BioEd Online

Teacher Background

Generally, when we think of stress, we think of being over-worked, mentally tired or overwhelmed by our daily lives. While too much stress can be detrimental to the body, too little of some kinds of stress can be harmful. Activities like walking, carrying packages and mopping the floor are physical stresses. Activities like doing crossword puzzles, balancing the check­book and reading are mental stresses. There also are emotional stresses, like receiving a bad grade on a test or walking into a surprise birthday party. Our bodies, including muscles and bones, require some physical and mental stresses to be healthy and grow.

Physical stress is created when bones and muscles are made to work against a force. It occurs when we pick up something heavy, like a 20-pound bag of cat litter. Gravity pulls down on the bag and we have to work to overcome that force to lift the bag. Swimming also causes stress because muscles and bones have to work against the resistance of the water to move the body. Gravity pulls on our bodies and our muscles and bones constantly work to counteract that force and keep us balanced.

Stress from physical activity is necessary for bone growth and maintenance. The body builds bone based on its needs. The need for any particular bone is dictated by the amount of stress placed on it. During the years a person’s bones are growing (birth to about age 25), physical stress on bones causes builder cells to work more, which makes bones grow. Builder cells produce collagen fibers that form the framework of bones. The framework is then filled in with minerals, producing a strong, thick bone (see the activity, “Bone Structure: Hollow vs. Solid"). Even after they stop growing, bones still need physical stress to maintain thickness and strength.

Muscles also rebuild and grow as a result of physical stress. Stress can lead to change in either muscle strength or muscle stamina (the ability to perform an activity for a long time without becoming tired). High-intensity, short-duration exercises (or stresses), like weight lifting, cause muscles to increase in strength. Low-intensity, long-duration activities, such as running and swimming, cause muscles to increase in stamina.

Objectives and Standards

Concepts

• Bones and muscles need exercise to be healthy.

• Muscles and bones are constantly changing.

• Stress tells bones and muscles how to change.

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Science, Health and Math Skills

• Observing

• Gathering data

• Drawing conclusions

Materials and Setup

Teacher Materials

• Overhead projector and screen

• Timers or clock with second hand)

Materials per Group of Students (see Setup below)

• 2 transparent plastic knives

Materials per Student

• Spring-hinged clip or clothespin

• Copy of the student page

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Setup

1. Place the materials for each session in a central area for Materials Managers to collect for their groups.

2. For Part 1, divide students into groups of 2–4 and give each group two plastic knives.

3. For Part 2, give every student one spring-hinged clip or clothespin and a copy of the student sheet.

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Safety

Please follow all school district and school laboratory safety procedures. It always is a good idea to have students wash hands before and after any lab activity.

Procedure and Extensions

Time: For Part 1, 10 minutes for set-up, 20 minutes for activity. For Part 2, 50 minutes on Day 1, five minutes every other day for two weeks, and 50 minutes on the final day to conduct activity.

Part 1: Stress Observations

1. Introduce the topic of stress by asking questions such as, What is stress? How can stress be a good thing? What are some good stresses? Explain that there are “good” stresses and “bad” stresses and that the body needs good stresses, like exercise, to be healthy.

2. Tell students that they are going to investigate how physical stress can affect bone—a hard material.

3. Have students compare the two knives to determine if they are the same or different.

4. Instruct students to mark one knife and bend it back and forth several times without breaking it.

5. Again, have the students compare the two knives. Ask, Is anything is different between them? Request a volunteer to bring up his/her group’s knives and place them on an overhead projector. Have students observe the knives and ask again if there is anything different between them.
   
   The students will be able to observe very thin opaque lines have developed only in the knife that was bent. Often, the lines are observable even without using an overhead projector. However, the projector will make the lines easier to see.

6. Discuss students’ observations. Explain that when they bent the knives or plastic strips, they applied physical stress and changed the appearance of the objects. Ask, If we wanted to break this knife, would it be easier to do so where we bent it before, or at another point? Why do you think it would be easier to break where we’ve already bent it? 
   
   The changes in the knives may look minor, but they are important to the objects’ structure. This concept is true for bones, too. Gravity and movement cause tiny, invisible stress patterns in bones. If we could see them, they would look very unimportant, but they tell the “bone construction crews” where to work to make bone thicker and stronger.

Part 2: Stress and Muscles

1. Explain to students that they will be exploring the effects of stress on the muscles in their hands.

2. The first trial will test each student’s initial muscle strength and stamina. Explain the exercise to students. Ask students to predict how many times they will be able to click a hinged clip or clothespin with their right (or dominant) hand during each of three, one-minute trials, and to record their predictions on their student sheets. 
   
   Have each student count the actual number of times he/she can click a hinged clip in one minute using his/her right (or dominant) hand, and record his/her results. Have students rest for one minute and then repeat the trials two more times.
   
   If students are working in pairs, have one student complete the trial while the other measures the time. Then have students switch roles. After students have completed all trials, ask, Did you feel your hand muscles burn? Were you more tired after each minute of clicking? Why do you think that happened?

3. Every other day for the next two weeks, have students repeat the exercise described above. This is the conditioning period. The stress induced by the hinged clip on the muscles of the hand will cause the muscles to become stronger and gain stamina. Have students predict and report their results each day.

4. The test of how well the stress conditioning worked comes on the last day of the two-week period. Again, have each student predict how many times he/she will be able to click the hinged clip during the timed periods and record his/her prediction. Have each student repeat the clicking-resting experiment again exactly as it is described in Step 2 and record the results.

5. Instruct students to write a paragraph (on a separate sheet of paper) about the results of their experiment. They should explain what happened and why they think things turned out the way they did.

6. Discuss results from the initial and final experiments. Students will discover that they were able to click the hinged clip more times (and with less muscle soreness) in the first one-minute period after the conditioning period. This shows that their muscles have grown stronger. Students also will discover that they are able to click more times in the second and third one-minute periods after the conditioning has taken place. This shows that the muscles have increased in stamina.  Ask students, Were you able to click more times in the third trial after two weeks than at the beginning of the experiment? Why do you think that happened? How did your results compare with your predictions?

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Extension

Have students graph their results to produce a visual representation of changes that occurred in the three bi-daily trials over the course of two weeks. They should create separate graphs for each one-minute period and record how the number of clothespin clicks changed over time. This will help students to understand how their strength and endurance increased.

Handouts and Downloads

Download color images from The Science of Muscles and Bones Teacher's Guide.