“Give me a lever long enough and a place to stand and I will move the Earth”
Levers are one of the basic tools that were probably used in prehistoric times. Levers were first described about 260 BC by the ancient Greek mathematician Archimedes (287-212 BC).
A lever is a simple machine that makes work easier for use; it involves moving a load around a pivot using a force. Many of our basic tools use levers, including scissors, pliers, hammer claws, nut crackers and tongs.
Lever first class , the pivot (fulcrum) is between the effort (force) and the load. Lever second class , the load is between the pivot and the effort (force) and Lever third class , the effort is between the pivot and the load.
Levers classified by positions of the forces
Bones, ligaments, and muscles are the structures that form levers in the body to create human movement. In simple terms, a joint (where two or more bones join together) forms the pivot (or fulcrum), and the muscles crossing the joint apply the force to move a weight or resistance. All three types are found in the body, but most levers in the human body are third class.
First-class levers in the human body are rare. One example is the joint between the head and the first vertebra.The weight (resistance) is the head, the pivot is the joint, and the muscular action (force) come from any of the posterior muscles attaching to the skull, such as the trapezius.
In the human body, an example of a second-class lever is found in the lower leg when someone stands on tiptoes The axis is formed by the metatarsophalangeal joints, the resistance is the weight of the body, and the force is applied to the calcaneus bone (heel) by the gastrocnemius and soleus muscles through the Achilles tendon.
There are numerous third-class levers in the human body; one example can be illustrated in the elbow joint The joint is the axis (fulcrum). The resistance (weight) is the forearm, wrist, and hand. The force is the biceps muscle when the elbow is flexed.
The fingers and hand are a class one, two or three lever system depending on the position of the load in the hand.
When the load is concentrated toward the end of the fingers, it is a class three lever system, flexible but weak. This would be similar to a pinch grip, such as holding a pencil. If the load is toward the wrist, and the fingers can curl around it, a power grip can be used. This is a class one or two lever system, stronger but less flexible.
If you have ever worked in a job that required lifting objects you will have come across signs telling you to lift with your legs, not you back. The reason for such signs is that by bending over from your hips with your back straight, even before you pick up the object,you will put about 4000N (400 kg) of compression on your backbone.
When you bend over the upper part of your body applies a torque about your hips that tends to turn the body downward (clockwise in the diagram). This torque is about that which would result from your upper body weight, typically 400N (40 kg), applied at a distance of about
300 mm (0.3 m) from your hips. The resulting torque about your hips is therefore about
This torque must be resisted by something, or your upper body just falls down. That something is, of course, the pull of your major back muscle between your hip and your upper backbone. This force, in turn, puts an equal force of compression on your backbone. These forces, the pull of your back muscle against the push of your backbone, together form a couple of the torque required to balance the torque from the weight of your upper body.
Another example of torques generating severe forces within the human body is that of supporting an object in your hand when you stretch out your arm. Here there are two major joints about which torques have to be resisted, that of the elbow and that of the shoulder.Because of the greater distance of the force from the shoulder, the torque at the shoulder will be greater than the torque at the elbow. Nature seems to know this by making the shoulder much sturdier than the elbow.As an example, suppose the distance from a 1 kg ball to your elbow joint was 35 cm and the distance from your elbow joint to your shoulder joint was another 35 cm. The torque that would have to be generated by your forearm muscle would then be about 3.5 Nm but the torque that would have to be provided by your bicep would be 7 Nm.
A little reminder- what is the torque or moment?