Wrist or chest: which heart rate to trust
The sensor in the watch is measuring blood flow through skin that is moving. That works better than it sounds — until the effort changes fast.
They are measuring different things
A chest strap reads the electrical signal that makes your heart contract. It is the same physics as a clinical ECG, and it is effectively ground truth — the beat either fired or it did not.
An optical wrist sensor shines light into the skin and measures how much comes back. Blood absorbs it, so the reflected signal rises and falls with each pulse. The watch then infers a rate from a waveform it is reading through moving tissue, in changing light, on a wrist that is swinging.
When to trust which
| Session | Wrist | Chest |
|---|---|---|
| Easy steady run | Fine | Fine |
| Long run | Fine | Fine |
| Intervals and hills | Lags 10–30 s | Accurate |
| Cold weather | Often poor | Accurate |
| Strength work, grip on bars | Unreliable | Accurate |
If most of your running is easy — and for most people it should be — the wrist sensor is genuinely enough. If you train by heart-rate zones on short intervals, the lag is not a small error: by the time the watch catches up the rep is over.
Getting the best out of the wrist sensor
Move it up your arm
A finger’s width above the wrist bone, or further for hard efforts. The bone stops the sensor making contact, and contact is the whole game.
Tighten it one notch more than feels natural
Snug enough that it does not slide when you shake your arm. A gap lets ambient light in and the reading falls apart.
Warm up your wrist in the cold
Cold skin means less blood at the surface and a weaker signal. A sleeve over the watch helps more than anything else you can do.
Keep the sensor window clean
Sunscreen and dried sweat both scatter the light. A rinse after a hot run is enough.
Every watch in the catalogue pairs with a chest strap over Bluetooth or ANT+, so this is not a buying decision — it is a per-session one.
What the number then feeds
Heart rate is the input to almost everything the watch claims to know about you: training load, recovery time, readiness, estimated threshold. A lagging reading on intervals does not just misreport that session — it feeds a wrong number into the week.
That is worth knowing before you take training readiness too seriously, and worth remembering if the watch keeps telling you an easy week was hard.
Is it worth owning a chest strap?
A decent strap costs about the price of a pair of socks relative to the watch, lasts years, and pairs with every watch in the catalogue. The question is not really cost — it is whether you will wear it.
The practical test: look at how many of your sessions in a typical month are hard intervals, hill reps, or anything where the effort changes inside a minute. If the answer is more than about four, a strap will make a visible difference to your data. If the answer is one or none, the wrist sensor is doing a fine job and a strap is something that will live in a drawer.
- Straps need damp electrodes. A dry strap on the first rep reads nonsense, which is the most common reason people give up on them.
- Wash it. Salt build-up is what makes a strap start dropping out after a year, and it is entirely preventable.
- Most straps also record while unpaired and sync afterwards, which is useful for pool sessions where the watch loses Bluetooth.
One thing worth knowing: if you are going to use a strap, set it up properly in the first week rather than months later. Pairing it once and forgetting is easy; retrofitting it to a habit is not.
Questions readers ask
- Is an armband sensor a good compromise?
- Yes — it is the same optical technology on the forearm or upper arm, where there is more tissue and less movement. In testing it sits much closer to a chest strap than a wrist sensor does, and it is more comfortable than a strap.
- Why does my heart rate read 170 during a warm-up?
- Almost always cadence lock: the sensor has lost the pulse and latched onto your footstrike instead. Check whether the number matches your steps per minute. Loosening and re-seating the watch usually fixes it.
- Does tattoo ink affect it?
- Dark ink absorbs the light the sensor relies on, and readings over a tattoo are often unusable. Moving the watch to unmarked skin or the other wrist is the fix.