Muscles and Joints
Lesson Overview
The purpose of this lesson is to teach some fundamentals about muscle architecture. The
children will construct a simple “arm” model out of tongue depressors and string, then
change the model to explore the effect of muscle and joint geometry on muscle function.
Finally, the children will observe as tutors dissect chicken wings and together they will
search for structures that are common to between their models and the wings.
Important Concepts to Convey
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Joints are the tissues that connect bones to each other. They give our skeletons
flexibility and allow us to move.
Muscles are the tissues that actually move our skeletons
A muscle is like a rope or spring. It is connected to two bones and pulls on them to
cause motion
Muscles are made up of smaller units called fascicles, which are made up of muscle
fibers (like the strands of a rope!)
A muscle’s strength depends on both its size and where it attaches to the bone.
o Thicker muscles (larger cross sectional area) can cause higher forces.
o Increasing the distance between a muscle and its joint allows it to resist more
weight.
o Decreasing the distance between a muscle and its joint increases the range of
motion of that joint
Learning Objectives
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Students will demonstrate that joints allow motion and muscles cause motion.
Students will demonstrate that muscles act in opposing pairs to flex and extend the
same joint.
Students will determine how a muscle’s line of action (distance from its joint)
determines its effective strength and range of motion.
Students will apply their findings to analyze a chicken wing and make inferences
about our own muscle architecture.
Materials
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Worksheet
Felt tip markers
Pre-drilled tongue depressors
9” lengths of string, pre-knotted
with loops on the ends
Rubber bands
Brads
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Thawed chicken wings
Trays for cutting wings
Knives (for tutors only)
Gloves (tutor and student sized)
Introduction ~10 min
1. Students fill out the pre-lab portion of the worksheet.
2. We’ve learned about the skeleton before, as well as its 5 major functions. If we
need the skeleton for support and protection, why don’t we have just 1 big bone?
3. What is a joint and why do we have them?
4. If joints allow us to move, what actually makes us move?
5. How do muscles move our bones?
6. Muscles are like ropes:
a. Can pull on bones by shortening in length. They do not push.
b. Muscle tissue is grouped into sub units called fascicles.
c. Muscles fascicles, like ropes, are made up of tiny strands called muscle fibers
7. How do you feel (and your muscles feel) when you exercise.
a. Muscles need oxygen and food to use energy. This is why exerting yourself
makes you tired and out of breath.
b. Exercised muscles feel sore partly because of small amounts of damage to the
fibers. Most of the time your muscles will heal and grow new fibers, making
it stronger.
Activity 1: Flexion/Extension Simon-Says
This activity will be done as a class. The lead tutor will lead the class in a special game of
simon says. All the calls will be to flex or extend specific joints. Depending on which class
is being taught and how comfortable the children seem to be with left vs. right, it might be a
good idea to make the activity right and left agnostic. It’s also best to avoid joints that don’t
have obvious flexion/extension like the foot, shoulder, and thumb. Instead, stick to obvious
joints like the elbow, knee, wrist, fingers, and hip.
Activity 2: Build and Study an Arm (see sketch below)
At this point the tutors should guide the students through the worksheet to create the
wooden arm.
1. Use a brad to pin the two sticks together at the holes labeled “E” to form an elbow.
2. Give the students 1 piece of string and ask them to attach it so that it can flex (bend)
the arm. Have them demonstrate flexing the arm.
Demonstrate antagonist muscle pairs
3. Ask them if they can extend the arm with the muscle they’ve attached. They can’t.
4. Hand them a second string and have them chose attachments that will allow it to
extend the arm.
Test the force amplification effects of the a muscle’s lever arm
5. Remove the extensor muscle (from step 4). If it’s not already there, move the flexor
muscle attachment to the hole closest to the elbow.
6. Attach a rubber band to the “hand” and have the tutor hold it fixed while the
students try to flex the elbow against it. Have them comment on whether it feels
easy or difficult.
7. Ask how you could make it easier to pull the rubber band with the muscle.
8. Add the second string in parallel to the first string and allow them to pull one string
with each hand, and note with them that it is easier.
9. Remove one muscle. Move the remaining muscle to the third hole from the joint.
Have them guess if it will be easier or harder to resist the rubber band. Repeat 6
and have them comment on whether it feels easier or more difficult to use a muscle
when it is farther away from the joint.
Test the effect of lever arm on range of motion
10. Make a 90 degree angle between the arm segments
11. Make the flexor muscle string taut. Use the magic marker to mark the point on the
string as it emerges from the through-hole.
12. Keeping the string taut, extend the arm 90 deg and mark the new through-hole
location on the string.
13. Move the muscle to the 3rd hole again. Have them guess whether the string will
travel farther this time. Repeat step s 10 through 12 and have them comment on
how increasing the distance of the muscle from the joint increases its change in
length during a motion. Since a muscle’s change in length is limited, this means that
a muscle with a larger lever arm has a more limited range of joint motion.
Activity 3: Analyze a Chicken Wing
1. The tutor should now make sure everyone has gloves and prepare a tray for
dissecting chicken wings.
2. Ask the students to identify where the “elbow” is.
3. Peel back the skin, but be careful to avoid cutting any tendons or muscle.
4. Point out the joint again, this taking note of the cartilage and ligaments if you have
time.
5. Have the students identify which muscles are flexors and which are extensors.
6. Have them estimate (or measure if you want to clean rulers) the ratio between the
distance from the elbow to the end of the arm, and the distance from the elbow to
the muscle attachments. The ratio should be very large, meaning that the muscles
are at a severe disadvantage. See the fun facts for more on this.
Fun Facts
1. Muscles are at a severe disadvantage for producing motion against outside forces.
Even everyday tasks like walking, climbing stairs, and reaching for something high
above your head require very high muscle forces.
2. When you run, the calf muscles that extend your ankle have to produce 6 to 8 times
your bodyweight. During jumping this can reach 10 to 12 times your body weight.
This means that an average man (165 lb) produces nearly 2000 lbf in his Achilles
tendon during a jump. As a result…
3. You can pick up and hang a compact car, like a Volkswagen Beetle, from your
Achilles tendon.
4. Muscles do more than just move your skeleton. They also pump your blood
throughout your body, move food through your digestive system, and produce heat
to keep you warm.
5. The “stranded rope” hierarchy applies to sever levels of muscle architecture.
Muscles are made up of strands called fascicles. Fascicles are made up of strands
called fibers. Fibers are made up of myofibrils, which are long strands of
sarcomeres. Sarcomeres are made up of the myofilaments actin, myosin, and titin.
Finished Arm