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  • Muscles and bones act together to form levers. A lever1 is a rigid rod (usually a length of bone2) that turns about a pivot3 (usually a joint). Levers can be used so that a small force4 can move a much bigger force. This is called mechanical advantage5.

    Rights: The University of Waikato Te Whare Wānanga o Waikato

    Mechanical advantage

    Levers can be used so that a small force can move a much bigger force. This is called mechanical advantage. In our bodies bones act as lever arms, joints act as pivots, and muscles provide the effort forces to move loads.

    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 lift6, 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 data7 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 tentative8.

    Types of levers

    Different classes of levers are identified by the way the joint and muscles attached to the bone are arranged.

    Rights: The University of Waikato Te Whare Wānanga o Waikato

    Pivot diagram of a Class 1 lever

    Different classes of levers are identified by the way the joint and muscles attached to the bone are arranged.

    For the Class 1 lever the pivot lies between the effort and load. A see saw in a playground is an example of a Class 1 lever where the effort balances the load.

    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 weight9 of the head (load). When the neck muscles relax, your head nods forward.

    Class 1 lever – nod your head

    Rights: The University of Waikato Te Whare Wānanga o Waikato

    Skull and neck

    Different classes of levers are identified by the way the joint and muscles attached to the bone are arranged.

    For a Class 1 lever the pivot lies between the effort and the load. A see saw in a playground is an example of a Class 1 lever where the effort balances the load.

    The place where your skull meets the top of your spine is a Class 1 lever. 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.

    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 tendon10 provide the effort when the calf muscle11 contracts. The load is your body weight and is lifted by the effort (muscle contraction12).

    Rights: The University of Waikato Te Whare Wānanga o Waikato

    Pivot diagram of a Class 2 lever

    Different classes of levers are identified by the way the joint and muscles attached to the bone are arranged.

    For the Class 2 lever 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.

    Standing on tip toes is a Class 2 lever. 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.

    Rights: The University of Waikato Te Whare Wānanga o Waikato

    Tip-toe

    Different classes of levers are identified by the way the joint and muscles attached to the bone are arranged.

    For the Class 2 lever 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.

    Standing on tip toes is a Class 2 lever. 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).

    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.

    Rights: The University of Waikato Te Whare Wānanga o Waikato

    Pivot diagram of a Class 3 lever

    Different classes of levers are identified by the way the joint and muscles attached to the bone are arranged.

    For a Class 3 lever 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. This type of lever system also gives us the advantage of a much greater speed of movement.

    A bent arm is a Class 3 lever. 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.

    Rights: The University of Waikato Te Whare Wānanga o Waikato

    Bent arm

    Different classes of levers are identified by the way the joint and muscles attached to the bone are arranged.

    For a Class 3 lever 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. This type of lever system also gives us the advantage of a much greater speed of movement.

    A bent arm is a Class 3 lever. 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.

    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 Newton13 (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 torque14 (or leverage15)

    Rights: The University of Waikato Te Whare Wānanga o Waikato

    Hamstring

    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.

    The formula for calculating the amount of torque is:

    torque = force x perpendicular16 distance to the pivot.

    The force is measured in newtons17 and the distance to the pivot is measured in metres18 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.

    Rights: The University of Waikato Te Whare Wānanga o Waikato

    Lifting heavy weights

    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.

    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.

    Related content

    Find out more about muscle performance – there are three are major factors that affect how well your muscles perform: strength, power and endurance. Muscle strength can be safely measured by estimating an athlete's one repitition maximum (1RM).

    Activity idea

    The Biceps curl activity models and measures the force in the biceps muscle.

    1. lever : A simple machine consisting of a rigid bar (a bone) that rotates about a pivot and is used to transmit a force.
    2. bone: A specialised form of connective tissue. The presence of the mineral hydroxyapatite helps to give bone its strength and density.
    3. pivot: The point about which a part of the body rotates that is usually a joint.
    4. force: A push or a pull that causes an object to change its shape, direction and/or motion.
    5. mechanical advantage: The ratio of the output force produced by a lever to the applied input force.
    6. lift: In aerodynamics, upward force produced by a difference in pressure due to airflow.
    7. data: The unprocessed information we analyse to gain knowledge.
    8. tentative: Not certain or fixed.
    9. weight: Force due to gravity acting on an object, measured in newtons.
    10. tendon: Stringy tissue that attaches muscle to bones.
    11. muscle: The tissue that makes it possible for an animal to move and to maintain its posture. Muscles also make the heart beat, force blood to circulate and move food along the digestive system. The human body has more than 600 muscles.
    12. contraction: 1. The reduction of the space matter occupies – by becoming smaller or shorter. 2. When muscles become shorter and pull.
    13. newton: The unit of measurement of a force (N), named after the famous English physicist Sir Isaac Newton whose laws of motion and gravity underpin much of modern day physics.
    14. torque: The turning or twisting force about a pivot.
    15. leverage: An alternative name for torque.
    16. perpendicular: At right angles (90 degrees).
    17. newton: The unit of measurement of a force (N), named after the famous English physicist Sir Isaac Newton whose laws of motion and gravity underpin much of modern day physics.
    18. metre: The base unit of length in the International System of Units (SI).
    Published 21 June 2007 Referencing Hub articles
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        lever

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      2. A simple machine consisting of a rigid bar (a bone) that rotates about a pivot and is used to transmit a force.

        force

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      4. A push or a pull that causes an object to change its shape, direction and/or motion.

        data

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      6. The unprocessed information we analyse to gain knowledge.

        tendon

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      8. Stringy tissue that attaches muscle to bones.

        newton

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      10. The unit of measurement of a force (N), named after the famous English physicist Sir Isaac Newton whose laws of motion and gravity underpin much of modern day physics.

        perpendicular

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      12. At right angles (90 degrees).

        bone

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      14. A specialised form of connective tissue. The presence of the mineral hydroxyapatite helps to give bone its strength and density.

        mechanical advantage

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      16. The ratio of the output force produced by a lever to the applied input force.

        tentative

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      18. Not certain or fixed.

        muscle

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      20. The tissue that makes it possible for an animal to move and to maintain its posture. Muscles also make the heart beat, force blood to circulate and move food along the digestive system. The human body has more than 600 muscles.

        torque

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      22. The turning or twisting force about a pivot.

        metre

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      24. The base unit of length in the International System of Units (SI).

        pivot

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      26. The point about which a part of the body rotates that is usually a joint.

        lift

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      28. In aerodynamics, upward force produced by a difference in pressure due to airflow.

        weight

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      30. Force due to gravity acting on an object, measured in newtons.

        contraction

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      32. 1. The reduction of the space matter occupies – by becoming smaller or shorter.

        2. When muscles become shorter and pull.

        leverage

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      34. An alternative name for torque.