Research Review: Underwater Heart Rate Error Analysis — Comparison of Optical Green-Light Sensors and Chest Strap Heart Rate Monitors in Swimming: International Research Literature Translation and Review Report (No. 363)

Source Journal Reference: 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) project green LED light onto the skin and detect changes in reflected light from blood flow to estimate heart rate. In swimming scenarios, there are three main sources of error:

• Water seepage 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 to the skin surface and weakening 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 skin-surface blood flow. Theoretically, 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 perspiration/water ingress, and some chest strap products lack sufficient waterproof ratings to withstand extended submersion.

Error Magnitude Comparison 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), use a chest strap heart rate monitor with sufficient waterproof rating
• Optical watches as trend reference: Despite measurement errors, optical sensing data 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 signal interference from 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 verify that the product explicitly specifies a waterproof rating suitable for underwater use and the signal transmission method (some chest straps using Bluetooth transmission lose signal underwater; models with onboard memory storage for underwater use should be selected).

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 Review: Underwater Heart Rate Error Analysis — Comparison of Optical Green-Light Sensors and Chest Strap Heart Rate Monitors in Swimming: International Research Literature Translation and Review Report (No. 1182)
• Research Review: Underwater Heart Rate Error Analysis — Comparison of Optical Green-Light Sensors and Chest Strap Heart Rate Monitors in Swimming: International Research Literature Translation and Review Report (No. 195)
• Research Review: Underwater Heart Rate Error Analysis — Comparison of Optical Green-Light Sensors and Chest Strap Heart Rate Monitors in Swimming: International Research Literature Translation and Review Report (No. 960)
• Research Review: Underwater Heart Rate Error Analysis — Comparison of Optical Green-Light Sensors and Chest Strap Heart Rate Monitors in Swimming: International Research Literature Translation and Review Report (No. 453)