Muscles Don't Push

It has been a while since I did a biology related experiment, so this week we are going to take a look at how your muscles work. In particular, we are going to examine the muscles that move your arm, to see what is really involved in moving your body.

For this experiment, you will need:

  • your arm

First, lets examine a muscle that most people are familiar with. Hold your arm out in front of you and make your hand into a fist. Bend your elbow and bring your hand near your shoulder. This should cause the muscles of your upper arm to tighten and bulge. Place your other hand around your upper arm. Straighten your arm and bend it again several times, noticing how the muscle changes as you do.

Muscles are wonderful things, but most people really do not know how they work. Your muscles move you by pulling or relaxing. That is all. Muscles cannot push.

Wait a minute. I can hear you saying that muscles must be able to push. How can pulling muscles open a door that says PUSH? (I frequently find myself pulling on push doors.) Lets think about the muscles. When a muscle is relaxed, it is longer and thinner. When you tense the muscle, it contracts, to get shorter and thicker. If the muscle was not attached to anything, this would not do much. When you tensed your muscles, you would just get lumpier. Luckily, our muscles are attached to our bones. When you tighten the muscles in your upper arm, they get shorter. They are attached to your shoulder at one end and to the bones of your lower arm at the other end. The muscle pulls your lower arm bones towards your shoulder, bending your arm.

What happens when you straighten your arm? Your muscles can only pull, so your upper arm muscles cannot simply push your lower arm away. Instead, you need a different set of muscles to pull your arm in the other direction. Try flexing your arm a few times and see if you can find the muscle that straightens your arm. (Hint: Try tensing your arm to hold it very straight. Then notice which muscles tighten.)

Did you find them? If not, try putting your other hand on your shoulder, against your neck. Your fingers should reach over your shoulder, onto your back. Now, straighten your arm and tense the muscles. You should feel the muscles on the back of your shoulder tighten.

How can muscles tightening in your shoulder make your arm straighten? This muscle, called the tricep, extends up your arm and across your shoulder. When the muscles of your upper arm tighten, they bend your arm. To straighten your arm, you relax the muscles of your arm and tighten the muscles in your shoulder. These muscles pull on the bones of your lower arm to extend them.

Your body works much like a puppet. You pull one string to bend your arm and pull the opposite string to extend it. Try to find the muscles that move your fingers. (Another hint: Most of them are not in your hand.) If you look closely at the front and back of your wrist as you move your fingers, you can probably see things that look like cords moving under the skin. These are tendons. They are long, string -like tissues that attach muscles to bones. These tendons attach your fingers to the muscles that move them. Put your fingers lightly around your wrist and I am certain you will be able to feel the tendons moving. Move your hand farther up your arm and you should find the muscles that move when you wiggle your fingers. When you have that figured out, then think about how muscles that can only pull let you stick out your tongue. That should keep you busy for a while.

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