[Research Review] Underwater Heart Rate Error Analysis: A Comparative Report of Optical Green-Light Sensors and Chest Strap Heart Rate Monitors in Swimming: Latest Academic Literature Review and Training Practice (No. 225)

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 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
• Repeated 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 to 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 cause electrode loosening due to skin sweating/water ingress, and some chest strap products lack sufficient waterproof ratings to withstand prolonged submersion.

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

Core Research Conclusions and Practical Recommendations

• Chest strap is the first choice for high-intensity training: If precise heart rate zone training is needed (e.g., interval workouts), a chest strap heart rate monitor with sufficient waterproof rating is recommended
• Optical watches as trend reference: Despite the errors in optical sensing data, 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, confirming 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 the optical signal being interfered with 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-point values.

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 will lose signal underwater; models with underwater memory storage functionality should be chosen).

References and Academic Citations

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

Further Reading

• Research Review: Underwater Heart Rate Error Analysis: A Comparative Report of Optical Green-Light Sensors and Chest Strap Heart Rate Monitors in Swimming: Latest Academic Literature Review and Training Practice (No. 765)
• Research Review: Underwater Heart Rate Error Analysis: A Comparative Report of Optical Green-Light Sensors and Chest Strap Heart Rate Monitors in Swimming: Latest Academic Literature Review and Training Practice (No. 930)
• Research Review: Underwater Heart Rate Error Analysis: A Comparative Report of Optical Green-Light Sensors and Chest Strap Heart Rate Monitors in Swimming: Latest Academic Literature Review and Training Practice (No. 738)
• Research Review: Underwater Heart Rate Error Analysis: A Comparative Report of Optical Green-Light Sensors and Chest Strap Heart Rate Monitors in Swimming: Frontiers in Exercise Physiology Research Progress (No. 1128)