Muscles and bones act together to form levers. A lever is a rigid rod (usually a length of bone) that turns about a pivot (usually a joint). Levers can be used so that a small force can move a much bigger force. This is called mechanical advantage.
There are four parts to a lever – lever arm, pivot, effort and load. In our bodies:
- bones act as lever arms
- joints act as pivots
- muscles provide the effort forces to move loads
- load forces are often the weights of the body parts that are moved or forces needed to lift, push or pull things outside our bodies.
Levers can also be used to magnify movement, for example, when kicking a ball, small contractions of leg muscles produce a much larger movement at the end of the leg.
Levers are able to give us a strength advantage or a movement advantage but not both together.
Nature of science
Scientists use data to back up their explanations of the world. These explanations add to a growing body of knowledge. For example, knowledge of levers underpins explanations of body movement. Remember that scientific knowledge continues to evolve and so is tentative.
Types of levers
Different classes of levers are identified by the way the joint and muscles attached to the bone are arranged.
This pivot exists in the place where your skull meets the top of your spine. Your skull is the lever arm and the neck muscles at the back of the skull provide the force (effort) to lift your head up against the weight of the head (load). When the neck muscles relax, your head nods forward.
Class 1 lever – nod your head
The pivot is the place where your skull meets the top of your spine. Your skull is the lever arm and the neck muscles at the back of the skull provide the force (effort) to lift your head up against the weight of the head (load). When the neck muscles relax, your head nods forward.
For this lever, the pivot lies between the effort and load. A see saw in a playground is another example of a Class 1 lever where the effort balances the load.
Nature of science
Scientists make models to demonstrate their explanations. Often models are constructed to demonstrate how things work. This model uses a physics idea of levers to provide an explanation for muscle/bone movement. The physics explanation of levers supports this model.
Class 2 lever – stand on tip toes
The pivot is at your toe joints and your foot acts as a lever arm. Your calf muscles and Achilles tendon provide the effort when the calf muscle contracts. The load is your body weight and is lifted by the effort (muscle contraction).
The load is between the pivot and the effort (like a wheelbarrow). The effort force needed is less than the load force, so there is a mechanical advantage. This muscular movement at the back of your legs allows you to move your whole body a small distance.
Class 3 lever – bend your arm
The pivot is at the elbow and the forearm acts as the lever arm. The biceps muscle provides the effort (force) and bends the forearm against the weight of the forearm and any weight that the hand might be holding.
The load is further away from the pivot than the effort. There is no mechanical advantage because the effort is greater than the load. However this disadvantage is compensated with a larger movement – a small contraction of the biceps produces a large movement of the forearm. This type of lever system also gives us the advantage of a much greater speed of movement.
Many muscle and bone combinations in our bodies are of the Class 3 lever type.
Nature of science
Laws of motion that scientists use today were proposed by Sir Isaac Newton (1643-1727). He is regarded by many as the greatest influence in the history of science, and the newton measurement of force acknowledges his contribution. His laws enable people to make predictions.
What is torque?
In the examples above, the effort and load forces have acted in opposite rotation directions to each other. If a load tries to turn the lever clockwise, the effort tries to turn the lever anticlockwise. Forces acting on a lever also have different effects depending how far they are away from the pivot. For example when pushing a door open it is easier to make the door move if you push at the door handle rather than near to the hinge (pivot). Pushing on the door produces a turning effect, which causes rotation.
This turning effect is called torque (or leverage)
The formula for calculating the amount of torque is:
torque = force x perpendicular distance to the pivot.
The force is measured in newtons and the distance to the pivot is measured in metres or centimetres, so the unit for torque will be either newton metres (Nm) or newton centimetres (Ncm).
You can increase the amount of torque by increasing the size of the force or increasing the distance that the force acts from the pivot. That’s why the door handle is far away from the hinge.
Forces from our muscles produce torques about our joints in clockwise and anti-clockwise directions. If the torques are equal and opposite, the lever will not rotate. If they are unequal, the lever will rotate in the direction of the greater torque.
In this diagram below, the load and weight of the lower leg produce a clockwise torque about the knee. The lower leg will rotate in a clockwise direction.
If the hamstring muscle at the back of the upper leg contracts with a strong force, it produces an anticlockwise torque that holds the leg up.
In this diagram, lifting the weight like the person on the left produces a greater torque about the lower spine (pivot) – the lifting force is at a greater perpendicular distance to the pivot. The back muscles must exert a huge force to provide a torque that balances the torque from the weight being lifted.
It is important to lift a heavy weight close to your body to reduce the torque produced around your lower spine.