Many thousands of people are thought to be prone to faints - a condition called vasovagal syncope.

It happens when blood pressure drops and the heart rate slows, starving the brain of oxygen.

It is thought to be a cause of many falls among the elderly, leading in some cases to fractures which cause permanent disability.

While sufferers can keep faints at bay by avoiding dangerous movements - such as standing up too quickly - and consuming enough salt and fluid, some doctors recommend exercises that patients can carry out if they feel a faint coming on.

If they cross one leg behind the other while standing, and then contract the thigh and buttock muscles, the theory goes, then blood pressure will be maintained for enough time to allow the patient to head for the nearest chair.

Now that theory has been tested by scientists at the Academic Medical Center of the University of Amsterdam.

You often see people standing with their legs crossed at cocktail parties - in our experience, patients can do this automatically after a while

Dr Wouter Weiling, University of Amsterdam

They picked 20 patients with a history of fainting fits - some had suffered up to 200 in their lifetime.

They were then taught the exercises, then placed on a "head-up" tilt table.

This allows the patient to be rested at an angle of 70 degrees - the perfect position for bringing on an attack.

If that wasn't enough, a drug which dilates the blood vessels was administered as well.

When the first signs of faint appeared, such as a decrease in heart rate, or feelings of light-headedness or nausea, the patients carried out the maneouvre.

Prevention

This, found the scientists, managed to stabilise the heart rate and blood pressure in all the patients, and completely prevented fainting in five out of 20 patients.

In the rest, the leg-crossing managed to delay the faint by an average of more than two minutes - long enough for a safe seat to be found.

In addition, when the trained patients were followed up a few months later, only a couple of them had suffered fainting fits.

Party trick

Dr Wouter Weiling, who led the study, said: "You often see people standing with their legs crossed at cocktail parties.

"In our experience, patients can do this automatically after a while. A great advantage of leg-crossing is that it can be done almost unnoticed."

Dr Martin Fotherby, a senior lecturer in medicine of the elderly at the University of Leicester, told BBC News Online that some frailer patients might find it difficult to use the technique - but it could be useful for others.

He said: "Older people who have a fall may not remember anything about it, suggesting they may have blacked out due to a syncope.

"Obviously these falls can be very damaging, leading to fractures."

The research was published in the journal Circulation.