Image caption Restrained: One of the volunteers is leant forward in the study

Forcing a detainee to bend over while seated can lead to death, a study into restraint techniques suggests.

Researchers found that the hold massively cut lung capacity.

The technique is not used in prisons but researchers say it might be used in some mental health or care homes, or by private security contractors.

The Ministry of Justice and the UK Border Agency are reviewing restraint techniques approved for use on aircraft.

Police are currently investigating why an Angolan man, Jimmy Mubenga, died shortly after being restrained on his deportation flight last year.

The circumstances surrounding the death of Mr Mubenga, 46, remain unclear. He collapsed on a plane at Heathrow in October 2010 and post-mortem examinations proved inconclusive.

Three security guards, then employed by G4S, were arrested and have been bailed until later this year. A different company, Reliance, now holds the contract to escort deportees out of the UK.

Most studies of restraint techniques have focused on what happens to a detainee where they are forced to the ground, such as when a police officer uses brute force to hold someone still.

In this research, published in Medicine, Science and the Law, a Royal Society of Medicine journal, the Coventry University team placed 40 volunteers in chairs and then leant them forward, bringing their face close to the lap.

At the same time, researchers used arm holds and applied a small amount of force to prevent the volunteer from attempting to return to a normal sitting position.

The study found that all the volunteers said they felt they could not breathe and many felt alarmed.

One female volunteer was so distressed the team had to immediately stop the experiment. Overall, the research found volunteers with a larger waist - although not necessarily overweight - found it hardest to breathe. In the worst cases, the lungs' capacity was almost halved.

'Vicious circle'

Dr John Parkes, the lead author, said that the research found that the position, rather than the restraint by itself, caused the restrictions in the lungs.

Image caption Jimmy Mubenga: Collapsed on a plane at Heathrow Airport

"When we were doing it, we were not applying pressure to people's backs," said Dr Parkes. "If someone was being restrained with considerable force in the back, that might be a different matter."

The team had initially considered applying up to 25kg of force to the back of volunteers, to simulate a strong security guard forcing someone down. They abandoned the plan when their early experiments on each other, using a sandbag, showed it was dangerous.

Dr Parkes said the danger lay in security staff misinterpreting the response from a detainee who is struggling to get air into the lungs.

"Imagine that from the perspective of the staff. They feel you struggle and they will feel that you are getting angry. They will apply more force to manage your resistance. It becomes a vicious circle.

"The people who would die are those who resist for a prolonged period of time. It is entirely possible that people will die where someone is using a level of force that a lay person may not consider to be excessive."