Types of Muscle Contraction

This lesson covers the types of muscle contraction as required by the AQA GCSE PE specification (3.1.1.1): isometric contraction and isotonic contraction (which is subdivided into concentric and eccentric contraction). You need to know the definition of each type, understand how they differ, and apply them to sporting examples.

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Overview: Two Main Types of Contraction

When a muscle is activated (receives a signal from the nervous system), it generates tension. However, the result of that tension depends on the forces acting on the muscle. This gives us two main categories:

Type of Contraction	What Happens	Does the Muscle Change Length?	Does Movement Occur?
Isotonic	The muscle changes length while generating tension	Yes	Yes
Isometric	The muscle generates tension but does NOT change length	No	No

graph TD
    A["Types of Muscle Contraction"] --> B["Isotonic"]
    A --> C["Isometric"]
    B --> D["Concentric"]
    B --> E["Eccentric"]
    D --> F["Muscle shortens"]
    E --> G["Muscle lengthens under tension"]
    C --> H["Muscle stays the same length"]

    style A fill:#4a90d9,color:#fff
    style B fill:#e67e22,color:#fff
    style C fill:#27ae60,color:#fff
    style D fill:#9b59b6,color:#fff
    style E fill:#9b59b6,color:#fff

Exam Tip: The prefix "iso" means "same" and "tonic" means "tension." So "isotonic" means the tension remains roughly the same while the length changes. "Isometric" uses "metric" (meaning "measure/length"), so "isometric" means the length stays the same while tension is generated.

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Isotonic Contraction

Isotonic contraction occurs when a muscle changes length while generating force, resulting in movement at a joint. There are two subtypes:

1. Concentric Contraction

Definition: A concentric contraction occurs when the muscle shortens while generating force. The muscle contracts and gets shorter, pulling the bone and producing movement.

This is the type of contraction most people think of when they hear the word "contraction."

How It Works

• The muscle fibres slide over each other and overlap more, causing the muscle to become shorter and fatter.
• The muscle pulls on the tendon, which pulls on the bone, producing movement at the joint.
• The agonist muscle is performing a concentric contraction when producing the primary movement.

Sporting Examples

Sporting Action	Muscle	Joint	Movement Produced
Upward phase of a bicep curl	Biceps	Elbow	Flexion — biceps shortens
Kicking a football (follow-through)	Quadriceps	Knee	Extension — quadriceps shortens
Jumping (take-off phase)	Gluteals, quadriceps, gastrocnemius	Hip, knee, ankle	Extension / plantarflexion
Push-up (upward phase)	Triceps	Elbow	Extension — triceps shortens
Sprint start (driving off blocks)	Quadriceps, gluteals	Knee, hip	Extension
Throwing a ball (release phase)	Triceps	Elbow	Extension — triceps shortens

Exam Tip: In a concentric contraction, the muscle is the agonist and it is shortening. The movement goes in the direction of the pull.

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2. Eccentric Contraction

Definition: An eccentric contraction occurs when the muscle lengthens while still generating force. The muscle is under tension but is being stretched because the external force (e.g., gravity, an opponent, the weight of a limb) is greater than the force the muscle is producing.

This is often described as a "controlled lengthening" or "braking" action.

How It Works

• The muscle fibres are being pulled apart (lengthened) even though they are still active and generating tension.
• This type of contraction is used to control movement, decelerate a limb, or lower a weight slowly.
• Without eccentric contractions, movements would be uncontrolled and jerky.

Sporting Examples

Sporting Action	Muscle	Joint	What Is Happening
Downward phase of a bicep curl	Biceps	Elbow	Biceps lengthens under tension to control the lowering of the weight
Landing from a jump	Quadriceps	Knee	Quadriceps lengthens to control the bending of the knee on landing
Lowering into a squat	Quadriceps, gluteals	Knee, hip	Muscles lengthen to control the descent
The backswing before kicking a ball	Quadriceps	Knee	Quadriceps lengthens eccentrically as the knee flexes
Walking downhill	Quadriceps	Knee	Quadriceps controls the bending of the knee as the body descends
Decelerating after a sprint	Hamstrings, quadriceps	Knee, hip	Muscles lengthen to slow the body down

Exam Tip: Eccentric contractions are the ones students find hardest to understand. The key point is that the muscle is still working (it is under tension) but it is getting longer, not shorter. Think of it as the muscle acting like a brake — controlling and slowing down a movement rather than producing it.

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Concentric vs Eccentric: The Bicep Curl Example

The bicep curl is the perfect example to illustrate both types of isotonic contraction:

Phase	Direction	Muscle Action	Type of Contraction
Upward phase (lifting the weight)	Weight moves up	Biceps shortens to lift the weight	Concentric
Downward phase (lowering the weight)	Weight moves down	Biceps lengthens to control the lowering	Eccentric