Inaccurate heart rate during training: where it comes from
How the reading works
The sensor lights the skin and measures the light bounced back by the blood moving underneath. Anything that lets outside light in, or lifts the sensor off the skin even for an instant, produces a nonsense reading.
That is why anomalies show up during exertion and almost never at rest: exertion is when things move.
Cause 1: the band is too loose
The most common. One finger should pass between strap and wrist, not two. And the forearm swells during exertion: what held during the warm-up can be floating twenty minutes later.
Cause 2: the device is on the bone
On the protruding wrist bone, the sensor lifts with every flex. The right position is two fingers above it, towards the elbow.
Cause 3: something is pressing on it
A bar, a handle, handlebars, a wrist strap. In those cases, moving the band up the forearm solves it without buying anything. See: WHOOP and weight training.
Cause 4: the band is tired
An elastic that no longer returns lets the device drift constantly, even with the clip fully tightened. The telltale symptom: data degrading gradually over several weeks with no change in habits. See: what wears a weave.
Cause 5: cold
In cold weather peripheral circulation drops and the signal weakens. A winter outing gives less stable readings than the same outing in June, with nothing faulty. A longer warm-up improves things noticeably.
The special case of weight training
Even perfectly set up, an optical sensor stays less reliable during heavy sets than during running: forearm contraction disturbs local circulation, and no adjustment cancels that. It is not a fault in your device, it is a limit of the technology.
The order to work through
Fit, position, what is pressing, age of the band. Four checks, two minutes, nothing to buy. If the data is still wrong after that, then and only then does the hardware question arise.
The band has a part in it
A weave that holds its tension keeps the sensor in place better than a loosened strap. The CoreKnit mesh is the firmest we make; Tide does the same job with more give.