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 arms 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 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 they will next investigate the more complicated movements associated with kicking a soccer ball. Ask for one student to come to the front of the room and demonstrate how he or she would kick an imaginary ball lying on the ground.

8. As the student kicks, ask the class to point out muscles that produce the kicking motion.

• For each muscle that students point out, have them explain where they think that the muscle attaches to the leg bones.
• Ask what happens when that muscle contracts.
• As necessary, have the student repeat that part of the kick featuring the muscle under discussion.

Keep this discussion relatively brief. Do not try to discuss all the muscles involved. The next step will address six muscle groups involved in kicking a ball. Guide the discussion so that muscles connecting the hip to the upper leg, the upper leg to the lower leg, and the lower leg to the foot are discussed.

9. Display a transparency of Master 3.4, Muscle Group Sequence. Explain that this transparency depicts six muscle groups needed for kicking a ball. The muscle groups are represented as shaded bands attached to the skeleton.

• Go over each muscle group, pointing out where it attaches to the skeleton.
• For each muscle group, ask the students, “What will happen when this muscle contracts?”

As the movements produced by muscle contractions are discussed, they can be drawn on the transparency as arrows indicating the direction of movement. By noting specific points of attachment of the muscle groups, students can assess the movement that contraction of a muscle group produces about the hip joint, knee, or ankle.

Note to teachers: The kicking motion will vary depending on how individual students kick. See also the annotation for Step 12 below.

10. Organize students into teams of three. Give each team one copy of Master 3.4, Muscle Group Sequence.

11. One member of each team should demonstrate the kicking motion as necessary. Instruct teams to review

• how each of the six muscle groups attaches to the skeleton and
• the movements produced by each muscle group.

12. After teams observe the kicking motion and review the movements produced by the six muscle groups, they should determine the sequence in which the six muscle groups function to produce the kicking motion. Students should write down their sequences in the table on Master 3.4, Muscle Group Sequence.

Because students will not necessarily demonstrate the same kicking motion, there is no single, correct sequence in which the six muscle groups will function to produce the kicking motion. For instance, some students may pull their leg back (D) before bending the leg at the knee (C), while others may reverse that order. There also may be variation in the point at which the foot is extended and flexed. It is important only that students present a reasonable sequence of muscle action that demonstrates muscles contracting in an order consistent with the kick investigated by their group.

13. After teams have determined their sequences, reconvene the class. Ask for volunteers to demonstrate their kicks, report their sequences of muscle contractions, and explain their reasoning. Collect copies of Master 3.4.

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 table on the right-hand 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 3.6.

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.”