Key Stage 3 science · Biology – Structure and function of living organisms
KS3 bones and muscles: the skeleton, joints and antagonistic pairs
In Key Stage 3 biology, pupils study how the skeleton and muscles work together. They learn the four main jobs of the skeleton (support, protection, movement and making blood cells), how bones, joints, tendons and muscles combine to produce movement, how muscles work in antagonistic pairs, and how the forces that muscles exert can be measured. This is often called biomechanics.
Skeleton and muscles, explained
The human skeleton has four main functions. It supports the body and gives it shape. It protects delicate organs: the skull around the brain, the ribs around the heart and lungs, and the vertebrae around the spinal cord. It allows movement, because muscles pull on bones. And it makes blood cells, since red and white blood cells are produced in the bone marrow, a soft tissue inside certain bones. Pupils often match each job to a body part and give an example.
Movement happens at joints. In a hinge joint, such as the elbow or knee, the bones move back and forth in one plane only, like a door. In a ball and socket joint, such as the shoulder or hip, the bones can move in many directions and rotate. Bones meet at joints with smooth cartilage covering their ends, to reduce friction and absorb shocks, and are held in place by ligaments, which connect bone to bone. Tendons connect muscle to bone. A fluid called synovial fluid lubricates the joint, much as oil does in a hinge.
Muscles can only pull, by contracting and getting shorter. They cannot push themselves back to a longer length. That is why they are arranged in antagonistic pairs, with one muscle on each side of a joint working against the other. In the upper arm, the biceps contracts to bend the elbow while the triceps relaxes, and the triceps contracts to straighten the arm while the biceps relaxes. Another pair is the hamstrings and quadriceps, which bend and straighten the knee. Pupils learn that the contracting muscle is the one doing the work, and the relaxed one is stretched as the limb moves.
Biomechanics looks at the body as a set of levers. The bone is the lever, the joint is the pivot, and the muscle supplies the force. The curriculum asks pupils to measure the force exerted by different muscles. Schools choose how, for example by pulling on a newton meter (force meter) with the forearm, pressing down on bathroom scales, or using a grip-strength meter. Pupils then compare different muscles and different people, and think about what makes the test fair. Readings can be compared, averaged and graphed, which links the topic back to working scientifically.
Words your child will hear
- Bone marrow
- The soft tissue inside some bones where red and white blood cells are made.
- Joint
- A place where two or more bones meet, such as the elbow, hip or knee.
- Ligament
- A strong, slightly stretchy band that joins one bone to another at a joint.
- Tendon
- A strong cord that joins a muscle to a bone.
- Antagonistic muscles
- A pair of muscles that work in opposite ways, so that one contracts as the other relaxes.
- Contract
- To shorten and pull. Muscles can do this but cannot push.
- Biomechanics
- The study of how forces act on the living body, especially the skeleton and muscles.
What the National Curriculum says
The programme of study for Key Stage 3 says:
- Pupils should be taught about the structure and functions of the human skeleton, to include support, protection, movement and making blood cells
- Pupils should be taught about biomechanics – the interaction between skeleton and muscles, including the measurement of force exerted by different muscles
- Pupils should be taught about the function of muscles and examples of antagonistic muscles
Objectives from the National Curriculum in England, © Crown copyright, used under the Open Government Licence v3.0.
Try it: explaining how the elbow bends and straightens
Explain how the arm bends at the elbow and then straightens again, using the terms antagonistic, contract and relax.
- Name the parts. The biceps is on the front of the upper arm, and the triceps is on the back. Each is joined by tendons to bones, one end in the shoulder area and the other in the forearm. The elbow is a hinge joint.
- Bending the arm. The biceps contracts, becoming shorter and thicker, and pulls the forearm up. At the same time the triceps relaxes and is stretched.
- Straightening the arm. The triceps contracts and pulls the forearm back down. At the same time the biceps relaxes and lengthens.
- Explain why two muscles are needed. Muscles can only pull, never push. One muscle on its own could bend the arm but could not straighten it again, so a second muscle on the opposite side is needed to pull it back. These two are an antagonistic pair.
- Link to biomechanics. The forearm is a lever, the elbow is the pivot and the muscle supplies the force. The muscle is attached close to the elbow, so it has to pull with a much larger force than the weight in the hand it is lifting.
To bend the elbow the biceps contracts and the triceps relaxes. To straighten it the triceps contracts and the biceps relaxes. Muscles can only pull, so they must work as an antagonistic pair.
Common mistakes and how to help
Thinking muscles can push bones as well as pull
Pushing a door open feels like something muscles do, and pupils picture a muscle “pushing” the arm straight.
What helps: Muscles only shorten and pull. Straightening the arm is done by a different muscle, the triceps, pulling on the other side of the joint.
Imagining bones are dry, dead and solid all the way through
The skeletons seen in classrooms and films are dried bone, and the hard outside gives no hint of what is inside.
What helps: Living bones have a blood supply, grow, repair themselves after a break and contain marrow. Talk about a broken bone healing.
Mixing up ligaments and tendons
Both are strong and white and the names sound similar, so they are easy to swap.
What helps: Remember “ligament: bone to bone, tendon: muscle to bone”.
Believing red blood cells are made in the heart or in the blood itself
Blood is so closely linked with the heart, and the idea that bones do anything other than support seems surprising.
What helps: Explain that blood cells are made in the marrow of certain bones, and then released into the blood. This is one of the four skeleton functions.
How to help at home
- Feel your muscles work. Hold a tin of beans in one hand and rest the fingertips of the other hand on the front of that upper arm, then slowly bend and straighten the elbow. Move the fingertips to the back of the arm and repeat. Which muscle goes hard and bulges as the arm bends, and which as it straightens? Try the thigh and knee as well.
- Make a model arm. Using two strips of thick card joined with a paper fastener, and two elastic bands or ribbons for the biceps and triceps, build a working arm. Pull on each band in turn to bend and straighten the arm. Ask your child what happens if one band is removed, and where the model is unlike a real arm.
- Measure muscle force on the scales. Put bathroom scales on a table and press down on them with the palm of one hand, then the other, keeping the arm in the same position each time. A reading of 10 kg means a push of roughly 100 N. Compare hands and family members, and ask what must be kept the same for a fair test. Then hold a light bag at arm’s length and close to the body to feel how the lever changes the effort needed. Stop if anything hurts.
- Count the joints. Move through a stretch routine in the living room and name each joint: hinge or ball and socket? Which muscles are pulling in each case? It makes a gentle warm-up before exercise or sport.
Related topics
Builds on
- Year 6: Classifying living things
More on biology – Structure and function of living organisms
- Key Stage 3: Cells and organisation
- Key Stage 3: Breathing, gas exchange and drugs
- Key Stage 3: Diet and digestion
- Key Stage 3: Reproduction in humans and plants
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