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How Does a Device Know When You’ve Fallen?

Fall detectors contain tiny sensors programmed to recognize unusual motion

a man riding a bicycle has fallen on a trail

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Nearly 36 million older Americans report falls each year, and a fifth of those result in injuries, according to the federal Centers for Disease Control and Prevention.

Those facts triggered a response from the technology industry: electronic fall detectors. These devices — mostly worn as pendants around the neck, on wristbands, clipped to pants or built into smartwatches — send an instant alert if they detect that the wearer has suddenly fallen to the ground or floor.

Electronic fall detectors “can be important in getting people to emergency or urgent care sooner and prevent complications that can arise from lying in one position for an extended time,” says Hilaire Thompson, a professor at the University of Washington who studies traumatic brain injury and injury prevention.

How a fall detector does its job

At the core of any fall detector is the accelerometer. Its tiny electronic sensors measure movement on three different axes:

  • Right and left
  • Forward and backward
  • Up and down

And the sensors register the unique motion of a fall:

  • A short loss of gravity or a sense of weightlessness
  • A jarring impact 
  • A period of motionlessness and a horizontal or tilted position

Accelerometers are sensitive and sophisticated. They can, for example, “distinguish a fall from the act of taking something off a shelf or walking down stairs,” says Marketing Director Kate Wahl at Medical Guardian, a Philadelphia-based medical alert systems company.

After the sensors detect the motion and hard impact of a fall, the device typically triggers an alarm that’s loud enough for others in an area to hear. Many devices can also dial 911 and selected personal contacts for help using Wi-Fi, cellular signals or even landline connections.

Plus, devices embed GPS coordinates. These can be relayed to emergency contacts to pinpoint your location and find you sooner.

Your iPhone can help prevent a fall

If your iPhone is running iOS 15, which debuted in fall 2021, it can use data on your walking speed, step length, double support time (what Apple calls the proportion of time that both feet are touching the ground when you walk) and walking asymmetry (the percentage of time that your steps with one foot are faster or slower than the other foot’s) to determine your walking steadiness.

You’ll have to give Apple’s Health app permission to gather the data — you don’t have to share it outside of your phone. And after the app records at least seven days of information, it will assess your stability while walking as “OK,” “Low” or “Very Low.”

The assessment, which is updated every seven days, can’t predict your fall risk, say, today. But if your stability is rated as low or very low, your fall risk within the next 12 months is elevated.


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The Health app suggests exercises to improve your mobility. But you also can share this information with your doctor to create a long-term plan for increasing your stability.

At the moment, Android phones, a little more than 2 in 5 of the smartphones in the U.S., don’t have an equivalent feature, but the same sensor is within them. Just about all smartphones and tablets in use today have triaxial accelerometers, according to information from Gary Weiss, head of the Wireless Sensor Data Mining Lab at Fordham University in the Bronx, New York, who does research into ways these sensors’ data can be used.

Most people take this sensor for granted. It’s what allows a phone to adjust its screen display when you change its orientation from vertical to horizontal.

Linda Dono contributed to this story. Lexi Pandell is a contributing writer who covers technology. Her work also appears in WiredThe New York Times, The Atlantic and other publications.

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