An IMU, or inertial measurement unit, is the motion sensor inside phones, watches and earbuds. It combines an accelerometer, which senses gravity and movement, with a gyroscope, which senses rotation, and sometimes a magnetometer, which senses direction like a compass. From these signals software works out how the device, and the body part it sits on, is tilted. IMUs are the basis of almost all wearable posture tracking.
What is inside
| Sensor | Measures | Useful for posture because |
|---|---|---|
| Accelerometer | Acceleration, including the constant pull of gravity | When you are still, gravity shows which way is down, so it gives tilt |
| Gyroscope | How fast the device rotates | Tracks quick turns and nods smoothly, even while you move |
| Magnetometer (some devices) | Direction of the Earth’s magnetic field | Gives heading, turning left or right; less important for posture |
From signals to an angle
Each sensor has a weakness. The accelerometer gives a stable sense of down, but it is confused by movement, because walking adds accelerations on top of gravity. The gyroscope follows movement well, but it measures rate of turn, and adding up small errors over time makes its angle drift. Software combines the two, called sensor fusion: the gyroscope for fast changes, the accelerometer to correct the slow drift. The result is an orientation in three directions:
- pitch: nodding forward and back, the one that matters most for slouching;
- roll: tilting toward a shoulder;
- yaw: turning left and right.
How accurate are IMUs for posture?
- A forehead IMU measured head posture within about 1° of a clinical reference device in still positions.1
- An IMU in a consumer earbud agreed very strongly with laboratory motion capture for forward and back head tilt during exercises, and moderately for side tilt.2
- A systematic review of 37 studies of spinal posture wearables, most built on IMUs, found good accuracy in clinical settings, with long term everyday use still to be validated.3
IMUs measure the tilt of the body part they sit on accurately in controlled tests. Accuracy over long, everyday use depends on how the device is worn and how drift is handled.3
Why calibration matters
An IMU knows how the device is oriented, not how your body is oriented. An earbud sits at a slightly different angle in every ear, and a strap shifts during the day. So posture apps take a reference, a moment when you sit or stand in your usual upright, and measure change from it. That is the idea behind a personal baseline.
Where IMUs sit
- Earbuds: follow the head, so they suit neck and head posture. See posture tracking with AirPods.
- Clip-on or adhesive sensors on the back: follow the trunk.
- Watches and phones: follow the wrist or the hand, which tells you about activity more than posture.
How IMUs compare with photos, X-rays and laboratory systems is covered in how posture is measured.
BodySignals reads the orientation that iOS reports from the motion sensors in AirPods with head tracking, sets your upright at the start of each session, and tracks pitch relative to it.
Footnotes
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