Apple Watch is one of the best heart rate monitors you can get for the price — and it's already on your wrist. Here's how it measures your heart rate, how it handles zone tracking, and how to get the most out of it with Underdog.
How Apple Watch measures heart rate
Apple Watch uses photoplethysmography (PPG) — a green LED light on the back of the watch that shines into your skin and measures light reflection. When your heart beats, blood flow increases in the vessels of your wrist, changing the amount of light absorbed. The watch's optical sensor detects these fluctuations and calculates your heart rate.
During workouts, Apple Watch measures heart rate every second. At rest, it measures periodically in the background. All of this data flows into Apple Health, where it's stored as timestamped heart rate readings.
How accurate is Apple Watch heart rate?
Multiple independent studies have found Apple Watch heart rate accuracy to be within 2–5% during moderate-intensity continuous exercise (like running or cycling) compared to chest strap ECG monitors. Accuracy decreases during high-intensity interval training or activities with significant wrist movement (like weight lifting), where movement artifacts can interfere with the optical sensor.
For Zone 2 and Zone 3 training — the ranges most relevant to aerobic base building — Apple Watch is accurate enough for practical zone training.
Does Apple Watch have built-in heart rate zones?
Apple Watch and the Fitness app show a heart rate summary after each workout, including a colour-coded zone breakdown. However, the native zones are fixed percentages and can't be customised easily. They're also tied to individual workout sessions — there's no weekly aggregate, no points system, and no competitive element.
Underdog reads the same underlying heart rate data from Apple Health but applies custom zone calculations and converts your time-in-zone into a competitive weekly score.
Setting up Underdog with Apple Watch
- Download Underdog from the App Store
- On first launch, grant Underdog access to Apple Health (heart rate and workouts)
- Wear your Apple Watch during any workout as you normally would
- Underdog syncs automatically — no manual imports, no extra steps
Your zone points update after each workout syncs to Apple Health. If you complete a workout while offline, the data will sync when your watch reconnects.
Which Apple Watch works with Underdog?
Underdog works with Apple Watch Series 3 and later, paired with an iPhone running iOS 16 or later. The app reads heart rate data from Apple Health regardless of which Apple Watch model you use — as long as your watch syncs workout data to Health, Underdog can read it.
What if I don't have an Apple Watch?
Apple Watch is recommended for the best experience, but Underdog can read heart rate data from any device that syncs to Apple Health — including Polar heart rate monitors, Wahoo sensors, Garmin devices (via the Garmin Connect app), and chest straps paired via Bluetooth to iPhone.
Getting the most from Apple Watch heart rate data
A few tips to get accurate zone data from your Apple Watch:
- Wear it snugly — a loose watch produces more motion artifacts. Position it about two finger-widths above your wrist bone.
- Start workouts in the Workout app — this triggers continuous heart rate sampling. Background measurement is less frequent.
- Clean the sensor — sweat and sunscreen can interfere with the optical sensor over time.
- Use wrist heart rate, not averaged data — Underdog reads the raw timestamped heart rate data from Apple Health, so every second of your workout zone time is captured accurately.