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[Research Briefing] Underwater Heart Rate Error Analysis: A Comparative Report on Optical Green-Light Sensors vs. Chest Strap Heart Rate Monitors in Swimming: Latest Academic Literature Review and Training Practice (No. 381)

Reference Journal Source: Journal of Sports Sciences • International Research Findings Briefing 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 LED light 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 seeping through the gap between the strap and skin alters the contact between the sensor and 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: In colder water, peripheral blood vessels constrict, reducing blood flow near the skin surface and weakening the optical signal intensity itself

Relative Advantages and Limitations of Chest Strap Heart Rate Monitors

Chest straps detect the heart’s electrical signals directly through electrodes (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 loosen electrode contact due to skin sweating/water ingress, and some chest strap products lack sufficient water resistance ratings for extended 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 precise heart rate zone training is needed (e.g., interval workouts), a chest strap heart rate monitor with sufficient water resistance 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 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, the Rating of Perceived Exertion (RPE) scale can be used 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 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 states its water resistance rating and signal transmission method suitable for underwater use (some chest straps using Bluetooth transmission will lose signal underwater; models with onboard memory storage should be chosen).

References and Academic Citations

1. Journal of Sports Sciences — Research direction 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.

Related Reading

• Research Briefing: Underwater Heart Rate Error Analysis: A Comparative Report on Optical Green-Light Sensors vs. Chest Strap Heart Rate Monitors in Swimming: Latest Academic Literature Review and Training Practice (No. 465)
• Research Briefing: Underwater Heart Rate Error Analysis: A Comparative Report on Optical Green-Light Sensors vs. Chest Strap Heart Rate Monitors in Swimming: Latest Academic Literature Review and Training Practice (No. 1044)
• Research Briefing: Underwater Heart Rate Error Analysis: A Comparative Report on Optical Green-Light Sensors vs. Chest Strap Heart Rate Monitors in Swimming: Latest Academic Literature Review and Training Practice (No. 78)
• Research Briefing: Underwater Heart Rate Error Analysis: A Comparative Report on Optical Green-Light Sensors vs. Chest Strap Heart Rate Monitors in Swimming: International Research Literature Compilation and Briefing (No. 180)