What is a lever?

A lever is a rigid bar that turns about a fixed point. In the body, bones are the bars, joints are the pivot, and muscles provide the effort. Every lever has three parts:

• Fulcrum — the pivot (the joint).
• Effort — the force from the muscle.
• Load (resistance) — the weight being moved.

The class of a lever depends on which part is in the middle.

First-class levers (fulcrum in the middle)

In a first-class lever the fulcrum is in the middle, between the effort and the load — like a see-saw.

In the body: nodding the head — the joint at the top of the neck is the fulcrum, with the neck muscles (effort) on one side and the weight of the head (load) on the other. Extension at the elbow by the triceps is also first class.

Memory: 1st class = Fulcrum in the middle. Think "F-1".

Second-class levers (load in the middle)

In a second-class lever the load is in the middle, between the fulcrum and the effort — like a wheelbarrow.

In the body: standing up on your toes / a calf raise — plantar-flexion at the ankle. The ball of the foot is the fulcrum, body weight is the load in the middle, and the calf muscle provides the effort at the heel.

Big point: second-class levers have a large mechanical advantage — a small effort moves a large load, but only over a short distance.

Third-class levers (effort in the middle)

In a third-class lever the effort is in the middle, between the fulcrum and the load. This is the most common lever in the body.

In the body: flexion at the elbow in a biceps curl — the elbow is the fulcrum, the biceps pulls (effort) close to the joint, and the weight in the hand is the load at the far end.

Trade-off: third-class levers have a mechanical disadvantage (large effort for the load) but give a large range and speed of movement at the far end.

Name that lever

Bicep curl lever

Mechanical advantage

Mechanical advantage tells you how effective a lever is at moving a load. It compares the distance of the effort from the fulcrum with the distance of the load from the fulcrum:

• Greater than 1 — the effort arm is longer, so a small effort moves a big load (second-class levers).
• Less than 1 — mechanical disadvantage, but faster, wider movement (third-class levers).

Mechanical advantage

Mechanical advantage again

Planes of movement

A plane is an imaginary flat surface that a movement travels along. There are three:

• Sagittal plane — divides the body into left and right; forward/backward movements (a somersault, a biceps curl, running).
• Frontal plane — divides the body into front and back; side-to-side movements (a cartwheel, star jumps).
• Transverse plane — divides the body into top and bottom; rotational/twisting movements (a spinning discus throw, a full turn).

Axes of rotation

An axis is an imaginary line the body rotates around. Each plane pairs with an axis:

• Transverse axis (side to side) — rotation in the sagittal plane, e.g. a front somersault.
• Sagittal (anteroposterior) axis (front to back) — rotation in the frontal plane, e.g. a cartwheel.
• Longitudinal (vertical) axis (head to toe) — rotation in the transverse plane, e.g. a spinning skater or discus turn.

Pairings to learn: sagittal plane ↔ transverse axis · frontal plane ↔ sagittal axis · transverse plane ↔ longitudinal axis.

Somersault movement

Put each one in the right group

Tap a sporting action, then tap the plane the movement travels in.

Spinning skater

Match each statement to its answer

Tap a movement on the left, then its correct plane or axis on the right.

The big ideas to know

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