Key concepts
Biology
Muscles
Movement
Nervous system

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 bodily movements. More specifically skeletal muscles, or the muscles attached to our bones, help us move our bodies—and even swallow and 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 your fingers touch something hot you automatically pull your arm back before even realizing you touched something hot. A sneeze is another example. These involuntary muscle contractions are helpful and protect us from harm. But some involuntary muscle contractions such as spasms are less desirable.

Sometimes the brain can suppress or override an involuntary muscle contraction. You might become aware of an impending sneeze and be able to 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 block the involuntary contraction signal before it reaches the muscles?

The involuntary muscle contraction explored in this activity is known as the Kohnstamm phenomenon. 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

  • Open doorway


Preparation

  • 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. If you have a shorter arm span, you 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?


Procedure

  • 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 test 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 the door frame 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.
  • 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 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? After you voluntarily contracted specific arm muscles for a minute or so they contracted by themselves. You were probably able to counteract this to keep your arms down, but it took effort to do so.

If you tried, you could probably make it happen with just one arm, too. You might have discovered you need to push for an extended time—and quite hard—before the muscles contract all by themselves once you have stopped pushing. Scientists know these spontaneous contractions, known as the Kohnstamm phenomenon, are due to involuntary orders sent by the brain. They are less sure about why these occur.

Researchers also have found the brain blocks the involuntary orders to suppress the Kohnstamm phenomenon. When you try to keep your arms down after you prompt the 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 for scientists to understand the mechanisms by which the body produces and suppresses spontaneous contractions, because this knowledge might help people, such as those afflicted with Parkinson’s disease, who experience illness-induced spontaneous contractions.

More to explore
The Science of the Floating Arm Trick, from Science
Speedy Science: How Fast Can You React?, from Scientific American
Spooky Science: Discovering the Eerie Colors behind Afterimages, from Scientific American
Science Activities for All Ages!, from Science Buddies

This activity brought to you in partnership with Science Buddies

Science Buddies