A look at the accuracy of commercial fitness trackers, as studies show factors like having darker skin or shuffling when walking can throw off sensors
Sara Harrison / The Markup : Tweets: @themarkup , @davezatz , @juliaangwin , and @themarkup Tweets: @themarkup : There are various ways your fitness tracker could go wrong, especially if you don't fit into a fairly narrow demographic: light skin tone, in your 20s or 30s, with an average fitness level and “purposeful” gait. https://themarkup.org/... Dave Zatz / @davezatz : From first hand experience, I can also tell you fast typing nets Fitbit steps. 💪🏼 https://twitter.com/... Julia Angwin / @juliaangwin : I was honestly shocked when I learned that the green light in Fitbits and other fitness trackers has a harder time penetrating darker skin. Our latest on the inaccuracies of fitness tracker, from @atsaraharrison, with a cameo by @suryamattu: https://themarkup.org/... @themarkup : New: How accurate is your fitness tracker? If you have darker skin, shuffle when you walk, or regularly push a stroller, your tracker's data may be unreliable. https://themarkup.org/...
Context & Ripple Effects
Wearable-accuracy skepticism is not new: a 2015 test of four activity trackers found substantial variation in calories-burned and distance readings the same month a review found Fitbit's Charge HR and Surge heart-rate sensors disappointing. What The Markup adds is that the error isn't random — it clusters around users with darker skin, shuffling gaits, strollers, or typing hands.
That reframes earlier gaps as one pattern rather than isolated quirks: Apple Health and Watch already drew criticism for ignoring pregnancy and other health conditions, showing both the sensor and software layers assume a narrow user.
First-order effects
- Users who don't match the light-skinned, purposeful-gait baseline — including Dave Zatz's fast-typing step inflation — get inflated or deflated step and heart-rate numbers right now, undermining the core promise they bought the device for.
- Fitbit, whose heart-rate hardware was already flagged in 2015 reviews, carries this accuracy debt into its health push even as it works with Google Research on a Gemini-based app for personalized recommendations built on that data.
Second-order effects
- Rivals entering health tracking face the same validation bar: Amazon's Halo Band review found invasive collection without much return, and a device whose measurements skew by skin tone makes that bargain harder to sell.
- Phones gain an edge in the value comparison — a prior study found phones track fitness about as well as wearables — so accuracy failures nudge buyers toward free phone-based tracking over paid bands.
Third-order effects
- As wearables position for medical and insurance-adjacent use, sensor bias against darker skin becomes a health-equity and regulatory exposure, not just a product-review footnote — the same demographic-blindness already visible in condition-blind software like Apple Health.
- If unvalidated measurement persists, the industry splits between casual step-counting toys and clinically validated devices, pressuring makers to publish per-demographic accuracy data the way they now publish battery life.
The trend: Consumer wearables are being repositioned from fitness gadgets toward health platforms while their underlying sensing remains unvalidated across skin tones, gaits, and body types.