In the 1987 routine, Michael leans from the ankle at a 45 degree angle, while keeping his body straight as a rod.

The illusion, which many have tried to copy, was thanks to specially designed shoes and the artist's core strength.

The spine experts warn others against attempting the potentially injurious but mind-boggling move.

Manjul Tripathi and colleagues from the Postgraduate Institute of Medical Education and Research in Chandigarh, India, say in the Journal of Neurosurgery: Spine: "Most trained dancers with strong core strength will reach a maximum of 25 to 30 degrees of forward bending while performing this action. MJ pulled off a gravity-defying 45 degree move that seems unearthly to any witness."

How MJ did it

If a person were to attempt the Smooth Criminal lean, they would notice that the bulk of the strain to strike the pose moves to the Achilles tendon in each ankle, rather than the erector spinae muscles of the back.

This allows for only a very limited degree of forward bend, even for someone matching Michael's strong athletic abilities, explains Assistant Prof Tripathi.

Michael got the extra degrees of tilt thanks to some fancy footwear.

A v-shaped slit in the bottom of each heel of his spats slotted onto a strong nail or "hitch member" driven into the ground, allowing the dancer to pivot and lean further forward, for the gravity-defying move.

Prior to the patented footwear invention, Michael had relied on supporting cables and a harness around his waist to create the illusion.

It's said that he and two Hollywood colleagues borrowed the footwear idea from US astronauts' boots, which can be docked to a fixed rail when working in zero gravity.

But even with specially designed footwear and the support of the hitch member, the move is incredibly hard to pull off, requiring athletic core strength from strengthened spinal and lower-limb muscles, say the doctors.

"Several MJ fans, including the authors, have tried to copy this move and failed, often injuring themselves in their endeavours," they caution.

Dr Tripathi said: "The chances of injury to the ankle are significant. You need strong core muscles and good support around the ankle. It's not a simple trick."