Levers of the Body

Introduction

A great deal can be learned about the human body when it is analyzed as a machine. It can be compared to a steam shovel, an electric plant, a furnace or other pieces of machinery. In this series of demonstrations the human body will be examined as a simple machine—a lever.

Concepts

• Muscle contraction/relaxation

• Lever classes
• Lever components

Background

Levers are rigid objects, usually in the shape of a bar, that can turn on one point or axis. This point is called the fulcrum in lever systems. A lever is used for the transfer and modification of force and motion. The human anatomy uses levers to make movement and strength/lifting/motions more efficient. The skeleton serves as a set of levers and the attached muscles supply the force. The net result of the muscle/bone interaction is normal body movement. The net result is that a small muscle contraction can result/provide a much larger movement or lifting potential because of the multiplying advantage of the levers. The movements can be fast, slow, long, short, coordinated and uncoordinated and can have various forms. To understand the many lever systems of the body, it is important to understand how levers work. The different lever types help visualize how the muscles and bones work together to move the body.

In a lever system the lever itself is always rigid—like a bar, rod, plank or bone. The load is whatever is being moved—a rock, a wheelbarrow, or an arm. The fulcrum is the pivot point around which the lever moves—in a skeleton it is a hinged joint where one bone moves against another one (e.g., elbow, knee). The force is anything capable of doing mechanical work—it may be a spring, a motor, a jet or a muscle.

Lever systems in action are useful in gaining speed, distance, precision or mechanical advantage. Mechanical advantage is defined as the ratio of work accomplished to the force applied. Each lever system has its own unique properties and has a balance among mechanical advantage and other traits. In general, there is a reverse relationship between mechanical advantage and both the amount and speed of movement. But there is no necessary relationship to precision.

The three basic lever types are diagramed in Figure 1. They are arbitrarily called Type I, II and III. Some texts call them A, B and C.

Properties and examples of each lever type are discussed with each demonstration. All of the demonstrations in this kit can easily be converted into individual laboratory exercises if sufficient equipment is available.

An individual skeletal muscle is separated from adjacent muscles and held in position by layers of fibrous connective tissue called fascia. This connective tissue surrounds each muscle and may project beyond the ends of the muscle fibers to form a cordlike tendon. Fibers in a tendon intertwine with those in the periosteum (a bone’s fibrous outer covering), thus attaching the muscle to a bone. Determining where on the bone a muscle is attached is critical in body lever analysis.

Several muscle features must be known when analyzing body mechanics. The most critical concept is that muscles can only contract and relax. Muscles do not lengthen to exert force. Muscles only exert force when they contract. This is why skeletal muscles are usually found in opposing pairs. One muscle contracts and pulls a lever in one direction while an opposing muscle is simultaneously relaxed. To move the lever in the opposite direction, the muscles reverse roles—the relaxed muscle contracts while the contracted muscle relaxes.

Materials

Safety Precautions

These laboratory demonstrations are considered safe. Follow all normal laboratory safety guidelines.

Disposal

All materials in these demonstrations can be recycled and reused many times.

Prelab Preparation

Secure a clean, empty gallon plastic jug with a secure handle and a piece of rope about 24" long prior to class. Locate the mailing tube that this kit was shipped in, and remove one of the end plugs.

Procedure

Demonstration 1. How Strong—How Long?

What happens to the force required to lift a load as the load is moved along the lever?

1. Fill the gallon jug with water and screw on the cap.
2. Tie a rope through the handle of the jug forming a loop that is large enough to slide along the arm (see Figure 2).

3. Select a volunteer—someone who thinks he or she is strong! Have the volunteer place his elbow on the edge of the table or desk. Place the rope loop near to the elbow and have the volunteer support the jug with his arm extended over the edge of the table. Move the jug successively further out on his arm until the jug is at the wrist. Have the volunteer describe the difficulty in supporting the jug as “harder” or “easier” along the length of the arm. (The volunteer should note that it requires more force to support the jug as it gets further away from the elbow—fulcrum.)
4. Now “extend” the volunteer’s arm by adding the mailing tube over the volunteer’s arm as illustrated. How far away from the elbow can the jug be placed and still be supported by the volunteer? Have the volunteer describe the muscle that is supporting the jug and what it is like as the jug gets further from the muscle.

Demonstration 2. Lever Type III

1. Introduce the terms fulcrum, force and load. Use the arm and jug in Demonstration 1 to identify the fulcrum (elbow joint), force (biceps) and load (jug). Make a diagram on the chalkboard (see Figure 3).

2. Use the materials to set up a lever simulating the biceps in an arm as shown in Figure 4.

3. Distribute Observation Record Sheet 1. Use weights to place a load near the end of the meter stick.
4. Using the spring scale, measure the force (in grams) required to keep the meter stick level. Take measurements for four different locations of the force relative to the fulcrum and the load. Vary the distances between the fulcrum and the spring scale. Use student volunteers to make the readings.
5. Have students record the demonstration data and calculate the mechanical advantage for each of the force locations.
6. Have students discuss and complete the questions on the Observation Record Sheet 1. Be sure students understand this lever system before going on to the next demonstration.
7. Distribute Observation Record Sheet 2. Conduct four trials where the fulcrum and force positions are held constant while the load is moved (like the arm and the jug).
8. Collect the data and complete Observation Record Sheet 2. Be sure students understand the lever system before going on to the next demonstration.

Demonstration 3. Lever Type I

1. Use the lever materials to set up a Lever Type I setup like the one shown in Figure 5. Discuss the positions of the force, fulcrum and load.

2. Distribute Observation Record Sheet 3.
3. Place a load at one end of the meter stick.
4. Using the spring scale, measure the force (in grams) required to keep the meter stick level for four different locations of the fulcrum. Vary the position of the fulcrum relative to the force and load. Have students record the data and calculate the mechanical advantage on their Observation Record Sheet.
5. Have students discuss and complete the questions on Observation Record Sheet 3.

Demonstration 4. Lever Type II

1. Use the lever materials to set up a Lever Type II like the one shown in Figure 6. Discuss the positions of the force, fulcrum and load.

2. Distribute Observation Record Sheet 4.
3. Place the load at various positions along the lever and measure the force in grams required to keep the load level.
4. Collect data for four different locations of the load and have students record the data on their Observation Record Sheet.
5. Have students discuss and complete the questions on Observation Record Sheet 4.
6. All materials in these demonstrations can be recycled and reused many times.

Student Worksheet PDF

10445_Student1.pdf

Teacher Tips

• This kit contains materials to repeat the demonstrations for many years.

• Make copies of the Observation Record Sheets for all students to work individually.
• Students may also notice that the mechanical advantage is related to the positions of the force and load in relation to the fulcrum.

Sample Data

Observation Record Sheet 1: Lever Type III

Effect of moving the position of the force when fulcrum and load are held in fixed positions.

Observation Record Sheet 2: Lever Type III

Effect of moving or increasing the load when the fulcrum and force are held in fixed positions.

Observation Record Sheet 3: Lever Type I

Effect of moving the fulcrum along the lever.

Observation Record Sheet 4: Lever Type II

Effect of moving the load along the lever

Answers to Questions

Observation Record Sheet 1

1. What happens to the force required to lift the load as the force gets further from the load?
   

   The force required to lift the load increases.
   

2. What happens to the mechanical advantage as the force gets closer to the fulcrum?
   

   The mechanical advantage decreases as the force gets closer to the fulcrum.
   

3. Is the mechanical advantage of the biceps muscle on the arm high or low? What is the advantage of its location?
   

   The biceps muscle has a poor mechanical advantage. The location allows a full range of motion of the arm and an increase in distance at the end of the arm for the distance traveled by the biceps.
   

4. What would be lost if the biceps were attached more in the middle of the forearm? What would be gained?
   

   It would have a greater mechanical advantage and therefore more lifting power with the same size muscle. The speed and range of motion would, however, be limited.

1. What happens to the force required to lift the load as the load is moved further from the fulcrum?
   

   The force required increases as the load is moved further from the fulcrum.
   

2. Relate this answer to the force the bicep must exert to lift a load. How does bicep “strength” depend on arm length? Explain your answer.
   

   The same biceps on two different length arms would require different amounts of energy to exert the same force. Longer arms would not be a necessary advantage.
   

3. How does the distance the force moves compare to the distance the load moves?
   

   When the force is close to the fulcrum, a short movement of the force will result in a larger distance moved by the load.
   

4. Identify other Lever Type III setups in the human body.
   

   Answers will vary but might include large leg muscles.
   

5. Which items (wheelbarrow, shovel or rake) utilize Lever Type III in their normal functioning? Explain.
   

   Rake. Fulcrum is established with upper hand and force is applied with lower hand with the load being the end of the rake.

Observation Record Sheet 3

1. In a Type I Lever, where is the fulcrum when the force and load are equal?

The fulcrum is right in the middle of the lever.

2. What is the relationship between the force needed and the position of the fulcrum?

As the fulcrum gets further from the load, the force required increases.

3. Diagram an arm and include a biceps and triceps muscle and explain how they move the arm as they work in opposition.

4. Based upon the size of the biceps and triceps and their apparent functions, explain how the two lever types work efficiently to operate the arm.

The biceps (Lever Type III) can exert force to lift an object or move the hand quickly with its attachment close to the fulcrum. The triceps (Lever Type I) can move the forearm back to an extended position when the biceps relaxes but usually is only the load of the arm. The biceps are much larger than the triceps and these two opposing muscles seem to function in a coordinated fashion.

5. Which items (wheelbarrow, shovel or rake) utilize Lever Type I in their normal functioning? Explain.

Shovel. The force is usually applied with the arm furthest from the shovel end of the shovel. The center hand is usually the fulcrum and the load is the dirt on the shovel.

1. Where would you place a load with this lever system to spend the least force to lift the load?
   

   When the load is closest to the fulcrum, the least force is required to lift the load.
   

2. Would Lever Type II be a good system for moving a load a long distance? Explain.
   

   No. When the load is close to the fulcrum, the force moves a great distance while the load hardly moves any distance. The lever can have a good mechanical advantage, but has no speed or distance.
   

3. Think of at least one common item that illustrates a Lever Type II system and explain how it works. What are the advantages and disadvantages of the device for the job?
   

   A wheelbarrow is a Type II lever system. Its good mechanical advantage allows lifting a heavy load with little relative force. The load, however, is not lifted very high off the ground if this were desired.
   

4. Diagram a person’s foot standing on the “ball” of her foot with her heel off the ground. Where is the force, fulcrum and load? What lever type is illustrated?
   

   The fulcrum is the ball of the foot. The load is the body weight supported on the long bones of the leg. The force is the contraction of the gastrocnemius (calf) muscle.
   

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