What is cadence lock in a wrist HR monitor?

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 to 200 steps per minute, the false HR reading lands in a plausible range, making it easy to miss without a reference device.

What is the dual-signal artefact seen in the Fitbit Air treadmill test?

Rather than locking onto a single wrong value, the Air’s optical sensor simultaneously detected two competing periodic signals in the PPG data during the treadmill session and alternated between them at sub-second intervals. The high signal averaged around 169 bpm, rising to 190 bpm, corresponding to foot-strike frequency. The low signal of around 111 bpm is a second mechanical artefact, most likely the treadmill belt or motor vibration. Neither signal is the true heart rate of 131 to 132 bpm.

Is the gym artefact the same problem as the treadmill artefact?

Same class of failure, different interference source. On the treadmill, the interference is sustained and regular, produced by a consistent mechanical source such as the belt or motor frequency. In the gym, interference is intermittent and irregular, arising from dynamic arm movement, grip pressure, and wrist flexion. The gym artefact resolves quickly once the movement stops, whereas the treadmill artefact persists for most of the session.

Does a cadence-locked HR reading affect training load calculations?

Yes, significantly. Metrics such as TRIMP, TSS, and any Karvonen-based calculations are entirely derived from the HR signal. A consistent 37 bpm overestimate will inflate every downstream load, recovery, and intensity figure.

Could the Fitbit Air firmware update cause this?

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.