How do we analyse a movement in terms of joints, levers, planes and axes?
The analysis of movement at joints, the three classes of lever and their mechanical advantage, and the planes and axes in which movements occur.
A focused WJEC A-Level PE answer on movement analysis, covering the three classes of lever and mechanical advantage, joint actions and muscle roles, and the planes and axes of movement used to describe sporting actions.
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What this dot point is asking
WJEC wants you to analyse a sporting movement: identify the joint actions and the muscles producing them, classify the lever operating at a joint and state its mechanical advantage, and name the plane and axis in which a movement occurs.
Levers and mechanical advantage
The three classes are defined by what lies in the middle:
- First-class lever: the fulcrum is between the effort and the load (for example, the head nodding at the neck, with the muscles at the back and the weight of the head at the front).
- Second-class lever: the load is between the fulcrum and the effort (for example, plantar flexion rising onto the toes). The effort arm is longer than the load arm, so the mechanical advantage is greater than one: a small force moves a large load.
- Third-class lever: the effort is between the fulcrum and the load (for example, the biceps flexing the elbow). The effort arm is shorter than the load arm, so it has a mechanical disadvantage (less than one), but it produces a large range of movement and high speed at the end of the limb. Most levers in the body are third class.
Planes and axes
To analyse a movement, decide the direction of the action: flexion and extension occur in the sagittal plane; abduction and adduction in the frontal plane; rotation in the transverse plane. Then name the matching axis.
Examples in context
Example 1. The Achilles and the second-class lever. Sprinters and jumpers rely on the calf muscles working a second-class lever at the ankle to drive off the toes. The mechanical advantage greater than one means the calf can exert a large force against the ground, a typical WJEC application of lever classes to power.
Example 2. The forearm as a third-class lever. A javelin thrower's elbow acts as a third-class lever, sacrificing mechanical advantage for a long lever and high speed at the hand, which helps generate release velocity. This shows why third-class levers dominate where speed matters.
Try this
Q1. State the three components of a lever. [1 mark]
- Cue. A fulcrum (pivot), an effort (muscle force) and a load (resistance).
Q2. Explain why most levers in the human body are third class and what advantage this gives. [3 marks]
- Cue. The effort lies between the fulcrum and the load; this gives a mechanical disadvantage but produces a large range of movement and high speed at the end of the limb.
Q3. Name the plane and axis used for a discus thrower's rotation in the circle. [2 marks]
- Cue. The transverse plane about the longitudinal (vertical) axis, because the rotation is a horizontal turn.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC 20194 marksIdentify the class of lever operating at the ankle when a performer rises onto the toes, and explain its mechanical advantage.Show worked answer →
Rising onto the toes (plantar flexion) uses a second-class lever at the ankle.
In a second-class lever the load lies between the fulcrum and the effort: the fulcrum is the ball of the foot, the load is body weight passing through the ankle, and the effort is the calf muscles pulling on the heel through the Achilles tendon.
Because the effort arm is longer than the load arm, a second-class lever has a mechanical advantage greater than one, so a relatively small muscle force can move a large load.
Markers reward identifying a second-class lever, the correct order of fulcrum, load and effort, and the mechanical advantage greater than one.
WJEC 20214 marksA gymnast performs a forward somersault. Name the plane and axis of this rotation and justify your answer.Show worked answer →
A forward somersault is a rotation in the sagittal plane about the transverse (frontal-horizontal) axis.
The sagittal plane divides the body into left and right halves, and forward and backward rotations (such as a somersault) occur in this plane.
The transverse axis runs side to side through the body, and the body rotates around it during a somersault, like a wheel turning.
Markers reward naming the sagittal plane and transverse axis, and justifying the choice by the direction of the rotation.
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