Lever Components
A lever is a rigid rod (in the body, a bone) that rotates about a fixed point when a force is applied, allowing the body to produce and control movement efficiently. Every lever system is made up of three components:
- The fulcrum (F) – the fixed point about which the lever rotates; in the body this is a joint
- The effort (E) – the force applied to move the lever; in the body this is the muscular force generated by a contracting muscle
- The resistance/load (R) – the weight being moved by the lever, made up of the weight of the body part itself plus anything else being moved (e.g. a held object)
First-Class Levers
In a first-class lever, the fulcrum sits between the effort and the resistance (order: R-F-E or E-F-R).
- Example in the body: the triceps brachii extending the elbow – the elbow joint is the fulcrum, sitting between the effort (triceps pulling on the forearm) and the resistance (the weight of the forearm/hand)
- A second example is at the neck: nodding the head forward and back uses the atlanto-occipital joint as the fulcrum, with the neck extensor muscles providing effort against the resistance of the head's weight
Second-Class Levers
In a second-class lever, the resistance sits between the fulcrum and the effort (order: F-R-E).
- Example in the body: standing up onto tip-toes (plantarflexion) – the ball of the foot acts as the fulcrum, the body weight pushing down through the ankle is the resistance, and the effort is supplied by the calf muscles (gastrocnemius) pulling up on the heel via the Achilles tendon
- Second-class levers are the rarest type found in the human body
Third-Class Levers
In a third-class lever, the effort sits between the fulcrum and the resistance (order: F-E-R) – this is by far the most common lever arrangement in the human body.
- Example in the body: flexing the elbow using the biceps brachii – the elbow joint is the fulcrum, the biceps inserting on the radius provides the effort, and the hand/any weight held in it is the resistance, positioned further from the fulcrum than the effort
- Other examples include the hamstrings flexing the knee and the hip flexors flexing the hip, both following the same F-E-R pattern
Mechanical Advantage and Disadvantage
The force arm is the distance between the fulcrum and the point where effort is applied; the resistance arm is the distance between the fulcrum and the resistance. Comparing these two distances determines how efficiently a lever operates.
- A lever has mechanical advantage when the force arm is longer than the resistance arm – a relatively small effort can then move a large resistance, though the resistance moves only a short distance and relatively slowly
- A lever has mechanical disadvantage when the resistance arm is longer than the force arm – a relatively large effort is needed to move even a small resistance, but in exchange the resistance moves a greater distance and at greater speed
- Most third-class levers in the body (the most common type) operate at a mechanical disadvantage, since the effort acts closer to the fulcrum than the resistance does. This is a trade-off the body makes deliberately: it sacrifices force for a large gain in speed and range of movement at the end of the limb, which benefits fast, powerful sporting actions such as kicking or throwing