My new trainer came in a nondescript black box. “WHOOP/4.0/ Unlock Yourself” was written on it in white letters. The content also looked very unspectacular at first glance: a small rectangle with a green glowing sensor and a narrow fabric wristband attached to it. As you may have guessed, this isn’t a real trainer, it’s a fitness tracker. And one that promises to be able to use the measured data to calculate how much you should train – and when it’s better not.
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