Arm Model: Levers in Your Body
Your bones and muscles are systems of levers.
By Lori Lambertson

About Levers:
This lever idea is from Eric Muller.

Using a 6 foot long 2" x 8" and a 2" x 2" fulcrum, can you lift another student off of the ground? Levers don't make you stronger, but they help you do more work with the strength you already have. All levers have three parts:

• the fulcrum (or pivot) which is the place where a lever rocks back and forth
• the load or resistance
• the effort

When the fulcrum is in the center of the lever (this is a first class lever), the amount of effort you push down equals exactly the amount of load you lift on the other end (like a seesaw). Moving the fulcrum closer toward the load (what you are trying to lift) will make the lifting require less effort, but the load won't be lifted as great a distance. We "trade" effort for distance, that is, lifting will require less effort, but we won't lift the load as high.

Look at the human arm. Can you identify the three parts of a lever? Can you identify what type of lever this is? The tendon that attaches the biceps to the radius acts as the effort in the arm. Notice that the fulcrum (the elbow joint) is outside the effort. This is a third class lever. The load is the lower arm and hand (and any weight added to the hand). Third class levers are very common in the human body.

Arm Model: Biomechanics of Biceps and Triceps

Materials:

• per group or per student
• 1 10" x 2" piece of heavy cardboard or foam board
• 1 14" x 2" piece of heavy cardboard or foam board
• white crayon or chalk
• duct tape (several small pieces)
• 1 brad (for foam core) OR a heavy duty stapler (for mat board)
• 2 pieces of string, about 40 cm each
• masking tape
• ball point pen (for foam core) OR a hole punch (for mat board)

Draw the humerus on the 10" long foam board, then draw the radius, ulna and hand on the 14" long foam board. Tape at elbow joint (tape both sides of the foam board), making sure that the elbow will still work as a hinge.

For Foam Core:

Push the brad through the foam board at the top of the radius (at least 3 cm from elbow hinge). Leave enough room to tie a string around the head of the brad and then flatten the ends on the other side. Punch a hole through the foam core at the top of the humerus with the ball point pen. Make sure the hole is the same diameter as the pen.

Hooking up the biceps: Tie a piece of string around the head of the brad and thread it through the hole at the top of the humerus (from front to back). Add a piece of masking tape to this string and label it "Biceps". Notice when the biceps contracts (when you pull on the string), the lower arm moves toward the upper arm.

Hooking up the triceps: Tie the second piece of string around the flattened pieces of the brad. Thread it through the hole at the top of the humerus from back to front. Label this string with masking tape, "Triceps". Hold the upper arm parallel to the ground, with the lower arm hanging straight down. Notice that contraction of the triceps causing elbow extension.

For Mat Board:

Staple one end of the string to the top of the radius, at least 3 cm from the elbow hinge. Use a hole punch or a drill to make a hole (approx. 3/8" diameter) at the top of the humerus. Hooking up the biceps: thread the string attached to the radius through the hole at the top of the radius (from front to back). Add a piece of masking tape to this string and label it "Biceps". Notice when the biceps contracts (when you pull on the string), the lower arm moves toward the upper arm.

Hooking up the triceps: Staple the second piece of string to the back of the top of the radius. Thread it through the hole at the top of the humerus from back to front. Label this string with masking tape, "Triceps". Hold the upper arm parallel to the ground, with the lower arm hanging straight down. Notice that contraction of the triceps causing elbow extension.

Extensions:

Have student pairs measure circumference of each other's upper arm when muscles are relaxed, and compare it to the circumference of the upper arm when muscles are contracted.

About Muscle:

Skeletal muscles are voluntary muscles (unlike cardiac muscle and smooth muscle, which are not under voluntary control). They are attached to bone by tendons. When skeletal muscles contract, they shorten in length, bringing the bones to which they are attached together. Muscles can shorten up to 60% of their resting length. Many muscles in our bodies work in antagonistic pairs - the biceps flexes the arm at the elbow, and the triceps extends the arm. Both actions are produced by contraction and the shortening of the muscle. The only way a muscle can lengthen is by relaxing (which is not an action, but the lack of the action of contracting).

These models show only one "head" or point of origin of the humerus. The biceps has two "heads" or points of origin, both of which wrap over the head of the humerus and attach to the scapula (top of the shoulder blade).

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