AQA · A-Level · Biology
Course version: 7402 · Version from Sept 2016
Muscles act in antagonistic pairs against an incompressible skeleton.
Full specification wording
Skeletal muscles are stimulated to contract by nerves and act as: Muscles act in antagonistic pairs against an incompressible skeleton.
Muscles can only pull.
Checked against the AQA 7402 specification (opens in a new tab) on .
- Point 1 of 6
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Muscles can only pull. A muscle shortens when it contracts and generates force, but it cannot push itself back to its original length, so something else must stretch it, and that something is another muscle pulling the same bone the other way. That is an antagonistic pair: as one muscle, the agonist or prime mover, contracts, its partner, the antagonist, relaxes and is stretched. At the elbow the biceps contracts to flex the joint while the triceps relaxes; to extend it, the triceps contracts and the biceps relaxes. The bones between them are incompressible, so the pull is transmitted rather than absorbed, and the joint acts as a pivot so the bone works as a lever. Tendons, made of collagen and almost inelastic, attach muscle to bone so that contraction moves the bone rather than stretching the attachment. Without a rigid skeleton the force would simply deform the body.
What earns the marks
- Stating that a muscle can only shorten and pull, and cannot push.
- Describing an antagonistic pair, with one muscle contracting while the other relaxes.
- The contracting muscle stretching its relaxed partner back to its original length.
- Naming a correct pair acting at the same joint, such as the biceps and triceps at the elbow.
- The skeleton being incompressible, so the force of contraction is transmitted and the bone acts as a lever.
Common mistakes and how to put them right
Saying the antagonist contracts in the opposite direction, when it relaxes.
Stating that a relaxed muscle pushes the bone back into position.
Claiming tendons are elastic and stretch during contraction, which would waste the force generated.
Naming a pair of muscles that do not act on the same joint.
Describing the skeleton only as support, without linking incompressibility to transmission of force.
Exam tips
- Answer in pairs: for every muscle you say contracts, name the one that relaxes.
- Use the word antagonistic explicitly, since it is often the marking word.
- If given an unfamiliar animal or joint, apply the same reasoning; the mark is for the logic, not the species.
Try the lesson
The start of the Skeletal muscles are stimulated to contract by nerves and act as: Muscles act in antagonistic pairs against an incompressible skeleton. lesson, made from checked teaching notes for this specification point.
Have a go first
Which statement describes how antagonistic muscles produce movement at a joint?
Listen
Muscles can only pull. A muscle shortens when it contracts and generates force, but it cannot push itself back to its original length, so something else must stretch it, and that something is another muscle pulling the same bone the other way. That is an antagonistic pair: as one muscle, the agonist or prime mover, contracts, its partner, the antagonist, relaxes and is stretched. At the elbow the biceps contracts to flex the joint while the triceps relaxes; to extend it, the triceps contracts and the biceps relaxes. The bones between them are incompressible, so the pull is transmitted rather than absorbed, and the joint acts as a pivot so the bone works as a lever. Tendons, made of collagen and almost inelastic, attach muscle to bone so that contraction moves the bone rather than stretching the attachment. Without a rigid skeleton the force would simply deform the body. A strong answer earns marks for these points. stating that a muscle can only shorten and pull, and cannot push. describing an antagonistic pair, with one muscle contracting while the other relaxes. the contracting muscle stretching its relaxed partner back to its original length. naming a correct pair acting at the same joint, such as the biceps and triceps at the elbow. Exam tip: Answer in pairs: for every muscle you say contracts, name the one that relaxes.
Key words
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antagonistic pairs
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agonist and antagonist
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muscles pull only
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incompressible skeleton
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See every definition
- antagonistic pairs
- Muscles that work against each other, so that as one contracts the other relaxes and is stretched.
- agonist and antagonist
- The agonist or prime mover contracts, while its partner relaxes and is stretched.
- muscles pull only
- A muscle shortens when it contracts and generates force, but it cannot push itself back to its original length.
- incompressible skeleton
- The bones are incompressible, so the pull is transmitted rather than absorbed and the force would otherwise simply deform the body.
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