[Research Review] Underwater Heart Rate Error Analysis: A Comparison of Optical Green-Light Sensors and Chest Strap Heart Rate Monitors in Swimming: Latest Academic Literature Review and Training Practice (Article 243)

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

• Water intrusion interference: Water seeps into the gap between the strap and skin, altering the contact between the sensor and skin and disrupting the stability of the optical signal
• Repetitive wrist muscle contraction: The pulling motion causes repeated compression of 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 strength 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 scenarios: 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 extended submersion.

Comparison of Error Magnitude 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 abandon it entirely 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 watch heart rate often fluctuate erratically during swimming?

A: This is mostly noise caused by optical signals being interfered with by water flow and wrist muscle movement, rather than actual dramatic heart rate swings. It is recommended to interpret the data using trend lines rather than point values.

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

A: No. You need to verify that the product explicitly indicates 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 capability).

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 noise processing of optical heart rate sensing signals.

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