[Research Digest] Analysis of Underwater Heart Rate Error: Comparing Optical Green-Light Sensors and Chest-Strap Monitors in Swimming: Quantitative Biomechanics Experimental Report (Episode 414)

This article examines sources of error in underwater heart rate measurement, comparing the accuracy of optical green-light sensors (PPG) against chest-strap heart rate monitors (ECG-based) in swimming.

Sources of Error in Optical Heart Rate Sensing

Optical heart rate sensors (used by most sports watches) shine green LED light onto the skin and infer heart rate from changes in reflected light caused by blood flow. In a swimming context, there are three main sources of error:

• Water infiltration: water seeping between the strap and skin changes how the sensor sits against the skin, disrupting the stability of the optical signal
• Repeated wrist muscle contraction: the stroking motion causes repeated compression of wrist muscles and subcutaneous tissue, easily contaminating the optical signal with muscle blood-flow noise that is hard to distinguish from the heartbeat signal
• Cold-induced vasoconstriction: in colder water, peripheral blood vessels constrict, reducing surface skin blood flow, which weakens the optical signal strength itself

The Relative Advantages and Limitations of Chest-Strap Monitors

Chest straps detect the heart’s electrical signal directly via electrodes (similar to a simplified ECG), unaffected by changes in surface skin blood flow, and are theoretically more accurate — they are also the “gold standard” comparison baseline used in most sports science research. However, chest straps still have limitations in swimming: they require good contact to maintain a stable signal, and prolonged swimming may loosen electrode contact due to sweating or water ingress; some chest strap products also lack sufficient waterproof rating to withstand prolonged submersion.

Illustrative Comparison of Error Magnitude Between the Two Sensing Methods

Core Research Findings and Practical Recommendations

• Chest straps for high-intensity training: if precise heart-rate-zone training is needed (such as interval sessions), use a chest strap with an adequate waterproof rating
• Optical watches as trend references: despite their error, optical sensing data can still serve as a reference for long-term training trends and rough intensity distribution — there’s no need to discard it entirely due to error
• Confirm device fit: regardless of sensing method, confirming that the device fits snugly but not too tightly before training is the most direct and effective way to reduce error
• Combine with perceived exertion: when underwater heart rate data is unstable, use a rate of perceived exertion (RPE) scale as a supplementary basis for pace adjustment

Frequently Asked Questions (FAQ)

Q: Why does my watch’s heart rate reading jump around so erratically while swimming?

A: This is mostly noise caused by water flow and wrist muscle movement disrupting the optical signal, rather than genuine drastic heart rate fluctuation — it’s best interpreted via trend lines rather than single data points.

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

A: No — you need to confirm the product is explicitly rated for underwater use, with an appropriate waterproof rating and signal transmission method (some chest straps use Bluetooth, which disconnects underwater, so a model with underwater memory storage is needed).

References and Academic Citations

1. Journal of Sports Sciences — research direction on comparing the accuracy of wearable device heart rate measurement.
2. IEEE Journal of Biomedical and Health Informatics — literature on noise processing for optical heart rate sensing signals.