Falls could be predicted in older people three weeks before they happen, according to new research using sensors in the home to monitor the occupants' gait.

The ground-breaking system, devised by scientists in the US, can alert medics to subtle changes, allowing them to intervene before a potentially lethal stumble.

And early tests show the system - in which movement sensors in the walls measure walking speed and length of stride - can help people stay in their own homes for longer.

Experts from the University of Missouri, found that even small changes can help health professionals predict if an elderly person is at risk.

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They found that when walking speed drops by 5.1 cm per second, older people have an 86 per cent chance of falling within three weeks, compared with 20 per cent if there is no change. Researchers also found that a drop in stride length of 7.6cm predicted a 51 per cent chance subjects tripping within three weeks.

Falls are one of the main causes of broken hips in the UK and can also point to undiagnosed health problems such as underlying infection and clashes of medication.

One in three pensioners has had at least one fall in the past year, at a cost of pounds 2.3 billion to the NHS. But this system could pick up problems and initiate changes before a potentially fatal tumble.

Lead researcher Prof Marjorie Skubic, from the University of Missouri, invented the sensor system after her mother-in-law had a bad fall. She recently fitted the system into her parents' home for her mother's 93rd birthday, in the hope it will allow them to remain at home and independent until death.

"You can make a big difference to how someone is going to age," she said. "What we are showing is there doesn't have to be a decline, that you can keep people up at a high level until they die. I pray that my parents die in their sleep, in their own beds, in their home."

Those monitored by the technology stayed in their own home for an average of 4.3 years, compared with 1.8 years for those who were not.Elsewhere scientists have begun a project to fit thousands of digital sensors to elderly people in the hope it could help detect the very first signs of Alzheimer's disease. The hi-tech wristbands which have been given 2,200 older people in Boston, in the US, measure everything from sleep, to balance and fall risk, to heart rate. Scientists hope the three-year project will reveal subtle physical changes that develop during the first stages of the disease and provide an alternative test for picking up the illness.

Currently it is difficult to diagnose Alzheimer's and requires a number of tests.