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Illusion: Lying down on a hard flat floor move arms up and down (as when doing a snow angel) while keeping them touching the floor. While repeating it a few times the floor begins to appear concave, with the deepest part where the arms as perpendicular to the rest of the body.

Questions: What is the origin of this illusion?

My guess: I think it might be related to the arms being harder to extend towards the back when they are perpendicular to the body than when they are at other angles.

Note: The name above is only the way I like to call it. I found this illusion when I was a kid, but I am sure others might have noticed it as well.

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It may be because, when we move our shoulders towards our the body, also know as abduction (Fig 1.) gives the central part of the body an elevation. when we rotate our arms more upwards (Adduction), the elevation will be lost.

So in other words, when we are making snow angels,

  1. Start Normal position (hands near thighs) = Back and shoulders on ground
  2. Middle position (hands in middle) = Back elevated and shoulders on ground
  3. Final Position (hands upward) = Back and shoulders on ground again.

Due to this motion, it seems as if the ground become concave.

Source:

Anatomical Terms of Movement

Articulating the Importance of Joints in Anatomy

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