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They’re wildly inaccurate for most anything outside of steps.
Fitness trackers are pretty popular these days. Whether you have a dedicated fitness tracker that is meant really just for tracking your fitness goals, or a watch that has a fitness mode as well, a huge percentage of people across the globe now have a strap on their wrist that supposedly tells them about their health and fitness. Whether it's a FitBit or an Apple Watch, don't trust it.
Fitness trackers have been around for just over 10 years now (at least mainstream), and they’ve come a long way since they first came out. At first, they really just tracked how many steps you were taking, but they’ve added all kinds of functionality since then, including tracking workout routines, heart rate, and calories burned.
Fitness trackers are generally good at tracking your steps and flights of stairs. They’re certainly not perfect in these cases, but they are pretty good overall. While every now and then it will pick up some extra steps, it gives a pretty decent idea of how far you’ve actually walked. Similarly, trackers that will track flights of stairs will give you a pretty good value of how many flights of stairs that you actually walk up.
Of course, these stats aren’t 100% accurate. You’ll have likely taken less steps then your tracker is telling you. The difference between these stats being inaccurate, and your heart rate and calories being inaccurate is that it’s not the end of the world if they’re inaccurate in other parts of your life. If you thought you took 11,000 steps, but actually only took 10,000 steps, there’s no real negative effect on the rest of your life. This may not be the case with the other stats.
Do not trust the calorie counts or heart rate values from your fitness tracker. They are not accurate, and believing that they are can have a negative impact on your life. You need to accurately measure your own heart rate and determine your own calorie expenditures. If you rely on your fitness tracker numbers, there is a good chance that you’ll not be able to make the progress that you want.
Yesterday, my fitness tracker told me that I burned 5,000 calories. There is no chance that I burned 5,000 calories and if I had eaten 5,000 calories since I believe that I did, I would be gaining weight extremely fast over the course of a week. I’m generally seeing a number around 4,000 calories on my tracker. I know that I should be eating 2,300 to 2,500 calories a day to maintain my weight - that’s not even close to what my fitness tracker is telling me. I’d be gaining a couple of pounds a week if I was trusting the number from my fitness tracker.
I don’t generally look at the heart rate number on my monitor because it’s also quite inaccurate. You can use it to see if your heart rate is relatively higher than it normally is during a workout, but don’t trust that the number is actually accurate. For whatever reason, they can be quite inaccurate when measuring your heart rate during your workout - you’re not getting an accurate measurement to know what your max heart rate is.
Overall, fitness trackers are useful tools as long as you don’t trust them completely. They can give you a decent idea of how you’re doing, but they’re not going to be exact. Use other methods for tracking your heart rate and your calorie expenditure.