Doctors have scanned the spine of a stage contortionist in a bid to work out why she is so flexible.
Contortionist Delia is incredibly supple
The results suggest both genes and intensive training enable the unusual body movements of such performers.
The researchers, led by Dr Richard Wiseman, found no evidence of deformed bone structures that might contribute - simply extra-stretchy ligaments.
The findings are being presented to mark the launch of the Dana Centre, a new public venue at London's Science Museum this week.
Contortionist Delia agreed to take part in the study and be scanned performing some impossible-looking manoeuvres whilst inside an MRI scanner.
This allowed doctors a clear view of her spine "in extremis" - allowing them to spot any differences between her and the average human.
Dr Wiseman told the BBC: "Whenever people see a contortionist perform the 'impossible' - bending right over backwards, they just want to know what's going on.
"We still don't fully understand what's happening, and part of the project is to explore this by looking at the skeleton and the ligaments, trying to work it out."
A spine expert examining the scans said she had been expecting to see some kind of bone deformity which allowed the spine to bend in an unnatural way.
Dr Carol Phillips, an orthopaedic research fellow, told the BBC: "She can get her spine into positions that 99% of the population would never be able to do.
"She has got what we call hypermobility - or 'double jointedness', but she has gone beyond that.
"Part of her ability is genetic, and part of it is down to training."
Everyone has tissues called ligaments which hold joints together.
In most people they are slightly elastic - but in people like Delia, genes may mean that they are much stretchier.
Dr Phillips said: "What was surprising was that her bones were beautifully aligned - there was no problem at all.
"It was her ligaments that were abnormal."