[Research Review] Underwater Heart Rate Error Analysis: Comparison of Optical Green Light Sensors and Chest Strap Heart Rate Monitors in Swimming — Frontiers in Exercise Physiology Research Progress (Article 1374)

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 differences between 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 seeps through the gap between the strap and skin, altering the contact state between the sensor and skin, disrupting 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, which are difficult to distinguish from heartbeat signals
• Cold-induced vasoconstriction: When water temperature is low, peripheral blood vessels constrict, reducing blood flow in the superficial layers of the skin, which weakens 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” comparison 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 contact to loosen due to skin sweating/water ingress, and some chest strap products have insufficient waterproof ratings to withstand prolonged submersion.

Error Magnitude Comparison Between the Two Sensing Methods (Illustrative)

Core Research Conclusions and Practical Recommendations

• Choose a chest strap 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 waterproof rating
• 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 errors
• 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 wildly and 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 fluctuations in heart rate. It is recommended to interpret data using trend lines rather than single-point values.

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

A: No. You need to confirm that the product explicitly states a waterproof rating suitable for water use and check the signal transmission method (some chest straps using Bluetooth transmission will lose signal underwater; you need to choose models with underwater memory storage capability).

References and Academic Citations

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

Related Reading

• Research Review: Underwater Heart Rate Error Analysis: Comparison of Optical Green Light Sensors and Chest Strap Heart Rate Monitors in Swimming — Frontiers in Exercise Physiology Research Progress (Article 1065)
• Research Review: Underwater Heart Rate Error Analysis: Comparison of Optical Green Light Sensors and Chest Strap Heart Rate Monitors in Swimming — Frontiers in Exercise Physiology Research Progress (Article 393)
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