Optical wrist sensors are good at steady efforts and unreliable at exactly the moments training decisions depend on.
Verdict
If your training is driven by heart rate zones, use a chest strap. Electrode-based straps measure the heart's electrical signal directly and track rapid changes accurately; optical wrist sensors infer rate from blood flow and are known to lag during intervals, drift in the cold and misread on rapid changes. For steady running and for general daily tracking, the wrist is fine.
The two technologies are not doing the same thing. A chest strap is a simplified electrocardiogram: it reads the electrical activity that triggers each beat. An optical wrist sensor shines light into the skin and infers a rate from changes in blood volume, which is a harder inference and one that is disturbed by movement, skin tone, temperature, sweat, tattoos and how tightly the watch is worn.
Validation studies consistently find good agreement at steady state and worse agreement during rapid changes in intensity, which is exactly where interval training lives.
~$199.00
Check Backcountry → Check Garmin Forerunner 165 Price| Polar H10 chest strap | Garmin Forerunner 165 | |
|---|---|---|
| Measures | Electrical signal | Blood flow, optically |
| Accuracy at steady effort | Excellent | Good |
| Accuracy during intervals | Excellent | Often lags or misreads |
| Cold weather | Reliable | Degrades |
| Comfort | Strap around the chest | Nothing extra to wear |
| Also gives pace and distance | No | Yes |
You train by heart rate zones, do intervals, or have ever looked at a graph and thought the numbers made no sense. The H10 is the reference device in a lot of published validation work, it pairs with a watch over both Bluetooth and ANT+, and it removes an entire category of doubt from your data. The cost is remembering to wear it and wetting the electrodes, which is the step people skip and then blame the strap for.
Check Backcountry → Check Polar H10 chest strap on Amazon
You run mostly at steady efforts, you want one device, or heart rate is background information rather than what your training is built on. A modern optical sensor is genuinely good at continuous steady output and at resting and sleeping rates. Wear the watch a finger's width above the wrist bone and snug enough not to slide, which is the single biggest improvement most people can make to optical accuracy.
Check Backcountry → Check Garmin Forerunner 165 on Amazon
Heart rate is affected by heat, dehydration, caffeine, illness, sleep and stress, so a session that feels harder than the numbers suggest usually reflects something real rather than a broken sensor. Cardiac drift means rate rises at a constant effort over a long run. If you experience chest pain, unexplained breathlessness, fainting, or a heart rate that is wildly out of line with your effort and stays there, stop and speak to a doctor rather than adjusting your zones.
At steady effort, generally yes. During intervals, rapid accelerations and cold weather they are noticeably less reliable, with lag and occasional misreading of cadence as heart rate. Validation studies consistently show this pattern. For zone-based training, a chest strap removes the doubt.
Common causes are a loose strap, wearing the watch too close to the wrist bone, cold hands reducing blood flow at the skin, sweat under the sensor and the sensor locking onto your cadence instead of your pulse. Tightening the band and moving it higher up the arm fixes most of it.
It helps, because zone 2 work depends on staying inside a fairly narrow band and an optical sensor that reads ten beats high will have you running the wrong session. If you are serious about aerobic base work, a strap is one of the cheapest meaningful upgrades you can make.
Most modern straps broadcast over Bluetooth and ANT+, which covers essentially every current running watch and most bike computers, and Bluetooth also covers phone apps. Check that your watch supports external sensors, which nearly all running-focused watches do.
Comparison based on published manufacturer specifications and the general consensus of owner reports. We have not tested these units side by side. Confirm current prices and specifications before buying. As an Amazon Associate we earn from qualifying purchases, at no extra cost to you.
We review it on its own too, in full detail: the Garmin fenix 8, whose training metrics all rest on that signal.