[Research Review] Underwater Heart Rate Error Analysis: Comparison of Optical Green-Light Sensors and Chest Strap Heart Rate Monitors in Swimming — Advances in Sports Physiology Research (Article 909)

Reference Journal Source: Journal of Sports Sciences • International Scientific 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 seepage interference: Water seeping between the strap and the skin alters the contact between the sensor and the skin, disrupting the stability of the optical signal
• Repetitive wrist muscle contraction: The pulling motion repeatedly compresses the wrist muscles and subcutaneous tissue, making the optical signal susceptible to contamination from muscle blood flow signals, which are difficult to distinguish from heartbeat signals
• Cold-induced vasoconstriction: When water temperature is low, 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 superficial skin blood flow. In theory, they offer higher accuracy and serve as the “gold standard” 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 loosening due to skin sweating/water ingress, and some chest strap products do not have sufficient water resistance ratings to withstand prolonged submersion.

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

Core Research Conclusions and Practical Recommendations

• Choose chest straps for high-intensity training: If you need precise heart rate zone training (such as interval workouts), it is recommended to use a chest strap heart rate monitor with sufficient water resistance
• 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 completely abandon it due to inaccuracies
• 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 errors
• 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 the data using trend lines rather than individual 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 water resistance rating suitable for underwater use and the signal transmission method (some chest straps using Bluetooth transmission will lose signal underwater; you need to choose models with onboard memory storage functionality).

References and Academic Citations

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

Related Reading

• Research Review: Underwater Heart Rate Error Analysis: Comparison of Optical Green-Light Sensors and Chest Strap Heart Rate Monitors in Swimming — Advances in Sports Physiology Research (Article 309)
• Research Review: Underwater Heart Rate Error Analysis: Comparison of Optical Green-Light Sensors and Chest Strap Heart Rate Monitors in Swimming — Advances in Sports Physiology Research (Article 945)
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