Scientists Investigate The Mechanisms Behind The Floating Arm Trick

Allan Ajifo, 'Brain Power,' via Flickr. CC BY 2.0

I’m sure many of you can remember spending school lunch breaks demonstrating the weird and wonderful floating arm trick. If you had a misspent youth, hold out your arms in front of you and ask a mate to push against your hands for thirty seconds while you push back against them as hard as you can. Put your arms down afterward and watch them magically float back up. If you don’t have a friend at hand and are desperate to give it a go, you can use a doorframe.

This trick is formally known as the Kohnstamm phenomenon, which is a long-lasting involuntary muscle contraction that causes continued movement of a limb as a result of sustained voluntary muscle contraction. The phenomenon was first described in 1915, but scientists know little about the underlying mechanisms of the effect. How brain networks inhibit this involuntary movement was also unknown.

To find out more, a group of researchers enrolled 39 volunteers into a study and performed various tests on them. One of the experiments involved asking the participants to hold their arms down after exertion against a resistance instead of allowing them to float up involuntarily. They also used a noninvasive method to stimulate a region of the brain called the motor cortex during the trick. This caused a transient interruption in the muscle contraction, followed by a rebound of muscle activity. This suggested to the researchers that something is persistently generating involuntary motor commands. Details of the study can be found in the journal Proceedings of the Royal Society B

After analyzing brain scans and muscle activity recordings, the researchers discovered that repressing the involuntary arm movement involves blocking the involuntary signal, the “lift” signal, before it reaches the muscle. This seems to refute another hypothesis proposed to explain the repression of involuntary movement, which suggested that the brain may send out a positive “push down” signal to muscles while the involuntary “lift” signal is being transmitted, cancelling out the latter. Taken together, this suggests that the inhibition of voluntary movement is actually a specific neural function, rather than the absence of positive voluntary commands.

While some of you may be thinking this research sounds like a waste of money, understanding the mechanisms behind the repression of involuntary movements may help people with conditions such as Parkinson’s or Tourette syndrome.

[Hat tip: Science]

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