosteum is the outer covering carrying blood vessels and nerves; the marrow is the inner tissue that makes blood cells and stores fat.

How to avoid these mistakes

Compare the three skeleton types with an animal example each.
Draw the arm and explain the biceps-triceps antagonistic action.
Learn the difference between tendon and ligament and between joint types.
Draw a fish and a bird side by side, labelling the structures that suit each one to its own habitat.
Make a two-column table contrasting bone and cartilage by hardness, flexibility and one example each.
Write a model answer stating a cause, an effect and a preventive step for arthritis and for osteoporosis.

Frequently asked questions

How do antagonistic muscles move the arm?

Muscles can only pull, 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 undoes the action of the other, the joint can move both ways.

What is the difference between a tendon and a ligament?

A tendon is a tough band that joins a muscle to a bone, so that when the muscle contracts it pulls the bone. A ligament joins one bone to another at a joint and holds the bones together while allowing controlled movement. Tendons connect muscle to bone; ligaments connect bone to bone.

What are the three types of skeleton?

A hydrostatic skeleton uses fluid pressure inside the body, as in an earthworm. An exoskeleton is a hard outer covering, as in insects and crabs, which also protects the body. An endoskeleton is an internal framework of bone or cartilage, as in humans and other vertebrates, which supports the body and provides attachment for muscles.