What Your Fitness Tracker Is Best & Worst At

These days, most of us in industrialized countries will wear a fitness tracker, usually in the form of a smartwatch of some kind. But, whether a smartwatch or a dedicated device, they generally have more or less the same technology, with the same potential for usefulness or leading us astray.

What they’re ok at

What they’re ok at is mostly the things that you’d expect them to be good at, for example:

• saying how many steps you have taken
• saying what your heartrate is like
• saying how well you slept

…and so on. They’re typically not amazing at these tasks, so expect a moderate margin of error. But they’re not terrible either, and will almost always be more accurate than if you were to simply guess.

What they’re worst at

They are, for the most part, and at the time of writing (we may presume technology will advance at some point), very bad at interpreting the data that they collect.

For example, many such devices and/or their associated apps will, nowadays, give aggregate scores based on the raw data from the previous category, for example:

• saying how well-rested you are
• saying how stressed you are
• saying how much energy you have

…and so on.

The problem, however, is that while trackers are very good at handling statistics, they have little-to-no understanding of the nuances of human life in general, let alone your life in particular.

Writer’s example: if I open my phone’s fitness app now and look at my “energy score”, it says my energy score is 43. There are no units for this (it’s not about my gravitational potential energy, or how many calories I contain, etc), it’s simply a score out of 100 to say how much energy it considers I have to take on my day. It is basing that number on:

• how much sleep it recorded for me last night (positive factor; I regained energy)
• how much exercise I have done today (negative factor; I expended energy)
• what my heart rate, heart rate variability, blood oxygen saturation, and other such metrics are like

However! It can easily be wrong about those metrics, for example:

• It can misjudge how much sleep I get (indeed, I’m sure this tracker overestimates my sleep, by counting wakefulness in bed as part of my sleep)
• It can misjudge how much exercise I get (for example, I will spend a lot of time at my desk; it certainly thinks I am sitting, but in fact I am standing, usually on one leg at a time, like one of those wading birds, which although I’m mostly not very conscious of it either because it’s just how I stand at my desk out of habit, I know for a fact involves a lot of stabilizing muscle use)
• It can overlook important factors such as the effects of food I eat (a snack may perk me up; a large meal may slow me down; it won’t know about either)
• It can overlook important factors such as what’s going on in my brain (meditation can be refreshing; mental stimulation is by nature uplifting, but a long period of intense focus can cause a crash afterwards due to exhaustion—remember our brain uses 20% of our daily calories just to keep it alive, let alone what we expend in mental exertion)

With such errors in any or perhaps all of those input variables, how reliable can its output possibly be?

Now, apply the same level of error to other aggregate metrics that your fitness tracker might give you, such as a “stress score”, for example.

Or, to quote what hard science has to say about such technology:

❝While metrics such as resting heart rate and heart rate variability are well-supported by empirical evidence, the proprietary algorithms underpinning CHS and the lack of methodological disclosure hinder reproducibility and independent scrutiny. Furthermore, the absence of rigorous validation across diverse populations raises critical concerns about generalizability and equity. The variability in measurement protocols and integration strategies further complicates comparability and reliability, ultimately limiting both scientific and clinical confidence in these tools. This lack of standardization undermines the scientific basis of CHS and limits their long-term potential for clinical adoption, personalized health management, and public health applications.

If CHS are to evolve beyond opaque consumer-facing features into scientifically validated health metrics, the field must shift from merely evaluating industry-led innovations to defining best practices for their development and deployment.❞

~ Dr. Cailbhe Doherty et al.

CHS = Composite Health Scores

Read in full: Readiness, recovery, and strain: an evaluation of composite health scores in consumer wearables

What they’re best at

Before you disconnect, disavow, or defenestrate your fitness tracker, let’s mention: there is one thing they’re very good at!

Establishing patterns. And this is critical for good health management!

For example:

• it might get your sleep score wrong (and report incorrectly how many hours sleep you got, or how long you spent in each phase of sleep), but it’ll be able to show clearly whether the amount of sleep you get is increasing or decreasing over time or remaining the same, or whether your sleep schedule is sliding forwards or backwards over time
• it might get your step count wrong (most do), but it’ll be able to clearly show which days you exercised more or less on, or whether your general trend is increasing or decreasing. Because although it’ll miscount, it’ll miscount by approximately the same percentage each day on average.
• it might get your heart rate wrong (it almost certainly will), but it’ll be able to show that your heart rate spikes when you wake up or after dinner or when you do an exercise session; it’ll also be able to show whether your resting heart rate is increasing or decreasing over time (because like with the step count, it’ll be wrong by approximately the same percentage each day on average).

Thus, to get the most out of your fitness tracker… By all means, let it track all the things. But also:

• take a moment to consider how your fitness tracker knows the various things it knows (i.e., and whether in fact it could be mistaken, and if so, to what degree)
• pay much more attention to the big trends, rather than to what the actual numbers on a day-to-day basis claim

See also: Thinking of using an activity tracker to achieve your exercise goals? Here’s where it can help—and where it probably won’t

Take care!