Fitness tracking has become a major industry, with new devices that uniquely combine form and function constantly entering the market. The products range from smart watches to necklaces, as well as clips for the hip and shoe, and even rings. These devices automatically track calories burned, steps taken, distance traveled, and more. But underneath the stylish exterior, it is often unclear exactly how these activity trackers work, and even more unclear whether they work well.
The physical components of the monitors are generally straightforward, with nearly all monitors relying heavily on miniature devices called accelerometers, which measure acceleration produced by movement of the body. Other components are commonly included to measure other characteristics of human movement, such as rotation (measured with gyroscopes), changes in elevation (measured with altimeters), or heat production (measured with thermometers). Some monitors measure heart rate using a new technology called photoplethysmography, which shines LEDs through the skin to capture the expansion and contraction of blood vessels as blood moves through.
While the general makeup of activity trackers is simple enough, it remains challenging to understand how accurately the devices translate basic movement (or lack thereof ) into convenient outcomes like calories burned or steps taken. A key cause of this uncertainty is that the algorithms governing the conversions are private, which makes it hard to evaluate whether they combine biology and math appropriately. Another issue is that manufacturers rarely describe how they tested their device’s performance before releasing it, which restricts information about the scientific rigor of the device’s development.
Considering how little we know about the algorithms or how the devices were developed and tested, the first thing for a consumer to do before buying a monitor is consider how important it is for the monitor to be accurate. For some, this will be highly important, while for others it will be less important. For example, a person who simply needs a motivational tool to help them be more active may be less concerned with accuracy than someone using the device as a clinical weight loss aid.
Those who need the most accurate monitor should gather their primary information from third-party experiments. Although the experiments are difficult to conduct (and are thus subject to limitations), they represent the best available source of knowledge about the accuracy of fitness tracking devices. Several studies reported through publicly accessible channels have performed head-to-head comparisons of a variety of monitors. (Including this one from the August 2017 issue of Medicine & Science in Sports & Exercise® .)
For those who are not as concerned with a monitor’s accuracy, the form and functionality may be more important considerations to consider. For example, some monitors make it easy to create challenges or competitions with friends, which can be a source of motivation for some. Other monitors provide tactile feedback to remind users to be active throughout the day. For more information on the motivational benefits of wearables, check out this article from the September 2017 issue of ACSM’s Health & Fitness Journal ®
Regardless of how an individual intends to use a fitness tracking device, it is critical to understand that the numbers an activity tracker shows are only estimates, and should be used as a guide, not the absolute truth. Consumers should select devices with care and use them to their full potential. This way, as the market continues to grow, the population can derive the maximum benefit from the services the new products provide.
Paul R. Hibbing, M.S.
Samuel R. LaMunion, M.S.
Lindsay P. Toth, M.S.