A lever 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 which are formed by our muscles and joints working together. An understanding of the lever systems 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 of lever system in the body, each is determined by the relevant positions of the fulcrum (F), effort (E) and load (L). A useful way of determining the different types of lever system 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 lever system.
For a 1st class lever system, F is the middle component.
For a 2nd class lever system, L is the middle component.
For a third class lever system, 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 lever systems allow different ranges of movement. This means that some lever systems are effective at overcoming a resistance, whilst others are able to generate speed.
First class lever system
In a first class lever system, the fulcrum is the middle component and lies between the effort and load. Examples of a first class lever system 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 lever systems can increase both the effects of effort and the speed of body.
There is sometimes more than one lever system operating at the joint and 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 system. 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 lever 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 plantarflexion 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, 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.