The floating arm trick and motor tricks in dystonia: could they share an underlying mechanism?

The "floating arm trick" is a phenomenon of a slow, tonic movement reflex that seems uniquely confined in normal humans to the deltoid muscles of the shoulders. To do this demonstration on yourself, follow the instructions here, or just stand in a doorway, press the backs of your hands firmly against the door frame with elbows straight, tensing around the shoulders, for at least 30 seconds to contract the deltoid muscles, then release hands to drop to sides. As you do so, you will feel a slight upward pull of your hands, as the deltoids try to pull your arms up again in a slow, almost dystonic contraction.

This phenomenon, called the Kohnstamm phenomenon, was investigated by a team of scientists in Zurich, who found that in order to override the reflex, the body has to send an additional motor signal to the arm to override the reflex, so that voluntary control does not stop the original signal to the muscle from occurring along with the voluntary cancelling. This was true even though, by transcranial magnetic stimulation, they were able to show the reflex movement comes from motor cortex,not the spine where the muscle stretch reflexes originate.

People with dystonias such as "wryneck" or writer's cramp have similar involuntary tonic movements. Perhaps the ability of such patients to briefly hold back their involuntary movement is similar in nature. So, perhaps there may be a treatment beyond Botox for dystonia in the floating arm trick. The dystonias have been shown to sometimes respond to "sensory tricks." Could there be a motor trick out there for dystonia? (HT: Science Magazine's Sifter.)



Proc Biol Sci. 2014 Nov 7;281(1794). pii: 20141139. doi: 10.1098/rspb.2014.1139.

Using voluntary motor commands to inhibit involuntary arm movements.

Ghosh A, Rothwell J, Haggard P.

A hallmark of voluntary motor control is the ability to stop an ongoing movement. Is voluntary motor inhibition a general neural mechanism that can be focused on any movement, including involuntary movements, or is it mere termination of a positive voluntary motor command? The involuntary arm lift, or 'floating arm trick', is a distinctive long-lasting reflex of the deltoid muscle. We investigated how a voluntary motor network inhibits this form of involuntary motor control. Transcranial magnetic stimulation of the motor cortex during the floating arm trick produced a silent period in the reflexively contracting deltoid muscle, followed by a rebound of muscle activity. This pattern suggests a persistent generator of involuntary motor commands. Instructions to bring the arm down voluntarily reduced activity of deltoid muscle. When this voluntary effort was withdrawn, the involuntary arm lift resumed. Further, voluntary motor inhibition produced a strange illusion of physical resistance to bringing the arm down, as if ongoing involuntarily generated commands were located in a 'sensory blind-spot', inaccessible to conscious perception. Our results suggest that voluntary motor inhibition may be a specific neural function, distinct from absence of positive voluntary motor commands.

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