Lever Systems

See figure 5-3 for an illustration of the three classes of levers.

a. First Class. In a first class lever, the weight to be moved is at one end of the lever, the applied force is at the other end, and the fulcrum (the pivot or turning point) is between the two.

b. Second Class. In a second class lever, the weight to be moved is between the applied force and the fulcrum. This type of lever enables a weight to be moved with less force than would be required without a lever. (Many feel that there are no second class levers in the human body.)

c. Third Class. In a third class lever, the weight to be moved is at one end of the lever, the fulcrum is at the other end, and the applied force is between the weight and the fulcrum. This type of lever provides speed, but a greater amount of force is required for a given weight. This is the most common type of lever in the human body.

FIRST CLASS SECOND CLASS THIRD CLASS

FIRST CLASS SECOND CLASS THIRD CLASS

Figure 5-3. Types of lever systems. 5-7. SIMPLE PULLEY SYSTEM

Figure 5-3. Types of lever systems. 5-7. SIMPLE PULLEY SYSTEM

a. In the human body when the tendon of a skeletal muscle slides over a round bony surface, the "system" acts like a simple pulley (figure 5-4). A simple pulley provides a change in the direction of the force or muscle pull. There is no change in the amount of force produced by the muscle. For example, the knee acts as a simple pulley by which the quadriceps femoris M. extends the leg.

Figure 5-4. A simple pulley (the human knee mechanism).

b. Sesamoid bones, such as the patella (kneecap), develop in tendons where pressure is applied to the tendon.

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