BioEd Online

Teacher Background

In a skeleton, the places where bones or external plates (as in insects) come together are called joints. Joints allow an animal’s body to flex and bend. Most animals, whether they have exoskeletons or endoskeletons, have joints.

In vertebrate skeletons, some bones, such as those in the skull, are connected at joints that do not allow movement. These “immovable” joints are called sutures. Most bones, however, are connected by ligaments at “moveable”?joints that permit bone movement. Of course, the moving is done by muscles, which are attached directly or by tendons to the bones.

Muscles move the parts of a joint by contracting (becoming shorter) and pulling two bones closer together. Since each muscle can pull in only one direction (and not push), muscles must work in pairs. One muscle or group of muscles bends part of a joint, a different muscle or group of muscles pulls it back to its original position. Muscle placement is very specific to optimize maneuverability and strength.

Our bodies can be thought of as machines. We lift, push and pull objects, and we work continuously to maintain posture and balance against the force of gravity. Bones, muscles, joints, ligaments and tendons all are necessary to do this work. In fact, there are simple machines within the body’s component parts. One example is the arm, which is a lever.

This activity allows students to explore how the arm’s bones and muscles work efficiently together. Students will see that muscles are positioned to achieve the most movement or power with the least possible effort.

Objectives and Standards

Concepts

• Bones come together at joints.

• The position of a muscle affects the amount of movement it causes a bone to make.

• Muscles work in pairs, in opposition to each other.

Science, Health and Math Skills

• Predicting

• Observing

• Drawing conclusions

• Modeling

• Inferring

Materials and Setup

Materials per Group of Students (see Setup below)

• 50-cm length of string

• 2 rulers with holes in the center (to make arm model)

• Clear tape

• Large brad

• Metal paper clip

• Pair of scissors

• Ruler (for measuring)

• Copies of the student sheets

Setup

1. Place materials in a central location.

2. Have students work in groups of two or three.

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

1. Tell students that they are going to build and explore a model of the arm. Have Materials Managers collect the rulers, string, paper clip, brad and tape for each group.

2. Tell students to follow the steps on the “Arm Model Instructions” sheet to make their model arms.

3. When each group has built its model arm, ask, In what ways does this model represent a human arm? Discuss the similarities and differences noted by students between their models and their real arms. Point out that actual muscles pull by contracting and becoming shorter.

4. Explain to students that they will investigate muscle attachment sites using their model arms. Have them continue with the instructions on the “Model Observations” sheet.

5. Discuss with students their data and conclusions about muscle attachment sites. Students will discover that moving the string on their arm model will move the bottom ruler different distances, depending on where the string is attached. Students also may notice that when the string was connected closer to the joint, it was harder to pull. Ask students how these concepts might apply to the placement of the biceps muscle in the arm. Explain that each muscle in the body has a precise attachment point. Muscle placement balances the movement of the bone with the effort of the muscle. The points at which muscles attach to bones allow muscles to cause a large movement with a relatively small amount of contraction.

6. Use the “Challenge” at the bottom of the “Model Observations” sheet to help students learn about how muscles work in pairs. After students have completed the “Challenge,” discuss the relationships between pairs of muscles. For instance, the biceps muscle bends the arm and the triceps muscle straightens it. Ask, How do we straighten the arm after bending it? Have students bend their arms at the elbow and feel their biceps muscles contract. Ask, Can you straighten your arm by contracting your biceps muscle? Where is the muscle that you contract to straighten your arm? It is the triceps muscle, located on the back of the upper arm. Have each student bend and straighten his/her arm and feel the triceps muscle contract and relax.

Handouts and Downloads

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