Lever Systems

In order to complete sports movements bones must moved so forces can be applied to the floor or the sports equipment we use. Movements are possible because of joints and muscular contractions. They operate in lever systems. This is where forces applied at one point of a bar (or bone in our case) move a resistance at second point, about a fulcrum or pivot. In our bodies, the muscles exert forces on the bones, moving one or both of these bones at the pivot point and these forces can be applied to other objects that they are in contact with (e.g. the floor, when we run & jump; or a rower pulling the oar against the water).

In a lever system there are three factors that must be considered.

1. Fulcrum: this is point at which the lever hinges or rotates.

2. Effort: this is the contraction of the muscle that pulls the bone, exerting a force.

3. Resistance: this is the load or mass the lever system must overcome. For example in lifting a weight during a bicep curl the resistance will be the mass of the weight and the lower arm, overcoming the force of gravity.

In class 1 levers the fulcrum lies in the middle between effort and load. If you think of a ‘see-saw’ in the local park, this is an example of class I lever system. If a friend sits on one end of the see-saw then you need usually lift them up by applying a force to the lever arm by sitting on the other side (of the fulcrum). Class I levers are not very common in the human body and the two main locations in the body are the neck (action of the upper fibres of the trapezius and levator scapulae in extending the upper vertebrae [tilting the head back]) and at the elbow (role of the triceps brachii in extension at the elbow joint).

In class II levers the fulcrum and effort lie at opposite ends of the lever arm with the load in the middle. An example of this is a wheelbarrow, where the fulcrum is where the wheel touches the ground, effort is applied to the handles to lift an object, maybe a collection of rocks, which are located in the barrow in the middle of the lever system. Class II levers are unusual in the human body and the most prominent example is the role of the gastrocnemius and soleus in calf raises.

In class III levers the fulcrum and resistance lie at opposite ends of the lever arm with the effort in the middle. Class III levers are very common in the body and the actions at the hip, shoulder, knee and elbow (apart from the example shown above) are usually this type. An example is the bicep curl where the dumbbell is lifted by contracting the Bicep Brachii which is inserted on the radius and ulna (lever arm), between the dumbbell and the elbow (fulcrum). This lever system is good for;

· Kicking – so the knee and hip joints operate using this lever system,

· Throwing – so the shoulder joint operates using this lever system.

In applying large forces to an object (e.g. our own bodies, a ball, racket or javelin) or moving the object rapidly, the different classes of levers are not the same. Class III levers are not advantageous for applying large amounts of force but they are good for accelerating objects quickly. The opposite is true for class II levers and the differences are caused by the distances of the ‘resistance arm’ and ‘effort arm’ in each one. When the effort arm is longer than the resistance arm, this is a mechanical advantage because the amount of force required to move the resistance is less than the force exerted by the resistance. This means that class II levers are a mechanical advantage. However, class III levers are a mechanical disadvantage because the effort arm is shorter than the resistance arm right.

Class I levers are not presented in table 9 above because they can be a mechanical advantage or disadvantage. It is a mechanical advantage if the length of the effort arm is larger than the length of the resistance arm. This is the case when tilting the head forwards (flexion of the cervical vertebrae) from an extended position so you would need to apply the same advantages and disadvantages from a class II system. Imagine a person with less weight sitting on the end of a see-saw, with a slightly heavier person sitting on the other side in the middle of the lever arm.

Class I levers can also be a mechanical disadvantage, when the length of the resistance arm is longer than the length of the effort arm. This is seen at the elbow joint and extending the cervical vertebrae. For these examples you would need to apply the advantages and disadvantages of a class III lever system.

Lever Length

Performers with longer levers have distinct advantages over those with shorter ones in some activities. For example, rowers are tall and have long arms and legs, which means that the oar can travel further during each stroke than those who are not as tall; Matthew Pinsent and Steve Redgrave are both around the 1.93 m (6’4”). Discuss how throwers and fast bowlers in cricket benefit from having a wide arm span. Although longer levers (limbs) take more force to move, if two limbs are travelling at the same angular velocity (measured in degrees or radians per second) a hand, foot or racket that is further from the pivot will travel at a faster velocity in those whose arm length is longer.