GPS Watch Optical Heart Rate vs. Heart Rate Strap: Which Is More Accurate? A Practical Guide for Taiwanese Runners

Introduction

Heart rate is one of the most important physiological metrics in road running training. Almost all GPS watches come with built-in optical heart rate sensors, claiming to “monitor heart rate accurately.” But during actual training, have you ever noticed the heart rate displayed on your watch suddenly spiking or dropping abnormally during interval runs? How accurate is optical heart rate, really, and when should you switch to a heart rate strap? This article walks you through the differences between the two technologies.

Measurement Principles: Why Can Optical Heart Rate Be Inaccurate?

How optical heart rate works (PPG, Photoplethysmography):
The LED light on the bottom of the watch (usually green light) shines onto the skin, and the sensor captures changes in light reflection as blood flows through the vessels to calculate heart rate. The problems with this method are:

• Motion Artifact: When running, arm swing and skin vibration on the wrist can cause unstable LED readings
• Skin Tone Effects: Darker skin has a lower reflectance for certain LED wavelengths, which may affect accuracy
• Wearing Position: Wearing the watch too loose or too close to the wrist bone reduces accuracy
• Temperature Effects: In cold environments, blood vessels in the hands constrict, weakening the optical signal

Heart rate strap (electrode-based ECG measurement):
The chest strap uses two electrodes to detect the heart’s electrical signals (just like an ECG). It is almost unaffected by motion artifacts, and its accuracy approaches medical-grade levels.

Accuracy Comparison Across Optical Heart Rate Brands

Based on multiple academic studies and runner community tests (using the Polar H10 chest strap as the gold standard):

Note: The values above are estimated ranges under typical conditions; individual variation can be significant

Scenarios Where Optical Heart Rate Performs Worst

Optical heart rate is particularly unreliable in the following situations:

1. High-intensity interval runs: When heart rate rises or falls rapidly, the optical sensor lags behind, often with a 10–30 second delay
2. Running in cold weather: In Taiwan’s mountainous areas during winter, readings can be severely off when hands are cold
3. Sprint segments: During the final 1 km sprint, vigorous arm swinging creates motion artifacts
4. Gripping treadmill handles: The fixed wrist position is unnatural and affects optical readings
5. Wearing the watch over the wrist bone: The sensor is not in the correct position

Heart Rate Strap Selection Guide

Recommended heart rate straps:

• Polar H10: Universally recognized as the gold standard. Supports Bluetooth + ANT+, can connect to multiple devices, and simultaneously records heart rate strap data to your phone (suitable for HRV measurement with the Elite HRV app). Priced around NT$2,800–3,200 in Taiwan
• Garmin HRM-Pro Plus: The best choice for Garmin users. Records running dynamics and automatically stores data when the watch loses signal. Priced around NT$4,500–5,000
• Wahoo TICKR: Lightweight and affordable (around NT$1,600–2,000), suitable for runners on a budget

How to wear a heart rate strap correctly:

• Position: Place the strap just below the sternum (directly above the xiphoid process), centered left-to-right
• Tightness: It should just barely not slip when you inhale, and not feel too constricting when you exhale
• Conductivity: Wet the electrode pads before running; in winter, apply a small amount of conductive gel

Practical Recommendations

• Daily easy runs: Optical heart rate is sufficient; no need to add the hassle of wearing a heart rate strap
• Lactate threshold runs and interval runs: Strongly recommend using a heart rate strap to ensure accurate heart rate zone control
• Marathon races: A heart rate strap is recommended. Race-day excitement can cause slight trembling in the wrist, affecting optical heart rate readings
• Improving optical heart rate accuracy: Tighten the watch band and wear it 2–3 cm above the wrist bone (on the forearm) to significantly reduce motion artifacts
• Double-checking optical heart rate: During easy runs, compare the watch against a heart rate strap. If the difference exceeds 10 bpm, you’ll know at which heart rate value your watch starts becoming inaccurate

Conclusion

The convenience of optical heart rate makes it a great companion for daily training, but for intensity workouts and races that require precise heart rate control, the heart rate strap remains an irreplaceable choice. Understanding the limitations of each and choosing the right tool based on your training purpose is the key to making heart rate data truly serve your training goals.

Related Reading

• Running Heart Rate Monitoring: An In-Depth Accuracy Comparison Between Heart Rate Straps and Optical Heart Rate
• Running Heart Rate Straps vs. Optical Heart Rate: Choosing Between Accuracy and Usage Scenarios
• Running Heart Rate Straps vs. Optical Heart Rate: Comparing Measurement Accuracy and Wearing Comfort
• Road Running Heart Rate Straps vs. Optical Heart Rate Watches: Accuracy Differences and Best-Use Case Analysis