What is a Whoop, the bracelet that Carlos Alcaraz was forced to remove? Why is it prohibited?

Before his match against Tommy Paul in the fourth round of the Australian Open, Carlos Alcaraz was forced to remove a device he was wearing on his right wrist, underneath his wristband. The request came from the chair umpire, Marija Cicak, and Alcaraz complied without protest.

It turned out to be a Whoop smart health and fitness band – a screenless device that tracks recovery, physical exertion and sleep, among other things. Through a subscription service, it also analyzes metrics such as heart rate and blood oxygen levels. It is widely used by professional athletes.

So why was Alcaraz not allowed to play with it? Because professional tennis restricts devices that enable data transmission off court or external communication, in order to prevent coaching violations and mitigate betting-related risks. What is not banned, for example, is playing with a watch that simply tells the time or does not connect to other devices such as phones, tablets or computers – like the one Rafael Nadal famously used to wear.

The curious part is that Alcaraz had competed with the band on in his three previous games, against Adam Walton, Yannick Hanfmann and Corentin Moutet.

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