This is a clever way to make sure that you’re only competing against yourself. Instead of measuring steps and calculating calories, the waterproof MZ-3 chest strap measures your heart rate as you work out. So it creates a level playing field by rewarding effort rather than fitness levels. In some health clubs (ask at yours before you buy), your effort data appears onscreen as you work out, and is also streamed to your mobile phone either while you exercise or afterwards, if the phone is absent. The battery lasts months between charges, and it's compatible with a range of Bluetooth devices and existing fitness apps.

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