Watch on the wrist and a chest strap paired to the same watch, recorded both for a fortnight. On steady runs they agree within a beat or two. On 400s and on anything in the gym they diverge wildly, and the wrist number climbs slowly and then sticks around a number that looks suspiciously like my cadence. I would like to stop owning two things if one of them is enough.
Small correction to the strap is truth framing. Chest straps produce their own garbage, particularly in the first five minutes of a winter run on dry skin, where you get an implausible spike into the 180s while your legs are still cold. Wet the electrodes before you put it on. A strap that is two years old with a stretched band gives you the same spikes even wet, and the fix is a replacement band rather than a replacement strap.
The dry skin spike is real and it is why people conclude their strap is broken in November. Also worth washing the strap regularly, salt and detergent residue both mess with conductivity.
You have already diagnosed it. The number that looks like your cadence is cadence lock, which is what optical sensors do when the blood flow signal gets noisy and the strongest remaining periodic signal in the data is your arm swinging.
Optical wrist sensing is genuinely decent at steady state and genuinely poor at sharp transitions, which is exactly what an interval session is made of. It lags going up and lags coming down, so your recoveries look shorter and your peaks look lower.
Keep both. Strap for anything with hard efforts in it, wrist for daily resting trends and sleep, where it is fine and the strap is unwearable.
Cadence lock has a name. Somehow that makes me feel better about the fortnight I spent wondering if my heart was broken.
The lag direction matters for training too. If you are running by heart rate zones, the wrist under-reads at the start of an effort, so you push harder to hit the number and end up above the zone you intended.
I trained by wrist heart rate zones for about six months and got slower. Every easy run was actually moderate, because the watch under-read at the start and I sped up to reach the zone, and I never recovered properly between sessions. Bought a strap, discovered my easy pace should have been over a minute per kilometre slower, and the next block was the best I have had. If you are doing any kind of polarised plan, the strap is not optional.
Before you write off the wrist entirely, fix the fit, because most people wear it wrong. Snug enough that it does not slide, a finger's width above the wrist bone rather than on it, and higher up the arm during hard efforts. Cold weather makes it much worse because peripheral blood flow drops. Wrist tattoos in the sensor area can defeat it completely. Mine went from useless to acceptable on easy runs purely by moving it two centimetres up my arm.
For lifting, wrist heart rate is decoration. Do not train around it, do not read anything into the calorie number it produces, and use rate of perceived exertion and the bar. Nothing worn on the wrist while gripping a bar is measuring what you want.
@proof_pilar · 6mo ago
Ran the comparison properly because I wanted to know rather than argue. Fourteen sessions, both sensors recording to separate files.
Easy runs: average heart rate matched within about two beats every time. Genuinely no reason to wear the strap.
Intervals: averages were within a handful of beats but the peaks differed by fifteen to twenty five, and the shape of the curve was completely different, rounded on the wrist versus square on the strap.
Lifting: not comparable. The wrist trace looked like a random walk.
So the honest summary is that the wrist is fine for the metric most people actually use, which is average and trend, and unusable for the metric interval runners care about.
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