The Human Body as a System

As already mentioned, the most common type of lever, in the human body, is a type 3 lever. This is where the force arm is shorter than the resistance arm, and lies within its length.

Bones as levers

The long bones of the body act as the arms of our levers. If we consider flexion of the elbow, as our example, then the radius and ulna combine as our resistance arm when we hold a weight in our hand. And, the distance from the elbow joint, to the insertion of the bicep makes up our force arm.

Joints as fulcrums

As we’ve seen, a fulcrum is the point of rotation for the lever. The rotation in the joints of the human body is an example of this. And, this rotation at the joints is what causes all movement of the body.

Muscles as forces

In order for our bones to rotate about our joints, the muscles of the body must pull on them, and create these movements. As such, the muscles become the force part of the lever system.

The combinations of these, in the human body, lead to some interesting effects. As far as force production is concerned, we can see that the type of lever most common in the body leads to a slight disadvantage, for overcoming forces. Fortunately, that can be worked on (and will be discussed later). Where this is made up for, is in the resultant speed of movement. Consider a rod spinning about one of its ends. Positions on the rod that are closer to the fixed end (point of rotation) are circling at a slower rate than points farther from the centre of rotation. When we apply this to the levers in our bodies, the point of attachment of the muscle rotates relatively slowly. However, that movement causes the distal end of the bone to move at a far greater rate.

A rather famous example of this speed advantage working in a person’s favour could be Usain Bolt. Yes, his extra height gives him a stride-length advantage. But, the rate at which his feet move at the end of the levers of his legs greatly increases his speed advantage.

In the illustration above, the levers are applied as follows: Force: Weight of forearm, or weight in hand Effort: The biceps muscle pulling from the biceps tendon insertion onto the radius Fulcrum: The elbow joint