If you've already got a smartwatch you like, here's what I'd get in the Prime Day sales — the Polar H10, the 'gold standard' electrical heart rate monitor

Every smartwatch and fitness tracker comes with heart rate tracking, but they're not all accurate, so we test them against a Polar H10 chest-mounted heart rate monitor. The Polar H10 is used by athletes all around the world, and can be linked to a smartwatch to provide incredibly accurate metrics using its electrode system. Now it's on offer in the Amazon Prime Day sales, and if you care about fitness tracking, you should probably get one.

• Browse the full Amazon Prime Day sale

The Polar H10 works with a strap that goes around your ribcage underneath your heart, and connects the device to the electrodes running along the underside of your band. These electrodes monitor the electricity generated by your heart when it expands and contracts.

This is very different to (and more reliable than) the way smartwatches and the best fitness trackersmeasure your heart rate on...

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