Floating Arms
Summary
Introduction
Have you ever wondered how a decision to move your arm can make your arm move? When your brain creates a command to move your arm, nerves pass along the command and muscles in the arm contract as ordered. These muscle contractions make your arm move. But could your arm move without a command from the brain? This activity is a fun and surprising way to find out!
Background
Contractions of muscles create body movements. More specifically, skeletal muscles, or the muscles attached to our bones, help us move our bodies—and even swallow or sneeze! Most of the time, skeletal muscles contract on command: the brain produces the order, which the nerves convey and the muscles execute. We call these voluntary contractions or voluntary movements.
Occasionally, skeletal muscles contract without an order from the brain. We call these involuntary muscle contractions. When our fingers touch something hot, we automatically pull our arm back before we even realize we touched something hot. A sneeze is another example. These involuntary muscle contractions are helpful and protect us from harm, but some involuntary muscle contractions, like spasms, are less desirable.
Sometimes the brain can suppress or overwrite an involuntary muscle contraction. You might become aware of an impending sneeze and suppress it, or you might deliberately touch something hot and your brain will order your arm to stay extended before you feel the pain. Neuroscientists are researching how this mechanism works. Does the brain order the muscles to extend when the involuntary signal was to contract, or does it block the signal for involuntary contraction before it reaches the muscles? Studies show that the latter happens when the Kohnstamm phenomenon is suppressed.
The Kohnstamm phenomenon is the involuntary muscle contraction explored in this activity. It was first described in 1915 and occurs when we have voluntarily contracted a skeletal muscle for an extended period of time. Try it out—and see how your arm seems to have a will of its own!
Materials
- An open doorway
Instructions
- Stand inside the doorway looking straight ahead into the room. Lift both arms until the backs of your hands are pressed against the door frame. Small children with a shorter arm span might need to push with just one arm against the door frame.
- In a moment, you will press the backs of your hands against the door frame while you count slowly to 60. Then, you will stop pushing, step out of the doorway, relax your arms, and observe. How do you expect your arms to feel just after you have stopped pushing?
- Try it out! Push for about a minute, step away, and relax your arms. How do your arms feel? Do you observe anything unexpected?
- If nothing surprising happened, try again. This time, try to press the backs of your hands a little harder against the door frame for a full minute. After that, step out of the doorway, relax your arms, and observe. Do you feel your arms floating up all by themselves?
- Do the experiment again, but try to keep your arms down, pointing to the ground, after you walk out of the doorway. Can you do it? How does it feel?
- Do you think you can make one arm float up by only pushing with one hand? Try it out and feel for yourself!
- Do you think you need to push for a full minute, or would half a minute or a few seconds be enough? Try a few times and see if you can find the minimum time required. Note that counting slowly to 60 takes about one minute.
- Do you think you need to push hard against the doorway for your arms to float up, or would a gentle push be fine too? Try it out and see how it feels.
EXTRA: See what happens when you rotate your hands and push with the insides of your hands up against the door frame. Do you feel different muscles working now? How do your arms feel when you walk out of the doorway?
EXTRA: Can you make your leg float up after pushing it sideways against the wall?
EXTRA: Press the backs of your hands against the door frame for about one minute, then step out, but deliberately keep your arms down for a few seconds before relaxing them. Do they still float up? How long do you need to counteract the phenomenon for it to disappear?
Observations and Results
Could you see and feel your arms float up when you walked out of the doorway? This is expected. After you have voluntarily contracted specific arm muscles for a minute or so, they contract all by themselves. You were probably able to counteract this and keep your arms down, but it took effort to keep them down.
If you tried, you could probably make it happen with just one arm, too. You might have discovered that you need to push for an extended time, and quite hard, before the muscles contract all by themselves once you have stopped pushing. This phenomenon is known as the Kohnstamm phenomenon. Scientists know that these spontaneous contractions are due to involuntary orders sent by the brain. They are less sure about why these occur.
Researchers also found that to suppress the Kohnstamm phenomenon, the brain blocks the involuntary orders. For example, when you try to keep your arms down after you have prompted the Kohnstamm phenomenon, your brain does not order your arms to extend to counteract the contraction; instead, it blocks the spontaneous order to contract the muscles.
It is important to understand the mechanisms by which the body produces and suppresses spontaneous contractions, as this knowledge might help people with illness-induced spontaneous contractions like Parkinson’s disease patients.
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Additional Resources
- What Are Reflexes?, from KidsHealth
- Speedy Science: How Fast Can You React?, from Scientific American
- Spooky Science: Discovering the Eerie Colors behind Afterimages, from Scientific American
- The Science of the Floating Arm Trick, from Science
- Science Activities for All Ages!, from Science Buddies
