1. Begin the activity by reminding the class of what they learned in Lesson 2: What Makes Bones Strong:

• Minerals and collagen affect the strength of bone.
• As shown in the bathtub model, bone is a dynamic system that gains and loses material in response to environmental factors such as growth, stress, and diet.

Ask the class, “On the basis of what you have learned about bone, what can you do to increase the strength and health of your bones?”

Write their responses on the board. Student responses will vary. Some will mention changing diet. Others may mention exercise. Make sure these two responses are on your list.

2. Explain that students will investigate how diet and different physical activities influence bone. Ask the class, “How can we tell if a bone is healthy or if it is strong?”

Students may mention that bone strength can be measured by having people perform fitness tests. To such a response, point out that these tests are largely measuring muscle, not bone, strength. If students respond that the bone itself can be subjected to a chemical analysis, mention the problem of having to remove it from the person being studied!

3. Explain that scientists use specialized X-rays to estimate the amount of minerals in bones without harming the person being studied. X-rays of bones being studied are compared with X-rays of bones that have known amounts of minerals in them.

Note to teachers: The specialized X-rays referred to here are called bone mineral density (BMD) scans. The most widely used method of BMD scan is dual X-ray absorptiometry (DEXA scan). DEXA scans use two different X-ray beams. Bone mineral density scans provide quantitative information about bone that cannot be obtained from standard X-rays.

4. Explain to students that they are going to read about a study that investigates the influence of diet and physical activity on the health of bone. Place a transparency of Master 5.1, Description of Milk Study, on an overhead and read it aloud to the class. Ask students whether they have any questions about how the experiment was performed.

The important points for students to understand are that teenagers in both groups experienced the same resistance training (bench pressing) and that those in the group that drank juice did not receive the extra minerals, especially calcium, which were consumed by those in the milk group.

5. Ask the class to predict the results of the experiment. Write their predictions on the board.

Student responses will vary. Some may predict that teenagers in the milk group will display greater strength and have more minerals in their bones than teenagers in the juice group. At this time, accept all answers.

6. Divide the class into teams of two. Give each team one copy of Master 5.2, Data from Milk Study. Explain that they have a few minutes to look it over and write down what conclusions they can make from the data.

Give the teams about five minutes to write down their conclusions. Students should notice that the amount of weight that study participants bench-pressed increased for both the juice group and the milk group. They should also notice that although the bone minerals increased in both groups, bone minerals increased more for the milk group than the juice group.

Note to teachers: Students may notice that in Week 6, the bench-pressed weight is slightly greater for the milk group than the juice group. Explain that you would expect the average bench-pressed weight to be similar but not necessarily the same for the juice group and milk group because of variation among individuals in this study. Emphasize how similar the values are, considering that each group had 15 teenagers who might respond differently to resistance training.

7. Remind the class of their predictions written on the board. Ask, “Were your predictions supported by the data? Why or why not?”

Some of the predictions will have been supported by the data, while
others will not.

8. Ask the class, “On the basis of the data from this study, what conclusions can you make about the effects of drinking milk on strength and the mineral content of bone?”

Student responses will vary. The discussion should bring out two main conclusions: 1) both groups of teenagers showed the same increase in strength (as measured by the amount of weight they could bench-press), and 2) the milk group’s increase in bone minerals was twice that of the juice group.

Some students may respond that since both the juice and milk groups showed the same increase in strength, calcium and other minerals found in milk are not important. Point out that the study measured bone-mineral content as well as strength and that the milk group showed a greater increase in bone-mineral content. If bone-mineral content goes up, then bones are stronger and less likely to break. Healthy bones with greater bone-mineral content may also help protect against disease and fractures later in life.

Tip from the field test: Some students may realize that milk contains protein as well as calcium, neither of which is present in orange juice. Explain that more studies would have to be done to determine whether the greater increase in bone-mineral content for milk drinkers compared with juice drinkers resulted from the calcium or protein in the milk. Use this as an opportunity to point out the tentative nature of science. Scientists must use results from many studies to establish the influences on bone-mineral content.

1. Ask students to recall the data presented in the previous activity. Ask the class, “How would resistance training alone affect bone minerals?”

Students should recognize that resistance training increases bone minerals, because the data in the previous study showed increases in bone minerals for those in the juice group as well as for those in the milk group.

2. Ask the class, “Do you think different kinds of physical activities affect bone minerals differently?” Explain to students that they will examine the results of a study that compares the influence of different physical activities on bone-mineral content.
3. Have the students assemble in teams of two, as in the previous activity. Give each team one copy of Master 5.3, Description of Sports Study. Have a student read it aloud to the class. Ask whether they have any questions about how the study was performed.

Answer any questions about how the study was performed. It is important to stress that students in the various groups were similar to each other except for their exercise habits.

4. Instruct teams to discuss the study and place the different groups in order of increasing bone-mineral content.

Students should write in the spaces provided on Master 5.3, Description of Sports Study. The group with the highest level of bone minerals should be labeled “1,” while the group with the lowest level of bone minerals should be labeled “4.”

5. Ask for volunteers to report their rankings and to explain why they put the groups in the order that they did.

Most students will conclude correctly that the control group (those who exercise very little) has the lowest level of bone minerals.

Students will probably not agree, however, on which sports produce the highest level of bone minerals. At this time, do not reveal the correct sequence. Students will confirm or refute their predictions in the next steps.

6. Give each team one copy of Master 5.4, Data from Sports Study. Allow the teams five minutes to review the data and reorder the groups, if desired.

Note to teachers: Students may ask why the study only provided bone-mineral content for the spine and hip and not bones in other parts of the body such as the leg or arm. Explain that the spine and hip are the most common areas tested for bone-mineral content because they generally have the most bone loss and are more likely to fracture when weakened by low mineral content.

7. After teams have completed their task, ask whether any teams have reordered their groups. Ask them to explain why.

After teams have completed their task, ask whether any teams have reordered their groups. Ask them to explain why.

1. basketball and volleyball (highest)
2. soccer and running short-distance track
3. swimming
4. control (lowest)

Students should explain that they reordered their groups based on the bone-mineral content data given in Master 5.4. These data showed that for both spine and hip measurements, the basketball and volleyball group had the highest bone-mineral content, followed by soccer and short-distance track, followed by swimming. The control group, which did not exercise, had the lowest bone-mineral content.

8. Ask teams to discuss where in the list they would rank the bone-mineral content of an astronaut who had spent time in space and then returned to Earth. They should record their prediction on Master 5.3, Description of Sports Study.

Make it clear that you are referring to an astronaut who has spent time in space and had his or her bone minerals measured after returning to Earth. Encourage students to think about what might be different for an astronaut in space as compared to someone on Earth.

9. Ask teams to share their predictions with the class and explain how they decided where to rank the astronaut.

Student answers will vary. Students are likely to rank the astronaut near swimming (3) and the control group (4) because astronauts cannot be as active as an individual playing basketball or soccer. Students might also rank the astronaut near 3 or 4 because they recognize that astronauts are in a weightless environment and thus are unable to do activities that put weight on their bones.

10. Explain to the class that the astronaut would be ranked 5, with bone-mineral content less than the control group because astronauts lose minerals from their bones while in space. Ask students why they think this happens.

Make sure students understand that astronauts lose minerals from their bones even if they exercise in space (such as ride a stationary bike). Students should recognize that astronauts are in a weightless (gravity-less) environment. Guide the discussion to bring out the importance of putting stress on bone and that weight is necessary to put stress on bone. In a weightless environment, astronauts put very little stress on their bones.

11. Remind the class that placing bone under stress from physical activity leads to minerals being added to bone. Weight-bearing activities put bone under more stress than nonweight-bearing activities.

Make sure students understand that weight-bearing activities include any activity in which feet and legs carry a person’s weight or any activity that involves carrying, lifting, or pushing a heavy object. Therefore, walking and jumping are considered weight-bearing activities.

12. Instruct students to revisit the data from the sports study. Facilitate a class discussion of whether any or all of the sports would be considered a weight-bearing activity. Include astronauts in your discussion.

Make sure students understand that basketball, volleyball, soccer, and short-distance track are weight-bearing activities. In both groups, the sports involve carrying a person’s weight while running and jumping. Swimming is not a weight-bearing activity because water supports the body. Students should recognize that astronauts are unable to do weight-bearing activities because they are in a weightless environment.

13. Explain to the class that osteoporosis is a disease characterized by low bone-mineral content and an increased risk for fractures (breaks). The disease usually affects people later in life. This study suggests that increasing bone-mineral content by playing sports while young may help lessen the effects of osteoporosis when older. Ask the class, “What type of physical activity (not a sport) do you think would lead to the greatest gains in bone-mineral content?”

On the basis of the study discussed in this activity, students should recognize that weight-bearing forms of exercise lead to gains in bone-mineral content. They may suggest running. You might suggest that students think about how basketball and volleyball differ from soccer and short-distance running. Remind them that the basketball and volleyball group had the highest bone-mineral content. Students might recognize that basketball and volleyball involve more jumping than soccer. If necessary, remind the class that placing the bones under high levels of stress helps to increase the amount of minerals in bone. Jumping has been found to be an especially effective form of exercise in this regard. As a result, jumping exercises are being recommended as a part of physical education programs in schools.

14. To conclude the activity, instruct students to write a brief explanation of why the bone-mineral contents of astronauts and each of the groups from the sports study are different.

As each sport is discussed, make sure that students explain why the bone-mineral content differs between groups. Look for explanations that point out that bones under weight-bearing stress, as from gravity and physical activity, will have increased mineral content.

• Astronauts—Astronauts can exercise in space, but very little stress is placed on their bones because of the weightless environment. Because their bones are not placed under stress, they lose bone minerals while in space.
• Control group—Students in the control group exercised very little. This lack of exercise accounts for these students having the second-lowest bone-mineral content of the five categories in this activity.
• Swimming—An environment such as water supports the body and partly relieves the stress from gravity. Therefore, swimming is not called a weight-bearing exercise and does little to increase bone-mineral content.
• Soccer and running short-distance track—Training for these sports involves running and jumping, mostly on grass or a soft track. Soccer and running are weight-bearing sports and lead to increased bone-mineral content in the spine and hip.
• Basketball and volleyball—Training for these sports involves running, jumping, and cutting (changing directions sharply and suddenly) on hard surfaces. Basketball and volleyball are also weight-bearing sports. However, they place greater stress on the bones than soccer and running short distances. This leads to higher bone-mineral contents.