Fitbit Air Has a Cadence Lock Problem on the Treadmill
Let’s not beat about the bush concerning this morning’s Fitbit Air test. I expected near-perfect HR results for a hot indoor treadmill interval session, even though it was fairly warm.
My reference chest strap completely failed, but luckily I had Polar SENSE as a backup reference, plus Apple Watch Ultra 3, Amazfit Helio Strap and Whoop MG. Yeah, greedy, right? I like backups – too many gadgets. But it saves time and the need to repeat a session when the chest strap decides not to play ball.
As you can see, Air alternates between the correct level and a higher level. Presumably, the higher level is my cadence – actually, it’s not, as my cadence is between 190 and 200spm on the fast parts. Let’s say they are cadence-linked! Or how about cadence-adjacent? I like that phrase.
I had not spotted this behaviour before. I’d not tested Air on the treadmill before either, and I did update the firmware yesterday. So maybe it is a new firmware bug? Quite a bad one. Fitbit Air’s data is based on TRIMP and a variant of the Karvonen formula (220-Age and HR reserve). Data like this means that EVERYTHING else that is inferred is wrong.
For those of you who dislike easy-to-read charts, here is a statistical summary excellently produced from DCRainmaker’s beta Analyzer tool
- Amazfit Helio, Apple Watch Ultra 3, WHOOP, and Polar SENSE show excellent accuracy, all agreeing closely around 131-132 bpm with minimal bias (<0.5 bpm) and tight limits of agreement.
- Fitbit Air is inaccurate, reading ~157 bpm — consistently ~37 bpm higher with poor agreement.
FAQ
Q: What is cadence lock in a wrist HR monitor?
A: Cadence lock occurs when the optical sensor picks up the rhythmic movement of the wrist or arm during running and interprets that motion frequency as a heartbeat signal. At running cadences of 160-200 steps per minute, the false HR reading lands in a plausible range, making it easy to miss without a reference device.
Q: Does a cadence-locked HR reading affect training load calculations?
A: Yes, significantly. Metrics such as TRIMP, TSS, and any Karvonen-based calculations (which use HR reserve against a 220-minus-age estimate) are entirely derived from the HR signal. A consistent 37 bpm overestimate will inflate every downstream load, recovery, and intensity figure.
Q: Could this be caused by the Fitbit Air firmware update?
A: Possibly. The behaviour appeared after a firmware update and had not been observed in previous treadmill sessions. Without a pre-update control test on the same session type, it cannot be confirmed as firmware-induced, but it warrants a follow-up test.
Last Updated on 27 May 2026 by the5krunner
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tfk is the founder and author of the5krunner, an independent endurance sports technology publication. With 20 years of hands-on testing of GPS watches and wearables, and competing in triathlons at an international age-group level, tfk provides in-depth expert analysis of fitness technology for serious athletes and endurance sport competitors. ID
