Counting steps without seeing your feet
An activity tracker does not detect steps directly. It infers them from motion. The key part is an accelerometer, a tiny sensor that measures how the device speeds up and slows down along three directions. When you walk, your arm or body moves in a rhythmic pattern, and that rhythm shows up as a repeating wave in the accelerometer signal.
Software looks for that pattern. Each time the signal crosses certain thresholds in a way that resembles a stride, it adds a step. The exact rules are the manufacturer’s own, and they are tuned on test data from people walking and running. A gyroscope, which measures rotation, is often used alongside the accelerometer to help separate real walking motion from other arm movements.
Because the device is guessing from a wave shape, accuracy depends on how closely your movement resembles what the software expects.
Where the count goes wrong
A few situations commonly cause errors, in both directions.
- Pushing a stroller, trolley or lawn mower. The wrist barely swings, so a wrist-worn tracker may miss steps.
- Gesturing, driving over bumps or brushing teeth. Repetitive arm motion can look a bit like walking and may add steps that never happened.
- Slow shuffling or very short steps. The signal can be too weak to cross the counting threshold.
- Treadmills and handrails. Holding on changes the arm swing.
- Cycling, rowing and swimming. Leg motion without the usual stride pattern means steps are often undercounted, and these activities typically need their own mode to be tracked properly.
Where the tracker is worn matters too. A device on the hip or in a pocket tracks body movement differently from one on the wrist, and results often differ between the two. For an everyday sense of how active a day was, a modest error is not important. For anyone chasing an exact number, it is worth knowing that different brands can disagree by a noticeable margin on the same walk, and no one of them is guaranteed to be right.
Distance, floors and speed
Distance is usually calculated by multiplying steps by an estimated stride length, based on your height and sometimes your recent walking. If your stride is unusual, the distance will be off even if the step count is right. Devices with built-in satellite positioning (often called GPS) can measure outdoor distance far more directly, though the signal can drift near tall buildings or tree cover.
Floors climbed generally come from a barometer, which senses small drops in air pressure as you gain height. It can be fooled by weather changes, air conditioning or a lift that moves quickly.
Calories are the least certain number
Calorie figures are estimates built from several inputs: your weight, age and sex entered in the app, your movement, and often your heart rate. Heart-rate-based estimates rely on the readings from heart rate sensors, which have their own variability. The tracker also has to guess your resting energy use, which differs between people in ways it cannot measure.
The result can be useful for comparing one day with another on the same device. It is a poor basis for deciding precisely how much to eat. If calories matter to you for health reasons, a doctor or registered dietitian is a better source of guidance than a wrist reading.
Active minutes and goals
Many trackers reward “active minutes” or similar scores. These are usually derived from movement intensity or heart rate crossing a threshold. The thresholds are product design choices, not clinical measurements, and different brands count differently.
Common goals such as a daily step target are popular because they are easy to understand, not because one number is correct for everyone. A figure that suits one person’s age, fitness and health could be too much or too little for another. If you have a medical condition or have not been active for a while, a health professional can help set goals that fit.
What a tracker is good for
Despite the limits, trackers do some things well.
- Showing trends. Whether this week was more or less active than last week is usually reliable, because the same device makes the same kind of error each time.
- Prompting movement. Reminders to stand or walk can interrupt long sitting stretches.
- Providing a record. Seeing a pattern over months can help someone notice that a habit has changed.
- Giving feedback on effort. Seeing how heart rate responds to a hill, for instance, can be interesting.
They are weaker as precise instruments, and treating the display as exact can create needless worry or false reassurance. Many people also find that numbers become stressful or lead to obsessive checking. If a tracker starts to feel like pressure, taking it off for a while is a reasonable choice.
Getting more consistent readings
A few habits tend to improve consistency.
- Enter your height, weight and birth year accurately in the app.
- Wear the band snugly but comfortably, a finger-width above the wrist bone.
- Wear it on the same wrist, and tell the app which wrist if it asks.
- Use exercise modes for workouts that do not involve walking.
- Keep the software current, since manufacturers sometimes refine their counting algorithms through software updates.
Remember also that the data does not stay on the device. Daily step counts, locations and routines are usually synced to an account, so it is worth glancing at app permissions and the privacy settings when you set up.