We haven't seen very many fitness trackers that include tap-to-pay NFC functionality built in. The ability to double as a mobile wallet is usually reserved for pricier smartwatches like the Apple Watch and smartphones themselves. But Jawbone seems to think that its wrist-worn health trackers are the perfect vehicle for NFC payments. Today, the company is announcing Up4, an upgraded version of the Up3 (which is finally, only now about to start shipping) that adds an NFC chip necessary to pay on the go. It'll ship this summer for $199 versus the $179 you'd pay for the Up3.

An American Express card on your wrist

The thinking behind this seems to be that since runners and other athletes often try to carry as little around with them as possible, it shouldn't take much to convince them to ditch their wallet. But Jawbone's entry into the mobile payments business has one big limitation: only American Express cardholders can use it. The company has partnered closely with AmEx on the new feature. Existing members will be able to use their regular account credentials to link their card with the Up4, but everyone else will be able to apply for an American Express card and (potentially) receive instant approval. But it's not exactly seamless; after registering your card with Jawbone's app, you'll have to jump over to the AmEx app to see recent transaction history and other account details. American Express' Leslie Berland told The Verge this split is meant to protect sensitive customer data.

And both companies say they're taking security seriously; like Apple Pay, AmEx's system uses tokenization to hide your true credit card number from merchants and replace it with a different one dedicated to mobile payments. And while it's theoretically as fast, Jawbone's system doesn't sound nearly as elegant as Apple's. Since there's no screen or any kind of speaker on Up4, you'll be relying solely on a store's payment terminal to verify that the NFC transaction went through.

The ability to pay with a tap is reserved exclusively for proper American Express credit card holders, with no prepaid alternative available for people with a spotty credit record. Meanwhile, Apple is regularly adding to the already sizable list of banks and credit card companies behind Apple Pay. Jawbone is unlikely to match that anytime soon, but if you're an American Express holder and you really want to buy things with your wrist, you'll soon be able to do it for a couple hundred bucks — and get what sounds like a pretty decent health tracker as part of the deal.