Antagonistic Muscle Action

This lesson covers antagonistic muscle action as required by the AQA GCSE PE specification (3.1.1.1). You need to understand that muscles can only pull (contract) — they cannot push. Because of this, muscles work in pairs to produce movement at a joint. When one muscle contracts, the opposite muscle relaxes. This is called antagonistic muscle action.

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Why Muscles Work in Pairs

A single muscle can only produce movement in one direction. To move a joint and then return it to its original position, you need two muscles working on opposite sides of the joint.

• When a muscle contracts (shortens), it pulls on the bone and produces movement.
• The muscle on the opposite side of the joint must relax (lengthen) to allow that movement to happen.
• To reverse the movement, the roles swap — the first muscle relaxes and the second contracts.

This is why muscles always work in antagonistic pairs.

flowchart LR
    A["Muscle A Contracts"] --> B["Produces Movement in Direction 1"]
    B --> C["Muscle B Relaxes to Allow Movement"]
    D["Muscle B Contracts"] --> E["Produces Movement in Direction 2"]
    E --> F["Muscle A Relaxes to Allow Movement"]

    style A fill:#4a90d9,color:#fff
    style D fill:#e67e22,color:#fff
    style B fill:#27ae60,color:#fff
    style E fill:#27ae60,color:#fff

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Key Terminology

Term	Definition
Agonist (prime mover)	The muscle that contracts to produce the movement
Antagonist	The muscle that relaxes to allow the movement to happen
Antagonistic pair	Two muscles that work on opposite sides of a joint, taking turns as agonist and antagonist

Exam Tip: The terms "agonist" and "prime mover" mean the same thing. Either term can be used in the exam. The agonist is always the muscle that is contracting to produce the current movement.

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Antagonistic Pairs at the Elbow

The biceps and triceps are the classic antagonistic pair at the elbow joint.

Flexion at the Elbow (Bending the Arm)

• Agonist: Biceps (contracts)
• Antagonist: Triceps (relaxes)
• Movement produced: Flexion — the angle at the elbow decreases
• Example: The upward phase of a bicep curl; pulling back the string of a bow in archery

Extension at the Elbow (Straightening the Arm)

• Agonist: Triceps (contracts)
• Antagonist: Biceps (relaxes)
• Movement produced: Extension — the angle at the elbow increases
• Example: Throwing a ball; the pushing phase of a press-up; a boxing punch

Movement	Agonist	Antagonist	Sporting Example
Flexion at elbow	Biceps	Triceps	Bicep curl (upward phase)
Extension at elbow	Triceps	Biceps	Throwing a javelin

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Antagonistic Pairs at the Knee

The quadriceps and hamstrings are the antagonistic pair at the knee joint.

Extension at the Knee (Straightening the Leg)

• Agonist: Quadriceps (contracts)
• Antagonist: Hamstrings (relaxes)
• Movement produced: Extension — the angle at the knee increases
• Example: Kicking a football; jumping; the upward phase of a squat

Flexion at the Knee (Bending the Leg)

• Agonist: Hamstrings (contracts)
• Antagonist: Quadriceps (relaxes)
• Movement produced: Flexion — the angle at the knee decreases
• Example: Drawing the leg back before kicking; bending the knees during a squat descent

Movement	Agonist	Antagonist	Sporting Example
Extension at knee	Quadriceps	Hamstrings	Kicking a football
Flexion at knee	Hamstrings	Quadriceps	Bending legs for a squat

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Antagonistic Pairs at the Hip

The hip flexors and gluteals are the antagonistic pair at the hip joint.

Flexion at the Hip (Lifting the Thigh Forward)

• Agonist: Hip flexors (contracts)
• Antagonist: Gluteals (relaxes)
• Movement produced: Flexion — the thigh moves forward and upward
• Example: Lifting the knee during running; kicking a football (the forward swing)

Extension at the Hip (Driving the Thigh Backward)

• Agonist: Gluteals (contracts)
• Antagonist: Hip flexors (relaxes)
• Movement produced: Extension — the thigh moves backward
• Example: Driving the leg back during sprinting; standing up from a crouched position

Movement	Agonist	Antagonist	Sporting Example
Flexion at hip	Hip flexors	Gluteals	Lifting knee in running
Extension at hip	Gluteals	Hip flexors	Sprinting (drive phase)

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Antagonistic Pairs at the Shoulder

The deltoid and latissimus dorsi are the antagonistic pair for abduction and adduction at the shoulder joint.

Abduction at the Shoulder (Raising the Arm Sideways)

• Agonist: Deltoid (contracts)
• Antagonist: Latissimus dorsi (relaxes)
• Movement produced: Abduction — the arm moves away from the midline
• Example: Raising the arm during a star jump; lifting the arm to serve in tennis