This May Be Why Your Fitness Trackers Are Inaccurate

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I lived in Los Angeles, where I went up to Griffith Park every morning with my dog, Ollie. Our climbs took, on average, about an hour and a half, and I was curious to see how much elevation I gained each week. So, I bought a fitness watch to track my activities and basic stats, like my distance. Since I didn’t want all the bells and whistles that some trackers offer, I went for the Fitbit Versa 2. I logged every step and started keeping track of my heart rate. mine and speed. I got really emotional—and started going for long runs to compete with my weekly standards.

When we moved to Philadelphia in 2021, I traded in the mountains and dry heat for wooded trails and less humidity. I got into cycling and running regularly and noticed that I sweat a lot on the East Coast, because of the thick, humid air out there. Even a quick run of two kilometers leaves me exhausted. When this happens, my watch slides up and down on my wrist and sometimes loses track of my heart rate—and I can’t help but wonder if my Fitbit so perfect when I’m covered in sweat and sunscreen.

I looked on Reddit, looking for answers to see if other athletes have suffered from this phenomenon, and found that tons of people also believe that their watch doesn’t work properly with sweat and sunscreen. around their arms. To get the much needed answers, Outside spoke to several experts to find out if sweat really affects the accuracy of a watch.

Do Sweat and Sunscreen Impair the Practice’s Correct View?

Your typical tracker uses a photoplethysmogram (PPG), a technology that uses infrared light to measure blood pressure and flow changes. A sensor located under the watch illuminates your skin to get a detailed view of how your blood is flowing, says Stephen Carter, an assistant professor in the Department of Kinesiology at Indiana University. Then, the sensor sends those measurements to an algorithm, which estimates how fast your heart is beating, he says.

Basically, anything that messes with the sensor can cause inaccuracy, says Carter. Sweat, makeup, cosmetics and repellents, for example, can clog the sensors and affect how they track your activity, says Michele Orini, Associate Professor of Health Engineering at King’s College London. Some evidence suggests that when something covers the sensor, it prevents the lights from picking up accurate heart rate, oxygen flow and respiration rate readings from your skin, Orini says.

Pictures of popular fitness watches provide guidance on how to care for your watch. Garmin clearly states that debris, such as sweat and makeup, can make it difficult for your watch to collect reliable heart rate data. Apple recommends keeping your watch clean and dry as water and sweat can cause inaccurate recording.

Orini and his research team conducted a study in 2022 where they dropped a saline solution (to simulate sweat) on the wrists of 17 people wearing fitness watches to investigate whether sweat can affect heart rate. how. They found that the fake sweat caused heart rate errors, but only when they were walking the way a person would during vigorous exercise.

This happens because vigorous exercise turns the clock around and allows sunlight, if you are outside, to shine between your skin and the sensor, which destroys the reading. Mistakes were made worse when the watches fit snugly rather than snugly in the wrist.

Orini’s biggest decision? “Sweat and makeup can reduce heart rate accuracy by acting as a barrier between the sensor and the skin.” They also act as a lubricant and cause the watch to float up and down your wrist, making it difficult to stay still and read accurately, he adds.

Other studies have found similar findings. A 2023 study concluded that proteins and lipids in your sweat can stick to the sensor and form a coating that, yes, blocks the sensor and prevents it from doing its job.

As for distance or steps, they are less likely to be affected by beads of sweat dripping from your hand because the built-in GPS watch and accelerometers track your location, not light sensors, says Orini.

There’s still a lot we don’t know, and more research is needed to find out how much blocking the sun or sweat from getting into the sensor can skew the readings and if they can affect the readings you how much you see on your watch.

What Should You Do If Your Watch Sweats?

Start with what you can easily control, such as how tight the watch is to your wrist. You want it to stay tight because accuracy can suffer if it’s too loose, Orini says, but a too-tight fit can restrict blood flow and damage projections.

To ensure that the group is safe but still comfortable, create a small space, just enough for your skin to breathe. The watch should also sit a finger or two above your wrist. (Click here to see what your fitness watch should look like.)

You also want to get into the routine of cleaning the sensor. Garmin and Fitbit, for example, recommend clearing it regularly with water and a cloth to remove the buildup—especially if you’ve been sweating a ton or wearing sunscreen or insect repellent.

Obviously, it’s difficult—if not impossible—to control how much you sweat, but the goal is to keep the sensor as clean and dry as possible, Orini says. Carter says most trackers will do a good enough job of tracking your exercise, even if you sweat more than usual.

If you’re worried that you’re not reading correctly, it might be a good idea to pause and dry it—and your hand—before continuing. COROS recommends walking this way if you are wet. But for many people—like Orini, who is a marathon runner—stopping mid-game isn’t really in the cards for most elite athletes.

When taking a break isn’t an option, it might be worth investing in a chest heart rate sensor like the Polar H10, which uses electrocardiogram sensors to record your heart’s electrical signals. Although wearing a chest belt may not be the most convenient tool to wear, a 2019 study found that they are more accurate than watches.

Research seems to be inconclusive as to whether sweat affects the accuracy of the chest pain sensor. However, some Reddit users claim that sweat interferes with their reading; others say sweat has no effect. (FYI: If you’re a super sweater, the Polar H10 isn’t waterproof.)

Fitness watches are amazing devices—they encourage people to exercise more and improve their fitness—but they’re not perfect, Carter says. Of course, there may be some bugs, but at the end of the day, this shouldn’t stop you from using the tracker. The best thing you can do is limit your needs.

So, if you’re in the middle of a grueling ride and your fitness watch says your heart rate is 11, don’t worry—you’re still alive; you may just have to clean your watch quickly.