Fitness sensors usually come in the form of smartwatches. They sit on your wrist and they track your steps and even your heart rate. They also calculate the steps that you take based on your speed. For example, whether you are running, doing a light jog or even a sprint. That brings about the question, how does your fitness tracker know all of this stuff?
Fitness trackers usually come with a 3 axis accelerometer. This helps you to track movement in every single direction you take. Some of them even come with a gyroscope as well. This helps you to measure the orientation and even the rotation of how you move. All of this data is collected and it is then transferred. You might be surprised at how many steps you take while on vacation since you do not have access to a car.