Accurate Heart Rate in Swimming

29.01.2021 - Harald Zumpt 

The accurate measurement of heart rate in swimming has always been a challenge. Either the transmitter belt slides down on every turn or the chemical content of the pool is so high that it kills every bug in sight including the heart rate reading. We don’t always get heart rate curves, showing each interval, like this.Transmission technologies have changed over the years with low power Bluetooth (BLE) or ANT+ being the favourites but they don’t transmit the signal under water.

With the introduction of wrist based optical heart rate (OHR), we assumed the solution had finally arrived – no transmitter belt and heart rate data that is recorded directly in the wrist device. Sadly, we could not celebrate just yet. Increased wrist tension/movement in swimming quickly showed that wrist based OHR was not very accurate and only “approximately correct” at best. Wrist based OHR is a convenient solution for sports with minimal wrist tension such as running and certainly delivers when tracking recovery during sleep using the measurement of resting heart rate and heart rate variability in features such as Nightly RechargeTM (more about that in a future post).

By moving the sensor onto the arm or even onto the temple under the goggle strap, the accuracy of the heart rate reading during swimming can be improved (it even works directly under the swim cap provided the hair was been wet). The Polar OH1 optical heart rate sensor has an internal recording option to record your heart rate data for later feedback and analysis.

Some of us however would like to check our heart rate during the session and then integrate the data with our swimming metrics from our wrist device. So how do we display our (accurate) heart rate on our device during the session considering that BLE transmission does not work under water?

OK, so this last statement is only partially true and depends on the transmission range. If you position your Polar OH1 sensor within a 2-3 finger width from your wrist device, the distance is short enough to transmit the signal under water (see image) - accurate, continuous heart rate data in the swim.

One of these heart rate measurement options during the swim will ensure accurate training load scores.