AS Level PE 8386 · Area 1 · Joints, Movements and Muscles

Functions of Muscles

A movement is never produced by one muscle. Cambridge wants you to explain three roles — agonist, antagonist and fixator — and to name them in a real action. Most students can define an agonist and then lose the fixator marks, because the fixator is the muscle doing nothing visible.

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Syllabus coverage

  • Explain the function of agonists using examples
  • Explain the function of antagonists using examples
  • Explain the function of fixators using examples
  • Analysis of actions (1.5.1) must identify working muscles and their functions as in 1.2.1, including named antagonists and fixators

Part of the AS Level PE 8386 syllabus 8386.

Antagonistic muscle action at the elbowFlexionagonist: bicepsantagonistFbicepstricepsBiceps contract (agonist), triceps relax(antagonist)The elbow angle decreases — the biceps shortenconcentrically, the triceps lengthen undercontrol.Extensionagonist: tricepsantagonistFbicepstricepsTriceps contract (agonist), biceps relax(antagonist)The elbow angle increases — the triceps shortenconcentrically, the biceps lengthen under control.Isotonic contractions move the joint (concentric shortens the muscle, eccentric lengthens it under load). Isometriccontractions hold the joint at a fixed angle.
An antagonistic pair at the elbow: the agonist shortens to produce the movement while the antagonist lengthens under control, with fixators holding the scapula steady.

Muscles work in teams, not on their own

A joint can only move if something pulls the bone. Muscles can only pull, never push, so a joint needs a muscle on each side of it: one to produce the movement and one to allow it.

Add the muscles that must stay still so the movement happens cleanly, and you have the three roles Cambridge examines: agonist, antagonist and fixator.

The same muscle can take different roles in different actions. Biceps brachii is the agonist when the elbow flexes in a pull-up action and the antagonist when the elbow extends.

  • Agonist: produces the movement
  • Antagonist: allows and controls the movement from the other side of the joint
  • Fixator: holds another part of the body still so the agonist works on one joint only

The agonist (prime mover)

The agonist is the muscle that contracts to produce the movement. Another name for it is the prime mover.

If a question asks you for the muscle responsible for a movement, it is asking for the agonist, and Cambridge has already told you which muscle it wants in the 1.1.3 table.

The agonist works by pulling on its insertion, the bone further from the body's centre. That pull is what creates the movement at the joint.

MovementJointAgonist
Elbow flexionElbowBiceps brachii
Elbow extensionElbowTriceps brachii
Knee extensionKneeRectus femoris
Knee flexionKneeBiceps femoris
Hip extensionHipGluteus maximus
Hip flexionHipIliopsoas
Plantar flexionAnkleGastrocnemius
Shoulder horizontal flexionShoulderPectoralis major
One mark is often available just for naming the agonist. Learn these muscle names cold — they are the cheapest marks in the whole area.

The antagonist

The antagonist is the muscle on the opposite side of the joint to the agonist, and it produces the opposite movement.

Its job is to relax and lengthen as the agonist contracts, so the agonist's pull is not resisted. Without the antagonist lengthening, the joint could not move at full range.

It has a second job. By lengthening under control, the antagonist also smooths and controls the movement — which is why a skilled performer's action looks fluid rather than jerky.

A useful sentence to remember: the agonist and antagonist are an antagonistic pair — they always lie on opposite sides of the joint and produce opposite movements.

Name the antagonist as well as the agonist whenever a question says "the muscles involved". A 2-mark question about a movement usually wants one mark for each muscle.

The fixator

A fixator is a muscle that stabilises a joint or a bone so that the agonist can work effectively. It holds part of the body still.

Think about a bicep curl: the biceps brachii pulls on the radius, but its origin is on the scapula. If the scapula moved every time the arm lifted, some of the force would be wasted moving the shoulder blade instead of the forearm. The muscles that hold the scapula steady are acting as fixators.

A fixator, then, is not producing the movement and not opposing it. It is removing unwanted movement elsewhere so the agonist's pull goes where it is needed. The result is a more accurate, more powerful action and a joint that is less at risk of injury.

  • Stabilises the origin of the agonist, giving it a fixed base to pull from
  • Prevents unwanted movement at another joint
  • Increases the accuracy and efficiency of the movement
  • Protects the joint by keeping it in the correct alignment
Some textbooks use the word synergist for a muscle that assists an agonist or cancels out an unwanted movement. Cambridge 8386 asks only for agonists, antagonists and fixators, so use those three terms in your answers.

All three roles in one action

Cambridge likes a whole-action question: describe the roles of the muscles in a named movement. The way to score full marks is to give one point per role, each one named and explained.

Take an Association football player extending the knee to strike a ball. The agonist is rectus femoris, contracting to extend the knee. The antagonist is biceps femoris, relaxing and lengthening to allow the knee to extend. The fixators are the muscles that hold the pelvis and trunk steady, so that rectus femoris — which crosses both the hip and the knee — pulls at the knee rather than moving the hips.

RoleMuscleWhat it is doingWhy it matters
AgonistRectus femorisContracting and shortening to extend the kneeProduces the force that strikes the ball
AntagonistBiceps femorisRelaxing and lengthening on the other side of the kneeAllows the knee to extend through its full range instead of being resisted
FixatorMuscles stabilising the pelvis and trunk (for example the abdominals)Holding the pelvis and trunk stillStops the two-joint rectus femoris wasting force by moving the hips, so the strike is accurate and powerful
Agonist (second joint)Gluteus maximusContracting to extend the hip in the follow-throughAdds the drive and power behind the strike

Turning muscle function knowledge into marks

Definitions alone will not score in an explain question. Each point needs the role, the muscle name and a consequence for the movement.

Compare these two answers about elbow extension. "The triceps brachii is the agonist" names a muscle and is worth one mark on an identify question but nothing on an explain question. "The triceps brachii is the agonist because it contracts and shortens to extend the elbow, and the biceps brachii is the antagonist because it relaxes and lengthens to allow that extension" contains two complete points.

Watch the wording of the question: "name", "state" and "identify" want the muscle only, while "explain" and "analyse" want the role plus the effect.

  • Name the muscle
  • Give its role (agonist, antagonist or fixator)
  • Say what happens to its length or the joint it holds
  • Say what that does for the performer

Key terms you must be able to define

Agonist
The muscle that contracts to produce the movement; also called the prime mover.
Antagonist
The muscle on the opposite side of the joint to the agonist, which produces the opposite movement and relaxes and lengthens to allow the agonist's movement.
Antagonistic pair
Two muscles on opposite sides of a joint that produce opposite movements, working together so the joint can move in both directions.
Fixator
A muscle that stabilises a joint or bone so that the agonist can work effectively, preventing unwanted movement.
Origin
The point where a muscle attaches to the more fixed, stationary bone, which acts as the base for the muscle's pull.
Insertion
The point where a muscle attaches to the bone that moves when the muscle contracts.
Prime mover
Another name for the agonist: the muscle mainly responsible for producing a movement.

Applied to your sport

Cambridge requires you to apply this knowledge to physical activities. Use these as models, then write your own for the sport you play.

Rugby union

During the drive of a scrum the quadriceps and gluteals act as agonists while the deep trunk muscles work as fixators, stabilising the spine so the pushing force goes into the opposition rather than bending the player's back.

Netball

In a chest pass the pectoralis major is the agonist for horizontal flexion at the shoulder, the posterior deltoid is the antagonist relaxing and lengthening on the other side of the shoulder joint, and the muscles holding the scapula still act as fixators.

Volleyball

When a player jumps to spike, gastrocnemius and the quadriceps are agonists for plantar flexion and knee extension, the opposing muscles lengthen as antagonists, and the abdominal muscles fix the trunk so the leg drive is not lost to a bending spine.

Badminton

Through an overhead smash the triceps brachii is the agonist for elbow extension and the biceps brachii is the antagonist; in the recovery swing the roles reverse as the biceps brachii becomes the agonist to flex the elbow again.

Olympic weightlifting

Pressing a weight overhead makes the muscles surrounding the scapula work as fixators, holding the shoulder girdle firm so the deltoids can drive the arms upwards.

Flashcards

Click a card to flip it. Mark honestly — the tracker uses this.

Q1What is an agonist?
The muscle that contracts to produce a movement; also called the prime mover.
Q2What is an antagonist?
The muscle on the opposite side of the joint to the agonist, producing the opposite movement, which relaxes and lengthens to allow the movement to happen.
Q3What is a fixator?
A muscle that contracts to stabilise a bone or joint so the agonist can work effectively without unwanted movement elsewhere.
Q4During elbow flexion, which muscles are the antagonistic pair?
Agonist: biceps brachii. Antagonist: triceps brachii.
Q5During knee extension, which muscles are the antagonistic pair?
Agonist: rectus femoris. Antagonist: biceps femoris.
Q6How many roles does one muscle have at one joint during one movement?
One. A muscle can only be the agonist, the antagonist or a fixator for that movement — but its role changes when the direction of movement changes.
Q7Why does the body need an antagonist at all?
Muscles can only pull, not push. Without the antagonist lengthening, the agonist's contraction could not move the joint through its full range and the movement would not be controlled.
Q8Give a sporting example of a fixator at work.
In a bicep curl or a chest pass, the muscles holding the scapula steady act as fixators so the agonist pulls only on the arm.
Q9Why does a fixator make a movement more accurate?
It removes unwanted movement at another joint, so the agonist's force is directed exactly where it is needed rather than being partly wasted.
Q10What is the difference between a muscle's origin and its insertion?
The origin is the attachment on the more fixed bone, acting as the stable base; the insertion is the attachment on the bone that moves when the muscle contracts.
Q11Which muscle is the agonist for shoulder horizontal flexion, and which is the antagonist?
Agonist: pectoralis major. Antagonist: posterior deltoid.
Q12In one sentence, explain the three roles working together in a kick.
Rectus femoris is the agonist, contracting to extend the knee; biceps femoris is the antagonist, lengthening to allow it; and the muscles fixing the pelvis hold it still so the rectus femoris works only at the knee.
0 of 12 marked as known

Exam-style questions

6 questions, 17 marks. Write your answer, then reveal the mark scheme and tick each point you actually made. Answering honestly here is what makes the rest of the site useful.

StateAO11 mark
State the term used for the muscle that acts as the prime mover of a movement.

Mark scheme — tick each point you actually made

Examiner tip: One mark, one word. If the question has already used the words "prime mover", the answer cannot be "prime mover".
DescribeAO12 marks
Describe the role of the antagonist during elbow flexion.

Mark scheme — tick each point you actually made

Examiner tip: Two marks means two separate ideas. "It relaxes and lengthens so the elbow can bend" repeated twice only scores once.
IdentifyAO12 marks
Identify the agonist and the antagonist at the elbow joint during extension.

Mark scheme — tick each point you actually made

Examiner tip: Label which is which. Writing the two muscles without saying which produces the movement can lose a mark.
ExplainAO23 marks
Explain the role of fixators during a Table tennis forehand drive.

Mark scheme — tick each point you actually made

Examiner tip: The fixator mark is usually worth more than naming it. Explain what it holds still and what that gives the performer.
ExplainAO24 marks
Explain how the agonist, antagonist and fixators work together when a Netball player performs a chest pass.

Mark scheme — tick each point you actually made

Examiner tip: One point per role per joint. Each mark needs the muscle named and its role stated.
AnalyseAO25 marks
Analyse the roles of the muscles involved when an Association football player extends the knee to strike a ball.

Mark scheme — tick each point you actually made

Examiner tip: Notice the two roles the biceps femoris takes in the same action. Analyse answers reward this kind of change over the phases of a movement.

Exam technique for this topic

  • Name the muscle, not just the role. "The fixator" alone scores nothing; "the muscles that hold the scapula still act as fixators" scores.
  • If the question says "the muscles involved", always give the agonist and the antagonist. They are an antagonistic pair and examiners expect both.
  • Use the word "fixator" whenever a muscle is holding something still rather than producing movement — it is the role most students forget to mention.
  • Learn the agonist for each movement from the 1.1.3 table, then the antagonist is simply the muscle that produces the opposite movement at that joint.

Why students lose marks here

A student wroteThe antagonist is the muscle that helps the agonist to contract more powerfully.
Why it loses marksThis describes a synergist, not an antagonist. The antagonist never assists the movement — it produces the opposite one.
The answer that scoresThe antagonist is the muscle on the opposite side of the joint to the agonist. It relaxes and lengthens to allow the agonist to shorten and produce the movement, and it controls the return of the joint.
A student wroteDuring elbow extension the biceps brachii is the agonist because it is the elbow muscle.
Why it loses marksNaming a muscle on the basis of where it sits, rather than what the movement needs, gives the wrong role. Biceps brachii flexes the elbow, so it cannot be the agonist for extension.
The answer that scoresDuring elbow extension the triceps brachii is the agonist, contracting to extend the elbow, and the biceps brachii is the antagonist, relaxing and lengthening to allow the extension.
A student wroteA fixator is a muscle that does not move at all.
Why it loses marksFixators are not inactive — they contract to hold a bone steady. Saying they do not move describes the joint, not the muscle's role, and misses the mark.
The answer that scoresA fixator contracts to stabilise the origin of the agonist, holding a bone or joint still so that unwanted movement elsewhere does not waste the agonist's force.
A student wroteThe same muscle is always the agonist in every action it takes part in.
Why it loses marksRoles are not fixed to muscles. A muscle's role depends on the direction of the movement, and students who memorise one role for one muscle lose marks on the follow-through phases of an action.
The answer that scoresA muscle's role depends on the movement. The biceps brachii is the agonist during elbow flexion in a pull action but becomes the antagonist during the return as the elbow extends.

Where this fits in AS Level PE 8386

This page is one topic of Cambridge AS Level PE 8386. AS Level PE 8386 is the course home: every area and every topic in Cambridge's own order, the papers you sit and what each one tests, and the exam-practice tools. The full topic list, with each topic linked to its revision page, is on the AS Level PE 8386 syllabus page.