We can all use a little help staying in shape. Regularly bringing exercise into busy lives can be tough and it’s often the first thing that gets thrown overboard when something crops up. The best fitness trackers provide us with friendly reminders to get up and out; they track steps, measure our heart rate, estimate calories burned, and give us a general read on how active we are. But how accurate are they (and does it matter)?
For most people the step counts are pretty good but check your step count on two different fitness trackers or compare it to your smartphone’s estimate and you’ll see differences. Step counting may have been more accurate with early clip-on devices that were designed to be worn on the hip. Wrist worn devices record a lot of movement that has little to do with physical activity. Even if you wear your fitness tracker on your non-dominant wrist, it’s going to register all of your hand movements. Manufacturers moved away from clip-on trackers because people would lose the devices, forget to attach them, or accidentally send them through the laundry affixed to pieces of sweaty workout wear. And the move to wrist worn trackers also opened the door for heart rate sensors.
How does it work? Well, the heart rate monitors used in most trackers work via a technology called pulse oximetry. An infrared light penetrates the skin and looks for subtle changes in the colour of the blood. These colours represent the oxygenation of the blood, which increases with each heartbeat.
The devices that measure heart rate tend to be pretty good at measuring heart rate at rest but have a lot more variability when measuring heart rate during exercise. If you’re running a marathon or training seriously for an event, then the heart rate sensor variability in a typical fitness tracker may be too high to make it really useful. In that case a chest strap is going to be much more accurate than a wrist worn tracker. But for most people looking to get a general sense of their heart rate, fitness trackers are probably accurate enough.
Most activity trackers are measuring the motion of your body and combining that with your height, your weight, your gender, and your age. Sometimes they might have asked you some lifestyle questions during setup and that data can be thrown into the mix. There are however other aspects that it is not taking into account. One glaring omission is muscle mass. Muscle is metabolically active — it requires energy to maintain and to use. Also, hormones have a big impact here. If you have a higher level of testosterone, you will burn more calories and build more muscle. Likewise, thyroid hormones, cortisol levels, insulin, and more all play a role.
What to look for in a fitness tracker
Definitely consider compatibility with your phone. The Apple Watch is going to be a great choice if you have an iPhone, but not so much if you don’t. There are also a lot of different features available and you need to decide if you want something just to track your activity or to serve as a smartwatch, too. Even the cheapest trackers have all of the most important features.
We recommend choosing a tracker that also measures heart rate. The heart rate sensor is important. It doesn’t just add heartbeat tracking. The meta-analysis makes clear that devices that employ accelerometry alone are also much less accurate at estimating energy expenditure than trackers with heart rate sensors.
Budget is obviously going to figure in your decision, too, but you don’t necessarily have to spend a lot. Check the lowest cost fitness tracker that has a heart rate monitor and then check if any of the more expensive trackers have features that are actually useful to you.