Antagonistic Muscle Action

This lesson covers antagonistic muscle pairs (also called antagonistic pairs) as required by the Edexcel GCSE PE specification (1PE0 — Topic 1: Applied Anatomy and Physiology). You need to understand how muscles work in pairs, identify the agonist and antagonist in any given movement, and apply this knowledge to sporting actions.

---

Why Muscles Work in Pairs

Muscles can only pull — they cannot push. When a muscle contracts, it shortens and pulls on the bone it is attached to. However, once the muscle has contracted, it cannot actively push the bone back to its original position. A second muscle is needed to pull the bone back the other way.

This is why muscles work in antagonistic pairs — two muscles that produce opposite movements at the same joint. When one contracts (shortens), the other relaxes (lengthens).

---

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 occur
Antagonistic pair	Two muscles that work opposite each other across the same joint
Isometric contraction	A contraction where the muscle generates force but does not change length (no movement occurs)
Isotonic contraction	A contraction where the muscle changes length and movement occurs
Concentric contraction	An isotonic contraction where the muscle shortens
Eccentric contraction	An isotonic contraction where the muscle lengthens under tension

Exam Tip: The agonist is the muscle doing the work (contracting to cause movement). The antagonist is the muscle relaxing to allow that movement. When the movement reverses, the roles swap — the agonist becomes the antagonist and vice versa.

---

The Four Edexcel Antagonistic Pairs

The Edexcel specification requires you to know the following four antagonistic pairs:

1. Biceps and Triceps (Elbow)

Movement	Agonist	Antagonist	Joint	Action
Flexion of the elbow	Biceps	Triceps	Elbow (hinge)	Biceps contracts, triceps relaxes
Extension of the elbow	Triceps	Biceps	Elbow (hinge)	Triceps contracts, biceps relaxes

Sporting example — Bicep curl: During the upward phase, the biceps contracts (agonist) and the triceps relaxes (antagonist) to flex the elbow. During the downward phase, the roles reverse — the triceps contracts (agonist) and the biceps relaxes (antagonist) to extend the elbow.

Sporting example — Throwing a javelin: During the release phase, the triceps acts as the agonist (extending the elbow to straighten the arm), while the biceps relaxes (antagonist).

2. Quadriceps and Hamstrings (Knee)

Movement	Agonist	Antagonist	Joint	Action
Extension of the knee	Quadriceps	Hamstrings	Knee (hinge)	Quadriceps contracts, hamstrings relax
Flexion of the knee	Hamstrings	Quadriceps	Knee (hinge)	Hamstrings contract, quadriceps relax

Sporting example — Kicking a football: During the kicking phase, the quadriceps contracts (agonist) to extend the knee and strike the ball, while the hamstrings relax (antagonist). During the backswing (preparation), the hamstrings contract (agonist) to flex the knee, and the quadriceps relaxes (antagonist).

graph LR
    A["Preparation:<br>Knee flexion"] -->|"Hamstrings = agonist<br>Quadriceps = antagonist"| B["Execution:<br>Knee extension"]
    B -->|"Quadriceps = agonist<br>Hamstrings = antagonist"| C["Ball is kicked"]

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

3. Hip Flexors and Gluteus Maximus (Hip)

Movement	Agonist	Antagonist	Joint	Action
Flexion of the hip	Hip flexors	Gluteus maximus	Hip (ball and socket)	Hip flexors contract, glutes relax
Extension of the hip	Gluteus maximus	Hip flexors	Hip (ball and socket)	Glutes contract, hip flexors relax

Sporting example — Sprinting: During the drive phase, the gluteus maximus contracts (agonist) to extend the hip and push the body forward, while the hip flexors relax (antagonist). During the recovery phase, the hip flexors contract (agonist) to bring the knee forward and up, while the gluteus maximus relaxes (antagonist).

Sporting example — Squatting: On the way down, the gluteus maximus lengthens under tension (eccentric contraction). On the way up, the gluteus maximus contracts concentrically (agonist) to extend the hip, and the hip flexors relax (antagonist).