Lever systems: WACE Physical Education Studies Unit 3
“Classify first, second and third class levers in the body and explain their mechanical advantage and effect on force and speed”
A lever has a fulcrum (joint), an effort (muscle force) and a load (weight moved). First class levers have the fulcrum in the middle, second class have the load in the middle, and third class have the effort in the middle. Most body levers are third class: they give a mechanical disadvantage for force but a large gain in speed and range of movement.
What this dot point is asking
WACE expects you to identify the three lever components in a body example, classify the lever, and explain the trade off between force and speed. The classification depends only on which component sits in the middle.
The three components
Every lever has three parts. The fulcrum is the fixed pivot, which in the body is a joint. The effort is the force applied to move the lever, which is the pull of the muscle at its insertion. The load, or resistance, is the weight being moved, which is the body part plus anything it carries. The arrangement of these three decides the lever class.
First class levers
In a first class lever the fulcrum lies between the effort and the load, like a seesaw. In the body, extension of the neck is a first class lever: the joint between the skull and the spine is the fulcrum, the neck extensor muscles at the back provide the effort, and the weight of the head in front is the load. First class levers can favour either force or speed depending on where the fulcrum sits.
Second class levers
In a second class lever the load lies between the fulcrum and the effort, like a wheelbarrow. The classic body example is rising onto the toes (plantar flexion): the ball of the foot is the fulcrum, the body weight through the ankle is the load in the middle, and the calf muscles pulling on the heel provide the effort. Second class levers always give a mechanical advantage, because the effort arm is longer than the load arm, so a small muscle force moves a large load.
Mechanical advantage exists when the effort arm (distance from fulcrum to effort) is longer than the resistance arm (distance from fulcrum to load). A long effort arm lets a small force move a large load, which is the case in second class levers. A short effort arm, as in third class levers, creates a mechanical disadvantage for force but lets the load end move a long way and quickly.
Third class levers
In a third class lever the effort lies between the fulcrum and the load, and this is the most common arrangement in the body. The biceps curl is the standard example: the elbow is the fulcrum, the biceps insertion just below the joint provides the effort, and the weight in the hand at the far end is the load. Because the effort arm is short and the load arm is long, third class levers are at a mechanical disadvantage for force, so the muscle must produce a large force. In return, a small muscle shortening moves the hand a long way very quickly. This speed and range is exactly what throwing, kicking and striking need.
Why the body favours speed
The body trades force for speed because muscles attach close to joints, giving short effort arms. A small contraction at the elbow or knee produces a large, fast movement at the hand or foot. This is ideal for sport, where the velocity of a bat, racquet or foot at the end of a long limb matters more than the raw force at the joint.
Students rote learn that the biceps curl is third class and the calf raise is second class, then misclassify a new example under pressure. The reliable method is to find the fulcrum, effort and load, then ask which one is in the middle. Fulcrum in the middle is first class, load in the middle is second class, effort in the middle is third class. Classify by position, not by memory.
Step 1: locate the three components
For rising onto the toes, find the fulcrum (the ball of the foot), the load (body weight acting down through the ankle) and the effort (the calf muscles pulling up on the heel through the Achilles tendon).
Step 2: decide which component is in the middle
Here the load (ankle) lies between the fulcrum (toes) and the effort (heel). Load in the middle means a second class lever.
Step 3: compare the lever arms
The effort arm (toes to heel) is longer than the resistance arm (toes to ankle). A longer effort arm means a mechanical advantage greater than 1.
Step 4: calculate and interpret
If the effort arm is 0.18 m and the resistance arm is 0.12 m, then . A value above 1 confirms the lever favours force: the calf muscles can lift the body weight using a force smaller than the load, though the heel moves a shorter distance than it would in a force-disadvantaged third class lever.
How this maps to the exam
A question may name a movement and ask you to classify the lever and state its advantage. Identify the three components, name which is central, give the class, then comment on whether it favours force (second class) or speed and range (third class). The reasoning earns more than the label alone.
Exam-style questions
Questions in the style of SCSA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
WACE 2021-style6 marksFor a biceps curl, identify the fulcrum, effort and load, classify the lever, and explain why this arrangement is well suited to producing fast movement at the hand despite requiring a large muscle force.Show worked answer →
A 6 mark answer needs the three components, the correct class, and the force-for-speed trade-off explained.
- Components
- Fulcrum: the elbow joint. Effort: the biceps pulling at its insertion just below the elbow. Load: the weight in the hand at the far end of the forearm.
- Class
- The effort lies between the fulcrum and the load, so this is a third class lever.
- Force required
- The effort arm (fulcrum to insertion) is short and the load arm (fulcrum to hand) is long, giving a mechanical disadvantage. The muscle must therefore produce a force much larger than the load it moves.
- Speed and range benefit
- Because the load is far from the fulcrum, a small, fast shortening of the muscle near the joint moves the hand through a large distance very quickly. This high end-point velocity is exactly what throwing, kicking and striking need.
Markers reward correct components, third class classification, the mechanical disadvantage for force, and the gain in speed and range at the hand.
WACE 2023-style4 marksA second class lever in the foot has an effort arm of 0.20 m and a resistance arm of 0.10 m. Calculate the mechanical advantage and explain what this value means for the muscle force needed to lift the load.Show worked answer →
A 4 mark answer needs the calculation and an interpretation.
- Calculation
- Mechanical advantage is the ratio of effort arm to resistance arm: .
- Interpretation
- A mechanical advantage of 2 (greater than 1) means the effort arm is twice the resistance arm, so the lever multiplies force. The muscle needs to apply only about half the force of the load to move it.
- Link to second class
- This is typical of second class levers (such as rising onto the toes), where the load sits between the fulcrum and the effort, always giving a mechanical advantage for force at the cost of speed and range.
Markers reward the correct ratio of 2, the meaning that less than the load force is needed, and the link to a force-favouring second class lever.