Two Ways to Measure Your Heart Rate
At the gym, in the pool, or out on a trail, your heart rate is one of the most valuable pieces of training data you can collect. But there are two fundamentally different technologies used to measure it — and they're not equally suited to every situation.
How Optical Wrist Sensors Work (PPG)
Wrist-based heart rate monitors use photoplethysmography (PPG): green LED lights shine through your skin and a photodetector measures how much light is absorbed. Blood absorbs more light than surrounding tissue, and since blood volume in your capillaries pulses with each heartbeat, the sensor can calculate your heart rate from those variations in light absorption.
Advantages:
- Always with you — no extra equipment needed
- Comfortable for all-day and sleep tracking
- Enables continuous background monitoring
- Works for swimmers (waterproof designs available)
Limitations:
- Motion artifacts during high-intensity exercise can cause lag or inaccuracy
- Tattoos, darker skin tones, and cold temperatures can affect signal quality
- Slower to respond to sudden heart rate changes (e.g., during sprint intervals)
How Chest Strap Monitors Work (ECG)
Chest straps use electrocardiography (ECG) — the same fundamental technology as clinical heart monitors. Electrodes on the strap detect the electrical signals produced by the heart with each beat. This is the gold standard for heart rate accuracy.
Advantages:
- Near-clinical accuracy across all intensities
- Instantaneous response to heart rate changes
- Excellent for high-intensity intervals and threshold work
- Provides heart rate variability (HRV) data for recovery tracking
Limitations:
- Requires putting on an extra piece of equipment before every workout
- Can be uncomfortable during long sessions if the strap shifts
- Needs to be moistened for good electrode contact
- Not suitable for swimming (most models)
Side-by-Side Comparison
| Factor | Optical Wrist Sensor | Chest Strap (ECG) |
|---|---|---|
| Accuracy (steady state) | Good | Excellent |
| Accuracy (high intensity) | Moderate | Excellent |
| Response time | 3–5 second lag | Near-instant |
| Comfort | Very comfortable | Moderate |
| All-day monitoring | Yes | No |
| Sleep tracking | Yes | No |
| Swimming compatible | Often (with 5 ATM rating) | Rarely |
| HRV accuracy | Moderate | High |
When to Use Each Type
Choose a Wrist Optical Sensor When:
- You want 24/7 passive health monitoring
- Your workouts are moderate intensity (Zone 1–3)
- You're a swimmer needing underwater heart rate
- Convenience matters more than clinical precision
Choose a Chest Strap When:
- You're doing HIIT, sprint intervals, or high-intensity cycling
- You're using heart rate zones for precise training control
- You want reliable HRV data for recovery monitoring
- You're a competitive athlete where accuracy is non-negotiable
The Best of Both Worlds
Many athletes use both: a wrist-based tracker for everyday health monitoring and sleep, and a chest strap for structured training sessions. Most modern fitness watches can pair with third-party chest straps via ANT+ or Bluetooth, giving you the accuracy of ECG combined with the data ecosystem of your preferred app.
Ultimately, the "best" heart rate monitor is the one that matches your training style, intensity level, and tolerance for wearing extra gear.