system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do wrist pedometers (like fitbit) actually know how many steps you've taken.
gpt
When you walk, every time your foot hits the ground it decelerates - going from moving to stationary very quickly. This sends a little shock wave of acceleration up your body that can be sensed anywhere. (Note: acceleration = change in velocity, whether speeding up or slowing down).

The Fitbit has an accelerometer, a tiny electromechanical device that can sense acceleration and generate an electrical signal. The Fitbit looks at that signal for the strong spikes that happen on each step and counts them. It's not 100% perfect, it will miss a few steps and count a few false-steps but overall they've tuned it to be pretty accurate.