Are Smart Watches Really That Smart?



Do you have a smart watch? Apple Watch? FitBit? Garmin? You likely do. If not, you 100% know someone who does. In fact, it is probably impossible to go through a day and not see someone sporting their smart watch.


Smart watches are devices that basically measure and monitor your activity, be it through steps, stairs, heart rate, energy expenditure. It does this through a fancy process called photoplethysmography (PPG for short). PPG is simply a technique that uses certain wavelengths of light to detect changes in blood volume from the skin surface. Your tissues and your blood absorb different wavelengths of light. PPG measures these differences and is how it detects your heart rate, which in turn determines your energy expenditure, sleep, and overall activity.


But just how accurate are these watches? If you have one, you’ve likely noticed inconsistencies or certain measurements that just don’t seem right.


Unfortunately, if you’ve doubted the accuracy of many of these watches, you’re likely correct.

The accuracy of smart watches depends on a couple of things including the type and intensity of activity, the device used (Apple v. FitBit v. Garmin), and even skin photosensitivity.


Unfortunately, the technology just quite isn’t there yet. PPG is a great tool and is accurate, but most often when sedentary…which defeats the purpose.


So what is some of this error that makes these watches not very accurate?


1. Skin pigmentation


That’s right. Melanin is a type of cell in your skin that determines your pigmentation. Lighter skin means a lower concentration of melanin and darker skin means a higher concentration of melanin. As you tan, your melanin concentration is increasing…which is why you tan in the first place. It’s not entirely certain why skin pigmentation would increase error in PPG, but it could be simply due to a block in signal from skin surface to blood or it could be from PPG development. If all of their studies were done on lighter skin individuals, then it would likely be more accurate for lighter skin people.


2. Type of activity and intensity


The lower the intensity of the activity, the lower the error. As soon as you start bringing in intensity, the accuracy plummets. In fact, the error for FitBit watches is 8% when sedentary, which then jumps to 16-40% error when active. First off, that’s quite a large range and secondly, that’s also a very high degree of error. I mean, you wouldn’t want to do a surgery that had a 40% error rate…your insurance probably wouldn’t even cover a surgery with error like that. In fact, FitBit has been involved in many class action lawsuits for inaccurate sleep and heart rate measurements.


Certain movements will falsely record steps. Biking is one of those. Even driving will sometimes record some steps due to bumpy roads. Group exercise (OrangeTheory, Total Body workouts, CrossFit), weight training, etc. have a lot of general, unpredictable movements. It’s not as consistent as running or walking, so it often leads to miscounting of steps. HIIT training and CrossFit exercises utilize intervals, which involves a large fluctuation in heart rate, something that smart watches aren’t as able to easily detect. The more vigorous the activity, the more inaccurate the smart watch is.


3. Type of device


All devices have their strengths and weaknesses, which brings about their own degree of error. FitBit is most accurate at heart rate measurements because it records heart rate every second, while Apple takes a reading every 10 minutes (except for when exercise). Apple has been reported to be more accurate at predicting energy expenditure and measuring sleep, compared to other watches.


4. Type of Measurement


Step and stair measurements have only a 4-6% error. In a study that compared three types of smart watches (Apple, FitBit, and Mio) to ECG measurements, heart rate measurements had an error (at rest) of 1-9%, while energy expenditure had an error of 9-43%. The error of sleep measurements aren’t indicated, however, many report general inaccuracy, and in a 2015 lawsuit against FitBit, it was shown that FitBit reports people as “awake” when very clearly they were asleep.


Okay…so they’re not that effective…why bother using them?


Smart watches aren’t 100% accurate…but that isn’t to say that they aren’t effective…


The biggest benefit that smart watches bring to the table is they get and keep people active. They make you aware of how active or inactive you are and they almost force you to increase that activity. They send reminders to get up and walk 250 steps. You become more aware of your elevated heart rate. You end up becoming a bit more connected with your body. You pay attention to your sleep. You pay attention to your movement each day. You gain a ton of motivation to exercise and to move from this external motivator. I couldn’t care less about the accuracy of the watch, because if they’re increasing activity and motivating people to get out and move daily, then that is all I need to know to be convinced of their success.


The second benefit that should definitely be considered is that despite the absolute inaccuracy, they are accurate relative to YOU. You can see which days you’re burning and expending more calories. You can see what days your sleep worsened. No it may not be accurate compared to your best friend, but it is pretty accurate when comparing other measurements of yourself. On days I work out, my watch will record 500-700 calories of activity. This may or may not be accurate. It generally will report that I burn 2800 calories/day. Last Monday, I did Murph which is a much more intense workout than I typically do. My watch recorded 1000 calories during the hour I completed Murph, and 3200 for the day. Now I know my maintenance is 2600 calories, so I know my FitBit isn’t accurate, but it showed me that I indeed did burn more calories and probably should take this into consideration as I’m planning my recovery and nutrition following.


The biggest downside I see to smart watches is how much emphasis people put on their smart watches, especially sleep and energy expenditure. It usually just brings unnecessary stress. FitBit, for example, in most cases underestimates sleep. It also will report times where you are awake when in reality you were sound asleep. In fact, with my FitBit, I often notice on my deepest nights of sleep that it will say I was awake for 1-2 hours and had 0% REM sleep… yet I remember my dreams which is a huge indication that I was in REM at least for SOME of the night. I’ll then spend too much time stressing about how little sleep I got… because it said 6.3 hours of sleep when I know I was in bed sleeping hard from 10pm-7am (9 whole hours). Well guess what? I got 9 hours of sleep…not 6.3.


The same goes for energy expenditure. People will use this number to determine how much food they should eat. In some cases, I find this expenditure to overestimate how much people are burning in a day. This is true for most people who do cardio or high intensity exercise. You’ll see that you burned 1000 calories exercising which puts you at a daily expenditure of 3000 calories…when in reality your maintenance is really only 2300. This means people will often times overeat and either not lose weight they’re wanting to lose or actually even gain weight. If you take someone who lifts weights and doesn’t do cardio, their watches will usually report a very low caloric expenditure, which just isn’t accurate. It’ll then have a lifter severely under-eating.


The takeaway from this is to not put all your eggs in one basket. Smart watches aren’t the end all be all. They are like anything, a tool. They can guide you and help keep you motivated and accountable. They get you up and moving and can generally be used to monitor day-to-day changes. They’re fairly accurate when it comes to steps, which honestly is the most important feature in my opinion because that directly relates to activity.


Instead of relying on your watch, feel free go old school.


1. To determine your heart rate, measure it by counting how many beats you feel in a 60s time frame. Don’t put so much emphasis on the daily heart rate you see. Practice deep breathing techniques if you feel your heart rate start to rise.

2. Calculate your total caloric expenditure following the guidelines I’ve laid out for you in my nutrition guide.

3. Record how long you were asleep by noting when you fell asleep and when you woke up and generally how you felt when you woke up (rested, groggy, tired).


It doesn’t have to be complicated!


If you want the accuracy of a smart watch, then consider hiring a coach. They’ll do the SAME thing as your fancy watch… but they’ll be a million times more accurate. Apply for a coaching call here.

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