[Research Review] Underwater Heart Rate Error Analysis: Comparison of Optical Green-Light Sensors and Chest Strap Heart Rate Monitors in Swimming — Report: Physiological Characteristics of Elite Athletes (Article 129)

Reference Journal Source: Journal of Sports Sciences • International Research Findings Review Series

This article explores the sources of error in underwater heart rate measurement, comparing the accuracy of optical green-light sensors (PPG) and chest strap heart rate monitors (ECG-based) in swimming environments.

Sources of Error in Optical Heart Rate Sensing

Optical heart rate sensors (used by most sports watches) estimate heart rate by shining green LED light onto the skin and detecting changes in reflected light from blood flow. In swimming scenarios, there are three main sources of error:

• Water intrusion interference: Water seeps through the gap between the strap and skin, altering the contact between the sensor and skin and compromising the stability of the optical signal
• Repeated wrist muscle contraction: The pulling motion causes repeated compression of wrist muscles and subcutaneous tissue, making the optical signal susceptible to contamination from muscle blood flow signals that are difficult to distinguish from heartbeat signals
• Cold-induced vasoconstriction: In colder water, peripheral blood vessels constrict, reducing blood flow to the skin surface and weakening the optical signal intensity itself

Relative Advantages and Limitations of Chest Strap Heart Rate Monitors

Chest straps use electrodes to directly detect the heart’s electrical signals (similar to a simplified ECG), and are unaffected by changes in skin surface blood flow. Theoretically, they offer higher accuracy and serve as the “gold standard” comparison reference used in most sports science research. However, chest straps still have limitations in swimming: they require good contact to maintain signal stability, prolonged swimming may cause electrode contact to loosen due to skin sweating/water ingress, and some chest strap products have insufficient waterproof ratings to withstand prolonged submersion.

Comparison of Error Magnitudes Between the Two Sensing Methods (Illustrative)

Key Research Conclusions and Practical Recommendations

• Choose a chest strap for high-intensity training: If you need precise heart rate zone training (e.g., interval workouts), a chest strap heart rate monitor with adequate waterproof rating is recommended
• Optical watches as trend reference: Although optical sensing data contains errors, it can still serve as a reference for long-term training trends and overall intensity distribution — there is no need to abandon it entirely due to inaccuracy
• Verify device fit: Regardless of the sensing method, checking that the device fits snugly but comfortably before training is the most direct and effective way to reduce error
• Combine with perceived exertion: When underwater heart rate data is unstable, use the Rating of Perceived Exertion (RPE) scale as a supplementary aid for pace adjustment

Common Research Q&A (FAQ)

Q: Why does the watch heart rate often fluctuate erratically during swimming?

A: This is mostly noise caused by optical signals being disrupted by water flow and wrist muscle movement, rather than actual dramatic heart rate fluctuations. It is recommended to interpret using trend lines rather than single data points.

Q: Are all chest strap heart rate monitors suitable for swimming?

A: No. You need to confirm that the product explicitly specifies a waterproof rating suitable for water use and the signal transmission method (some chest straps using Bluetooth transmission lose signal underwater; models with onboard memory storage for underwater use should be selected).

References and Academic Citations

1. Journal of Sports Sciences — Research related to accuracy comparisons of wearable device heart rate measurement.
2. IEEE Journal of Biomedical and Health Informatics — Literature related to noise processing of optical heart rate sensing signals.

Further Reading

• 【Research Review】Underwater Heart Rate Error Analysis: Comparison of Optical Green-Light Sensors and Chest Strap Heart Rate Monitors in Swimming — Report: Physiological Characteristics of Elite Athletes (Article 672)
• 【Research Review】Underwater Heart Rate Error Analysis: Comparison of Optical Green-Light Sensors and Chest Strap Heart Rate Monitors in Swimming — Report: Physiological Characteristics of Elite Athletes (Article 678)
• 【Research Review】Underwater Heart Rate Error Analysis: Comparison of Optical Green-Light Sensors and Chest Strap Heart Rate Monitors in Swimming — Report: Physiological Characteristics of Elite Athletes (Article 774)
• 【Research Review】Underwater Heart Rate Error Analysis: Comparison of Optical Green-Light Sensors and Chest Strap Heart Rate Monitors in Swimming — Report: Exploring the Relationship Between Clinical Medicine and Athletic Performance (Article 741)