iPhone Apps Can Tell Many Things About You Through the Accelerometer

Nearly every modern smartphone is equipped with an accelerometer, which, as the name implies, is a sensor that measures acceleration. It’s most commonly used for detecting the device’s orientation. It also has many other uses, whether as a game controller in racing games, as a pedometer for counting daily steps, or to detect falls as seen in the Apple Watch. There also have been some research to develop novel accelerometer applications: estimating heart rate, breathing rate, or even as a rudimentary audio recorder using just the accelerometer. Currently, iOS allows any installed app to access accelerometer data without explicit permission from the user. Curious apps might be able to learn a lot about users through the accelerometer and without their knowledge or permission.

https://www.mysk.blog/2021/10/24/accelerometer-ios