Fitness Trackers: Accuracy and Features
Modern wearable fitness trackers offer far more than simple step counting. They monitor heart rates, blood oxygen saturation levels, sleep quality, and even menstrual cycles. Driven by decades of innovation from several companies, these devices have become remarkably accurate, according to Lisa Cadmus-Bertram, a researcher at the University of Wisconsin-Madison.
Cadmus-Bertram, the director of the Wisconsin Physical Activity Epidemiology Lab and an associate professor of kinesiology at the UW-Madison School of Medicine and Public Health, has been studying the accuracy of fitness trackers since 2011.
“I think I bought a first-generation Fitbit near the end of 2010 and have been using them ever since,” Cadmus-Bertram said. “It’s been fun to watch this group of technologies change and evolve over the years.”
Her research centers on understanding how physical activity relates to chronic disease risks, and she notes that the technology in fitness trackers can help people who have undergone cancer treatment adopt and maintain healthy lifestyles, although this requires thorough work in the lab to determine the accuracy of the devices.
“How good are they at accurately measuring steps? How good are they at measuring your heart rate when you’re resting? How good are they at measuring your heart rate during exercise? How well can they correctly classify what type of activity you’re doing?” Cadmus-Bertram recently discussed on WPR’s “The Larry Meiller Show.”
This interview has been edited for brevity and clarity. “I wear my Apple Watch when I’m golfing,” said Meiller. “And if I take a particularly strong swing, it’ll ask, ‘Oh, did you fall down? Should we call emergency services?’”
Cadmus-Bertram noted that fitness trackers can sometimes fall short, especially concerning movement patterns. She added that, when they first appeared, fitness trackers weren’t great at counting steps when users’ hands were stationary—such as when pushing a grocery cart or a stroller. Cadmus-Bertram highlighted efforts by companies to improve their pattern recognition algorithms to better differentiate between activities like jogging and using an elliptical machine. Cycling poses another challenge for these devices because the tracker is on the wrist, an area of the body not primarily engaged in the motions of cycling.
“There are inherent challenges when you’re trying to measure the activity of a human body which could be moving in different directions at different speeds and in different patterns [at the same time], yet you only have one sensor in one place,” Cadmus-Bertram explained.
What To Look for in a Fitness Tracker
Meiller asked how active a person needs to be before buying a fitness tracker.
“Zero activity,” Cadmus-Bertram replied. She emphasized that, regardless of one’s goals, knowing your resting heart rate and step count is crucial. This data can help to set goals.
When asked about what to look for in a fitness tracker, Cadmus-Bertram recommended brands with a long track record and strong research support, such as Fitbit (now a subsidiary of Google), Garmin (known for watches for endurance athletes), and Apple Watch. All three, she said, provide decent accuracy for key metrics like resting heart rate, physical activity minutes, intensity, and steps.
For runners, Garmin offers excellent devices, often paired with a chest strap for the most accurate heart rate measurement during exercise. Weightlifting and yoga are challenging for trackers due to the irregular and non-repetitive movements, although apps can help by allowing users to program and record routines.
When discussing security and ease of use, Cadmus-Bertram noted that most Fitbits are user-friendly with many features. She pointed out the importance of considering the company’s terms of service, especially if you are concerned about sharing location data.
“Most Fitbits are very easy to use, although they do have a lot of features and they keep adding more. Fitbit used to be its own company. It’s now a subsidiary of Google, [so users are subject to the company’s] terms of service,” Cadmus-Bertram noted. “That’s something to consider if you’re concerned about sharing your location data. You may want to choose a fitness tracker that doesn’t include GPS. Although, if you’re carrying a cell phone around with you, your location is available through that anyway.”
For sleep tracking, solid choices include the Oura Ring, Apple Watch, and Fitbit. Cadmus-Bertram mentioned that she uses a Fitbit herself, finding it helpful. Many devices now offer a “readiness” metric based on heart rate and sleep data. By measuring resting heart rate, sleep patterns, and overall sleep quality, the “readiness” metric gives users a snapshot of how recovered the body is and ready to exercise.
When discussing what users can do with heart rate and resting heart rate data, Cadmus-Bertram shared her experience. During phases of regular exercise, her resting heart rate is typically lower than when she takes a break. She added that the resting heart rate varies from person to person, but paying attention to changes, such as whether it is increasing or decreasing over time, can be very helpful.
The Oura Ring provides similar functions as a watch-style tracker, using the same technology. Cadmus-Bertram mentioned that the ring, which looks like a thick wedding band, may be convenient for some, depending on how they use their hands and whether they prefer to view data directly on the tracker display or via an app on their phone. For her, viewing the data on the tracker itself is preferable.
