Lever Systems In Biomechanics

first second third class lever

A lever system is a rigid bar that moves on a fixed point called the fulcrum when a force is applied to it. Movement is made possible in the human body by lever systems that are formed by our muscles and joints working together. An understanding of the levers in the body helps us to understand how movement is possible.


Levers in biomechanics

Muscles are attached to bones via tendons and the bones of the skeleton act as levers which muscles pull on to create movement. A lever system is made up of three parts; an effort, a load, and a fulcrum. In the human body, the effort is provided by the muscle (the muscles point of application/insertion), the load is the weight of the body and any additional resistance and the fulcrum is the joint itself.

Types of lever

There are three types found in the body, each is determined by the relative positions of the fulcrum (F), effort (E) and load (L). A useful way of determining the different types of lever systems operating during a particular movement is to remember the following rhyme.

“For 1, 2, 3 think F, L, E” This rhyme helps us to identify the middle component of each system.

  • For a first-class lever, F is the middle component.  
  • For a second class lever, L is the middle component. 
  • For a third-class lever, E is the middle component.

Functions of a lever system

Lever systems have two main functions; firstly to increase the resistance that a given force can move and secondly to increase the speed at which a body moves. In addition, different types allow different ranges of movement. This means that some are effective at overcoming resistance, whilst others are able to generate speed.

First-class lever system

In a first-class lever, the fulcrum is the middle component and lies between the effort and load. Examples of a first-class lever in the body are rare as few exercises utilise a first-class lever system although extension (straightening) at the elbow is one example. Extension at the elbow can be seen during a throwing action or tennis stroke.

In the image below, the triceps is the effort, the fulcrum is the elbow joint and the load is the weight of the arm and Javelin. First-class levers increase both the effects of effort and the speed of the body.

There is sometimes more than one lever system operating at the joint. The elbow joint is one example. During extension of the elbow, the effort is created by the triceps via its point of insertion on the ulna, so is a first-class lever. However, during flexion at the elbow, as in a bicep curl, the effort comes from the point of insertion of the biceps on the radius, this is an example of a third-class system.

Second class lever system

In a second class lever system, the load is the middle component and lies between the fulcrum and the effort. Exercises involving plantar flexion at the ankle (going up on your toes) are second class lever systems, such as a calf raise or when jumping upwards whilst performing a layup in basketball. Second class lever systems only tend to increase the effect of the effort force, in other words, they are effective at overcoming resistance as opposed to generating speed.

In the example of plantarflexion at the ankle joint, the ball of the foot and toes are the fulcrum, the weight of the body is the load and the effort is applied by the gastrocnemius muscle.

Third class lever system

The majority of movements in the human body are classified as third-class lever systems. In a third-class lever system, the effort is the middle component and lies between the fulcrum and load.

There are many examples of third class lever systems, including both flexion and extension at the knee joint. These movements are involved in running, jumping and kicking. During flexion at the knee, the point of insertion of the hamstrings on the tibia is the effort, the knee joint is the fulcrum and the weight of the leg is the load. Third class lever systems are used to increase the speed of a body and allow a wide range of movement.

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