[Research Review] Underwater Heart Rate Error Analysis: Comparison of Optical Green Light Sensors and Chest Strap Heart Rate Monitors in Swimming — Report: Physiological Characteristics of Elite Athletes (Article 1197)

Reference 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, there are three main sources of error:

• Water seepage interference: Water seeps through 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 swimming stroke 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: In colder water, peripheral blood vessels constrict, reducing blood flow to 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 detect the heart’s electrical signals directly via electrodes (similar to a simplified ECG), and are unaffected by changes in superficial skin blood flow, theoretically offering higher accuracy and serving as the “gold standard” comparison 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 contact to loosen due to skin perspiration/water ingress, and some chest strap products have insufficient waterproof 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 precise heart rate zone training is needed (e.g., interval workouts), it is recommended to use a chest strap heart rate monitor with a 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, perceived exertion scales (RPE) can be used 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 interfered with by water flow and wrist muscle movement, rather than actual dramatic fluctuations in heart rate. It is recommended to interpret the 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 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 capability should be chosen).

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 signal noise processing in optical heart rate sensing.

Further Reading

• Research Review: Underwater Heart Rate Error Analysis: Comparison of Optical Green Light Sensors and Chest Strap Heart Rate Monitors in Swimming — Report: Physiological Characteristics of Elite Athletes (Article 1209)
• Research Review: Underwater Heart Rate Error Analysis: Comparison of Optical Green Light Sensors and Chest Strap Heart Rate Monitors in Swimming — Report: Physiological Characteristics of Elite Athletes (Article 693)
• Research Review: Underwater Heart Rate Error Analysis: Comparison of Optical Green Light Sensors and Chest Strap Heart Rate Monitors in Swimming — Report: Physiological Characteristics of Elite Athletes (Article 1215)
• Research Review: Underwater Heart Rate Error Analysis: Comparison of Optical Green Light Sensors and Chest Strap Heart Rate Monitors in Swimming — Report: Physiological Characteristics of Elite Athletes (Article 372)