Usually it is the sensor, not your heart. In the first minutes of a run the optical sensor can pick up the rhythm of your steps instead of your pulse, especially when your skin is cold or the band is loose. The watch then shows a number close to your step rate, often 160 to 180, and drops to something believable a few minutes later.
How does the sensor read my heart rate?
Apple Watch shines green light into your wrist and measures how much comes back. Blood absorbs green light, so each heartbeat changes the signal slightly. It works well when there is plenty of blood flow under the sensor and the watch sits still against the skin. The start of a run is the moment when neither is true.
Why does it happen at the start of a run?
- Cold skin. Blood flow in your wrist is low until you warm up, so the pulse signal is weak. Apple lists skin perfusion as one of the things that affects readings.
- Arm swing. Your arm moves in time with your steps. With a weak pulse signal, that rhythm can be the strongest thing the sensor sees. Runners call this cadence lock.
- A loose band. If the watch slides, light leaks in and the signal gets noisy.
Tattoos under the sensor can also block the light.
How do I tell a false spike from a real one?
| A sensor error | A real high heart rate |
|---|---|
| Appears in the first 5 to 10 minutes, at easy effort | Builds with the effort |
| Sits close to your cadence | Does not match your cadence |
| Drops suddenly by 30 beats or more with no change in pace | Falls gradually when you slow down |
| You feel fine | You feel the effort |
How do I fix it?
- Wear the watch snug, one finger above the wrist bone. Tighten it one notch for runs.
- Warm up before you start the workout, or keep the watch under your sleeve in cold weather.
- Keep the back of the watch and your skin clean and dry.
- If it keeps happening, pair a Bluetooth chest strap with your watch. Apple suggests this when the wrist sensor cannot get a consistent reading, and the strap’s readings go into the same workout.
Does it matter?
For a single run, not much. For your numbers, yes. Heart-rate zones and training load are both built from the recorded heart rate, so ten minutes at a false 175 looks like hard work that never happened.
Kiiro calculates training load and time in zones from the heart rate saved in Apple Health, so a bad recording carries through. The fix is at the source: a better signal on the wrist.
When should I take it seriously?
If you see high readings at rest, or you feel palpitations, dizziness or chest pain, that is not a sensor question. Talk to a doctor.
Related questions
- How does Apple Watch set my heart rate zones, and can I change them?: Apple Watch gives you five heart rate zones, calculated automatically from your maximum heart rate and your health data. You can replace them with your own. Here is how, and why your zones may look different in Kiiro.
- Why is my VO2 max not updating on Apple Watch?: Apple Watch only estimates VO2 max during outdoor walks, runs and hikes on fairly flat ground, with good GPS and heart-rate signal and enough effort. Here is what to check.
