Over the last decade or so, a growing number of companies have developed tech that measures physical data. This is usually done for the purpose of tracking health and helping people come up with exercise plans.
That being said, when users look down at their Fitbit, Apple Watch, or other smartwatches, they might scratch their heads, wondering what the data represents. From steps and bloop pressure to pace and calorie burning, keep reading to learn what those metrics really mean and how they're measured.
One of the most popular uses for a smartwatch or a Fitbit tracker is to help determine how many calories a person is burning during a given workout or as part of a larger weight loss journey.
Since this metric depends on a body's energy expenditure, it's hard to get uniform results because people have different metabolic rates. As Patrick Slade of Stanford University told CNET, "People can't necessarily just scale the smartwatch estimates by a fixed amount to make them accurate for everyone."
How do smartwatches measure calorie burn?
According to a 2019 study in the International Journal of Environmental Research and Public Health, the most accurate way to determine the number of calories burned is to measure the difference between oxygen and carbon dioxide levels in each breath. Naturally, smartwatches can't do this, so they instead measure a user's heart rate against data they provide, such as age, weight, height, gender, and usual levels of physical activity.
But while some devices get closer to the truth than others, none of them can be as accurate as the unwieldy and expensive tech that uses breaths to measure this.
While some smartwatches will use a wearer's heart rate to measure calorie burn, others will offer this information on their own directly.
As a 2019 study in the International Journal of Environmental Research and Public Health explained, such devices will track a user's resting heart rate, as well as the average and maximum changes to that rate during various physical activities. Athletes tend to find this information helpful when determining how strenuous their workouts are.
How do smartwatches measure heart rate?
In a 2020 study published in JMIR Mhealth Uhealth, researchers found that smartwatches typically have back panels that contain contact electrodes, which use contact with the user's bare wrist to read and determine ECG signals.
Another feature often used in combination with these electrodes is a Photoplethysmography (or PPG) sensor. This was identified in a 2018 study in the International Journal of Biosensors and Bioelectronics as a device that uses a light source and a detector at the surface of the skin to measure changes in volume as blood circulates through the body.
One of the most common uses of smartwatches and fitness trackers like the Fitbit is to track the number of steps a person will take both in a given day and over larger periods of time, like months or years.
The popularity of this measurement comes from its ability to give users a quantifiable way to track and hold themselves accountable for their fitness journeys.
How do smartwatches measure steps?
As Alfred Poor wrote for The Verge, they accomplish this step tracking through sensors called accelerometers that determine when the device is moving. Some will also bolster these results with a gyroscope that tells the device the direction the user is moving in and whether they're rotating.
But while they're supposed to ignore non-walking motions, certain actions like hammering nails, riding a bus, or even washing hands can create enough vibrations to fool the motion sensors into recording non-existent steps.
Stairs and overall height climbed
Fitness tracking software that measures steps will typically also determine how high those steps took users.
UCHealth Foot and Ankle Center director Dr. Kenneth Hunt told US News and World Report that stair climbing not only makes for an effective low-impact cardio workout but also strengthens the body's muscles. That goes a long way to explain why this metric would be worthwhile for these devices to track.
How do smartwatches measure stairs climbed?
As Alfred Pool put it in an article for The Verge, they make use of the same motion sensors that track the vibrations in our steps. However, the data from these sensors is bolstered by air pressure detectors.
The idea behind this design decision is that as a person climbs higher, the air pressure around them tends to decrease. Unfortunately, that's not all that causes air pressure changes. As a result, smartwatches can treat trips on an elevator, entering and exiting tall buildings, or even sudden weather changes as the number of stairs climbed.
Although this isn't a feature offered by all fitness trackers and smartwatches, some will purport to track their users' blood oxygen levels.
According to the National Center For Biotechnology Information, this is a measurement of how much of the hemoglobin in blood is bound to oxygen molecules compared to how much hemoglobin exists in the body at all. The closer to 100% this reading gets, the healthier the level of oxygen saturation.
How do smartwatches measure blood oxygen?
As researchers found in a 2020 study published in JMIR Mhealth Uhealth, smartwatches determine blood oxygen levels by measuring volume changes in blood circulation using a PPG light system. This is typically found in the watch's back plate.
However, those researchers also found that smartwatches are not yet capable of accurately measuring oxygen saturation in a useful way. Users should then be skeptical of the data from wearable fitness trackers that claim to do this.
A smartwatch that tracks a user's steps can also typically be expected to track the total distance that a person walked, ran, or rode a bicycle over a given period of time.
These measurements are calculated according to the most understandable unit of measurement for the users, who can typically choose whether they want their distance in miles or kilometers.
How do smartwatches measure travel distance?
Smartwatches and other fitness trackers with this capability measure a person's distance using GPS location tracking software that derives its data from satellites.
As blogger DC Rainmaker found after conducting a series of tests on four different smartwatches, both a given environment's ease of satellite lock and the quirks in a given smartwatch's software can affect the device's accuracy. So if someone's hiking through a dense forest and they're making a lot of twists and turns, the readings may be off.
Maximum oxygen consumption
Some high-end sports watches and similar wearable tech will include features to measure how much oxygen a person consumes during a workout. This value is also called maximum oxygen uptake and is often displayed as a user's "VO2max."
In a 2007 study in Medicine and Science In Sports and Exercise, this value was explained as defining the limits of a given person's cardiorespiratory system.
How do smartwatches measure oxygen uptake?
According to a 2019 Environmental Research and Public Health study, smartwatches can vary in how they measure this metric. Some, such as the Garmin Forerunner 920XT, track the user's heart rate during what they consider a comfortable run and measure it alongside the personal data they provide.
Others, like the Polar V800, use a test developed by Dr. Fritz Schellong to determine maximum oxygen uptake. As a 2019 study in Clinical Autonomic Research explained, this involves comparing changes in a person's heart rate and blood pressure from when they're lying down to when they're standing.
Active zone minutes
Although Fitbit coined the specific terminology "active zone minutes," other fitness trackers will use similar metrics, whether they're called active minutes or intensity minutes.
Fitbit defines this metric as tracking a user's " heart-pumping " minutes during a given week. The metric also typically categorizes such activity as "moderate" or "vigorous."
How do smartwatches measure active minutes?
As Simon Jary wrote for Tech Advisor, fitness trackers compare a user's resting heart rate to heart rate "zones" that person would experience during moderate and vigorous workouts.
In the case of a Fitbit, the default settings recommend 150 minutes of moderate activity and 75 minutes of vigorous activity per week. That's based on the recommendations of both the American Heart Association and the World Health Organization.
Systolic blood pressure
According to the CDC, systolic blood pressure refers to the pressure that builds in the arteries at any time a person's heart beats.
To further provide context for this metric, a person who receives a blood pressure reading of 120 over 80 will have a systolic blood pressure of 120.
How do smartwatches measure systolic BP?
According to a 2020 study published in JMIR Mhealth Uhealth, fitness trackers often measure systolic blood pressure by using ECG readings and determining volume changes during blood circulation with the light detectors in a PPG device.
Once this data is gathered, the tracker then uses it to determine the pulse transit time or how long it takes the pulsation from a heartbeat to travel along the arteries. Unfortunately, that study found fitness trackers often measure this value with questionable accuracy.
Diastolic blood pressure
As the CDC explained, diastolic blood pressure is the pressure that builds in the arteries when the heart is resting between beats. So if a person receives a blood pressure reading of 120 over 80, they have a diastolic blood pressure of 80.
If it seems silly to distinguish between these two blood pressure metrics, it's worth noting that not all fitness trackers provide both and usually favor systolic blood pressure when they don't.
How do smartwatches measure diastolic BP?
Although the researchers behind a 2020 study published in JMIR Mhealth Uhealth were certainly interested in knowing exactly how an Everlast smartwatch they examined measures diastolic blood pressure, it seems the company was not forthcoming with that information.
However, the fact that they found a PPG sensor in the back plate makes it likely that the watch uses its light source and the relevant detector to measure volume changes in blood circulation. This is likely used to help determine pulse transit time as with systolic blood pressure measurements, but it's unknown whether ECG is used as well.
There are many ways a smartwatch could potentially measure a user's pace, but they typically calculate it by a runner's average pace per mile unless commanded to do otherwise.
As Angela Moscaritolo wrote for PCMag, some watches will also calculate the user's threshold pace, which she described as "The fastest pace at which the body does not accumulate lactic acid significantly in the body during the workout."
How do smartwatches measure pace?
Based on the contents of Angela Moscaritolo's PCMag article, tracking pace is likely to require data from the watch's GPS systems to measure distance, it's timekeeping systems to measure how quickly that distance is traveled, and the step-measuring motion sensors to detect the average length of a person's stride.
As for the threshold pace, it's only through a smartwatch's PPG sensor that tracks changes from within the body with light that a body's lactic acid levels could plausibly be determined while running.
Some fitness trackers shouldn't be used underwater at all, but as Jae Thomas wrote for Mashable, others can not only track lap times during open water and pool swimming but also keep track of underwater heart rates.
While the Apple Watch Series 7 is capable of this, Thomas cited Garmin Vivoactive 4 and the Polar Grit X Pro as being specifically made for this kind of tracking.
How do smartwatches measure swimming?
First, it's worth checking the manual as to whether the watch in question is water-resistant at 50 meters. But the ones that fit that bill and can measure underwater heart rates will have electrodes modified to account for how water tends to interfere with that kind of data collection.
If a watch can determine lap times in open water, it uses its GPS capabilities to do so, and Jai Thomas wrote in a Mashable article that many fitness trackers rely on accelerometers to measure calorie burning while swimming.
It seems hard to imagine that a smartwatch could be capable of measuring something as hard to define as sleep quality, but some do this by examining a few key factors that tend to affect a good night's sleep.
These can include the motion and position of a body during sleep, as well as the characteristics of the user's sleeping environment such as noise, light, and temperature.
How do smartwatches measure sleep quality?
In a 2018 study from the Proceedings of the Association for Computing Machinery on Human-Computer Interaction, researchers who developed their own sleep-tracking smartwatch did so by using an accelerometer, a gyroscope, and an orientation sensor to track the user's posture, movements, and propensity to toss and turn while sleeping.
Also included was an onboard microphone to determine ambient noise levels in the sleeper's environment, as well as light and heat sensors to measure these relevant factors.
Getting the most out of a smartwatch
No smartwatch is perfect for measuring these metrics, and they vary in accuracy, but there are nonetheless a few tips to ensure they work as reliably as possible.
And while the first step may seem obvious, it's nonetheless important to follow the manufacturer's instructions as written in the enclosed manual carefully while setting up the device.
A smartwatch goes on the non-dominant hand
As Alfred Poor wrote for The Verge, this is because the dominant hand is more active during the day and, thus, more likely to engage in activity that is accidentally misread as steps.
Since these activities can range from tightening a screw to stirring a pot, it can be surprising how easily wearing a smartwatch on a dominant wrist distorts data.
Secure a smartwatch firmly, but don't overdo it
As Jimmy Westernberg wrote for Android Authority, a fitness tracker will have a much harder time sensing its user's movements and especially their heart rate, if it's moving too much on the wrist.
For that reason, the device should be secured tightly enough that it remains snug and motionless on the wrist but not so much that it's uncomfortable to wear.
A single workout's data is a drop in the bucket
One of the most important pieces of advice Jimmy Westernberg had to share in his Android Authority article was to focus more on long-term trends that can be observed in a fitness tracker's data than anything that comes from one day.
That way, those distorted readings that come from stepping onto an elevator or a major repair job will present themselves as what they are: Anomalies in a larger, more helpful fitness trend.