1. Ask students to hold their right arms straight out in front of them with the palm facing up. Then ask them to flex that arm—that is, they should bring the extended hand upward toward their heads while bending their elbow.
2. Ask students to use their left hands to feel what the muscles of their right upper arm are doing.

Students should examine both the biceps and triceps. Some students may not know the names of these muscles. Point out to students that the biceps is on the top (or front) of the upper arm and the triceps is on the bottom (or back) of the upper arm.

3. Ask students to extend their right arms back to the starting position and continue to feel what the biceps and triceps of their right upper arms are doing.
4. Ask students to describe what they feel happening to the muscles of their right upper arms.

Students generally can feel the biceps of the right arm shorten (contract) and become less soft and more firm as the arm is flexed. At this point, the triceps is relaxed. As the arm is extended, particularly if it is held straight out or straight down, students can feel the biceps stretch and become relaxed while the triceps becomes more firm (it is difficult to feel the triceps contract).

5. Ask students the following questions:

• “Which muscle allows you to flex your arm?” or “Where on your arm is the muscle that allows you to flex your arm?”
• “Which muscle allows you to extend your arm?” or “Where on your arm is the muscle that allows you to extend your arm?”

Students should recognize that the biceps flexes the arm while the triceps extends it. Some students might believe erroneously that the biceps pulls (contracts) to flex the arm and then pushes to extend it. However, they should have learned from their own muscles that the biceps is very relaxed and the triceps begins to tighten when the arm is extended.

• “Why do you need both biceps and triceps to move the lower arm up and down at the elbow?”

Students should conclude that muscles act by contracting (shortening). Help them realize that muscles work in opposing pairs to produce opposite (for instance, push-pull) movements.

Note to teachers: The major objectives of this lesson are to stress that muscles produce movement when they contract and that they are arranged in opposing pairs. You should not get distracted with details of anatomy. Questions may come up, however, about how muscles attach to bone. You may wish to explain that

• where two bones meet is called a joint (for example, the elbow is a joint);
• special connective tissue called tendons attaches muscles to bone; and
• ligaments are another type of connective tissue. They connect bone to bone; that is, ligaments hold the bones of a joint together.

Students will probably understand that the biceps and triceps should each connect the upper arm to the lower arm. Although the anatomy is more complex than students need to understand, the biceps originates in two places, the collarbone and the upper arm bone (humerus) and then attaches to the radius of the lower arm. The triceps also originates in the collarbone and the humerus and then attaches to the ulna of the lower arm.

6. Explain to students that moving the arm about the elbow is a simple motion involving two muscle groups. Show the transparency of Master 3.1, Muscles, and explain that more complicated movements, such as running, involve more than two muscle groups.

Point out to students that the human body has more than 660 skeletal muscles, which includes 56 total in both legs.

7. Explain that students will next investigate six muscle groups involved in kicking a soccer ball. To simplify the situation, they will be able to see the muscle groups represented as colored bands. They will also see how the muscle groups connect the bones of the hip, leg, and foot.

By noting specific points of attachment of the muscle groups, students can assess the movement that contraction of a muscle group produces about joints at the hip, knee, and ankle.

8. Organize students into teams of three.

Give each team one copy of Master 3.2, Anatomy of a Kick Results.

Explain that they will use this handout to record the locations and movements produced by the six muscle groups involved in the activity.

Direct students to their computer stations.

9. Ensure that students’ browsers are open to the URL http://science.education.nih.gov/supplements/bone/student and ask students to click on the “Lesson 3—Anatomy of a Kick” link.

After the opening animation plays, the activity screen is divided into three sections:

• In the upper left area, users can watch an animation of a character kicking a ball. Two views are provided. One shows the character with its complete muscle system. The other shows a skeleton with colored bands representing the six different muscle groups. A slider is provided so that the user can control the progress of the animation and even stop motion so that the movement of the leg and foot can be studied in detail.
• In the upper right area, the leg bones are depicted with colored bands of muscle attached to them. These bands represent larger groups of muscles and make it easier for students to see how the muscles attach to the bones around a joint. Users may select front, back, and side views to visualize the attachment locations of the muscle groups. When students click on the letter representing a muscle group, the movement produced by that muscle group is shown.
• The bottom portion of the screen features six boxes where students enter the sequence of muscle contractions necessary to kick the ball. To help students begin, the first muscle group used is already filled in. The figure to the left of the boxes shows the lower leg pulled back as the character begins its kick. The muscle group responsible for pulling the lower leg back is “D,” and that letter is entered in the first box.

10. Instruct teams to begin by investigating how each of the six muscle groups attaches to the skeleton. They also should observe the movements produced by each muscle group. Students should fill out Master 3.2, Anatomy of a Kick Results, as they investigate.

Students should do the following:

• View the leg in the upper right-hand panel from all three views: front, back, and side. This helps them understand where each muscle group attaches to the bones of the hip, leg, and foot.
• Click on each muscle group to observe the movement it produces. By clicking on a muscle group, students see movement around a joint (hip, knee, or ankle) that is in only one direction. This emphasizes that muscles only contract to produce movement in one direction.

11. Instruct students to view the animations of the figure kicking the ball in the upper left-hand panel.

These animations show the leg movements used to kick a ball. Students can play the animations at normal speed or use the slider to control the animation manually.

12. After students investigate the activity of the six muscle groups and observe the kicking motion, they should determine the sequence in which the six muscle groups function to allow the character to kick the soccer ball. Students should enter their sequences in the boxes at the bottom of the screen. They can test their sequences by clicking on the “Test” button.

The first muscle group to contract (D) is already entered on the screen. Students enter muscle groups functioning second through sixth during the kick. If the sequence entered is correct, an animation of the character kicking the ball plays. If a sequence entered is not correct, the boxes are cleared from the first incorrect box to the end. The contraction of muscle groups A and F occurs almost simultaneously during the kick. For this reason, students can either enter AF or FA within the sequence and be considered correct. Therefore, the correct sequences are D B F A C E and D B A F C E.

Tip from the field test: If students would like a larger view of the animations, they can right click with their mouse and use the zoom feature.

13. Reconvene the class. Ask students, “What did you learn about muscles from this activity?” Collect Master 3.2.

Student responses will vary. Bring out in the discussion that muscles attached to bones on both sides of a joint produce movement about the joint. Students should also recognize that by being arranged in opposing pairs, muscles, which can only contract, move bones back and forth.

14. Ask students, “Why does the body need so many skeletal muscles?”

Answers will vary. Focus student thinking on the many movements the human body is capable of performing. For example, the hand and fingers are capable of a wide variety of different movements.

15. Explain that students are now going to apply their knowledge of muscle function to an analysis of how muscles move bones in the arms. Display a transparency of Master 3.3, Relax! I’m Contracting.

Master 3.3 depicts a human skeleton in three panels. In the first panel, the skeleton has its arms extending straight out from the body. The second panel depicts movement about each elbow and wrist joint. In the third panel, the arms have returned to their original positions. In Panel A, eight muscles are shown. Each muscle attaches on either side of a joint and is identified by a number.

16. Ask students to compare the positions of the arms among the figures in the three panels. What movements have occurred to change the arm positions from Panel A to Panel B? From Panel B to Panel C?

Students should mention the following:

• In going from Panel A to Panel B, there has been movement about the elbow and wrist joints.
• In going from Panel B to Panel C, there also has been movement about the elbow and wrist joints, but this time in reverse of that moving from Panel A to Panel B.

17. Give each student one copy of Master 3.3, Relax! I’m Contracting. Instruct students to

• consider the movements needed to go from Panel A to Panel B;
• indicate in the left-hand table whether each of the eight labeled muscles was relaxing or contracting;
• consider the movements needed to go from Panel B to Panel C; and
• indicate in the right-hand table whether each of the eight labeled muscles was relaxing or contracting.

Give students about five minutes to complete the task. The correct responses are shown in Figure 3.3.

18. Ask students which muscles were relaxing and which were contracting. Write their responses on the transparency.

If a student makes a mistake, ask the class if they agree. Guide the discussion to bring out the correct muscle movements.

19. Ask students if they can relate the pattern of relaxing and contracting muscles when they compare the results between the two tables.

Students should notice that as movement goes from Panel A to Panel B, the pattern of muscle contraction is the opposite of that going from Panel B to Panel C. This is because the movement depicted in the panels goes from a starting position to a new position and then back to the original position.

20. Conclude the lesson by asking students to write a sentence or two describing how muscles cause our arms or legs to move. Instruct them to include the ideas of contraction and opposing pairs in their descriptions.

The following is an example of what a student might write: “Opposing pairs of muscles contract to cause movement in opposite directions.”