Human Arm Model

I am a physics teacher, and I have many students who are interested in pursuing careers in medicine. I really wanted to add some lab experiences related to the human body or medicine so I decided I would like to design and build a class set of human arm models to investigate the use of the arm as a lever/pulley device.

Materials Needed

• cardboard for making a model first
• bolts, nuts, washers
• plastic sewing machine bobbins
• chalkboard paint
• cord
• screw eyes
• conduit clamps

Before I began I examined the construction of my own arm and attempted to make a working model in cardboard. This went through several variations before I developed one that I was happy enough with to reproduce in plylwood.

Here is one of the earlier iterations...

Once I was happy with the cardboard model, I cut out a paper model to use to more easily transfer the shape to the plywood.

To cut the pieces for the arm, I outlined the paper pattern on the plywood sheet. Then I securely clamped the plywood onto my work bench and cut out the shapes with a jigsaw. If I had a laser cutter it would have been much neater!

At first I thought it would be neat to paint the arms to look like muscles, but some of my physics students can be a bit squeamish and that would limit the number of times I could use the models. Then I looked in my left over paint shelf in the garage and saw that I had some chalkboard pain.

First I gave each piece a thorough coat of primer, then I sprayed them with two coasts of black chalkboard paint.. The chalkboard paint gives a very tough surface and students will be able to mark on the arm with chalk if needed during their investigations. This really helped with the appearance especially since I used plywood.

There are quite a few holes needed for this model, I drilled them as I went along using a drill press. Since the pieces are not very thick I stacked three at a time, clamped them, and then drilled the holes. (I was making a set of arms at the same time.)

Make sure that the holes are slightly smaller than the bolts you are using so they fit in tightly.

When the model is threaded with cord so the muscles can be manipulated they need to go around something like a pulley. I looked around for pulleys and they were all rather expensive or difficult to mount onto my flat pieces.

For the pulleys, I used plastic sewing machine bobbins. To attach them to the piece I put washers on the top and bottom of the bobbin and threaded the whole thing through a bolt. I fastened the bot with a nut on the other side of the plywood and left it loose enough that the bobbin could turn freely.

Since you are not actually using the bobbins with a sewing machine it doesn't matter which size or model you purchase but some bobbins have seams inside the spindle, I avoided those because I was concerned that the cord might catch on that.

There are three pieces that fit together to form the arm: the shoulder, the upper arm, and the lower arm. While the arm model is being used, the pieces have to move past each other without rubbing.

To accomplish this, I put a bolt through the top piece, tightened a nut to the bottom of the first piece, put the end of the bolt through the second piece of wood and then another nut on the bottom. I put washer under the head of the bolt and under each nut to allow the joint to easily rotate.

One end of the cord that pulls to simulate muscle action is tied to a small screw eye. One screw eye will be the base of the muscle along the back of the arm and other the front of the arm. I put in four different screw eyes to be used for the front of the arm muscle to be used when investigating the length of a lever arm.

Be careful to choose screw eyes that will not stick out the back of the plywood because the pieces will be sliding past one another and also the tip of the screw eye is pointy and could be a safety hazard if it sticks out the back.

Once all the parts are assembled, the pulleys attached, and the screw eyes attached it is time to put in cord that can be pulled to activate the muscle action of the arm.

One goes along the back of the arm and the other the front of the arm. I tied loops on the end of the string to allow a force scale to be easily hooked onto the ends of the cords to monitor how much force is required to different motions of the arm.

To stabilize the model during use, I mount two conduit clamps on the back of the "shoulder" piece so it can mounted onto the vertical pole of a ring stand. The stands I have in the lab are fairly large, make sure you pick the size that will clamp securely onto your size poles. These are 1/2 inch clamps.

I pre-drilled holes in the plywood and used washers, bolts, and nuts to attach the clamp.

To make sure the two clamps are aligned with each other, I put a marker through the two clamps before marking where to drill the holes for the bolts.

I wanted the head of the bolt to be on the front of the model so it was rather difficult to tighten the nut, you could reverse this but I didn't want the students to fiddle with the nut and keep taking off the clamp just with idle hands during their investigations.

This works best when each lab group can do the investigation themselves so get to work and make a few more!

Now for the fun part!

Here are just a few ideas I have, I plan to introduce the models to the students will a structured inquiry activity and then lead them to open inquiry. I plan to combine the arm model with force probes and video cameras so students can record the force required and use video analysis to analyze the motion and energy of the "hand" of the arms.

• Investigate how the length of the arm effects the force required to left an item.
• Determine the work done to lift something to elbow level.
• Determine the power developed when lifting an object.
• Determine the work done when throwing a ball underhand vs. overhand and compare the kinetic energy imparted to the ball in each case.
• Compare the work done by the bicep vs. the tricep for different motions of the arm.
• Determine the mechanical advantage of the arm when the cord is tied to different screw eyes.
• Determine which motions of the arm (lifting, pulling, pushing, curl, throwing overhand, throwing underhand) use which of the muscles of the arm and if they use both when ratio of muscle use is used.
• Figure out the function of each of the joints and muscles.
• Which actions of the arm are examples of first class, second class, and third class levers and what are the advantages of each of these types of levers?

I think there could be more uses in rotational motion too when considering the angle at which the elbow rotating, but I need to see them in action first.

I'm thinking of...

• a set of human leg models
• a working ear
• a human jaw