Lesson focus: Antagonistic muscles move bones because muscles pull and produce turning effectHow much a force makes something turn, like pushing a door handle far from the hinge. around joints.
Explain flexionBending a joint, such as when your arm bends at the elbow., extensionStraightening a joint, such as when your arm straightens at the elbow. and stabilisation at the elbow.
Build on earlier learning
Movement: jointsRevisit tendons and joint structure before tracing muscle force.
You will be successful when you can…
- State that muscles pull rather than push.
- Describe biceps/triceps action in flexion and extension.
- Explain tendon force transmission.
- Recognise that turning effect depends on force and distance from joint.
Words that carry the science
- contraction
- Active development of tension, often with muscle shortening.
- antagonistic pair
- Muscles producing opposite actions at a joint.
- flexion
- Movement decreasing the angle at a joint.
- extension
- Movement increasing the angle at a joint.
- turning effect
- Rotational effect depending on force and perpendicular distance from pivot.
Turn muscle tension into joint movement
01Muscles pull through tendons
When a muscle contracts it develops tension. A tendonA strong cord of tissue that joins a muscle to a bone. transmits the pull to bone, producing a turning effectHow much a force makes something turn, like pushing a door handle far from the hinge. about the joint. Muscles do not push.
02Opposite actions need opposite muscles
During elbow flexionBending a joint, such as when your arm bends at the elbow., biceps contracts and triceps relaxes. During extensionStraightening a joint, such as when your arm straightens at the elbow., triceps contracts and biceps relaxes. They form an antagonistic pairTwo muscles that work in opposite directions: when one contracts, the other relaxes, like the biceps and triceps..
03Stability uses balanced moments
Both muscles can develop tension to stabilise a joint. Stability depends on balanced turning effects, not necessarily equal forces, because attachment distances differ.
Antagonistic muscles at the elbow
Muscles can only pull. The biceps contracts to bend the arm and the triceps contracts to straighten it, so they work as an antagonistic pair.
Darker red: the muscle that is contracting. The biceps is on the front of the upper arm, the triceps at the back.
Controls
Lengths are illustrative. Tendons join muscle to bone; the bones meet at the elbow joint.
Explain elbow flexion
A person lifts the forearm towards the upper arm.
- Biceps contracts and develops tension.
- Its tendon pulls on the radius/forearm.
- The pull produces a turning effect around the elbow.
- Triceps relaxes sufficiently, allowing flexionBending a joint, such as when your arm bends at the elbow..
How to make the reasoning visible: Name muscle action, connection and resulting rotation.
Movement sequence
Complete the opposing actions.
| Movement | Agonist action | Antagonist action | Joint angle |
|---|---|---|---|
| Elbow flexion | Biceps contracts | Triceps relaxes | Decreases |
| Elbow extension | Triceps contracts | Biceps relaxes | Increases |
| Hold position | Both may develop tension | Turning effects balance | Constant |
| Lower load slowly | One muscle controls lengthening | Opposition is coordinated | Changes under control |
Why can a muscle not push a bone?
Why need stabilising forces not be equal?
Use the table. During elbow extension, which muscle contracts and which relaxes?
Check your understanding
Answer from memory, then check against the suggested answer.
Which muscle contracts in elbow flexion?
Which muscle contracts in elbow extension?
What transmits force to bone?
What is an antagonistic pair?
What happens to a muscle when it contracts?
A biceps tendon ruptures. Predict the effect on flexion.
Explain co-contraction during joint stabilisation.
Why do muscles have to work in pairs?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
Describe how an antagonistic muscle pair extends the elbow.
(4)Total for this question: 4 marks
Command word: Sequence the two muscles and the resulting joint movement.