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Angela Harrigan, a personal trainer at Life Time Fitness, sees each client maybe two hours per week, but she knows exactly how much of the other 166 hours they’ve spent furthering — or undoing — their efforts.

With a glance at her phone, Ms. Harrigan can tell, among other things, who stayed up too late, who has hit the gym — and whose only steps have been to the office vending machine. That’s because like all trainers at Life Time, which has gyms in 24 states, Ms. Harrigan encourages clients to buy a wearable activity tracker (the gym began selling Fitbits in 2012), then accept her friend request, which allows her to monitor every move.

“She’ll text me and say, ‘So-and-so is ahead of you in steps; what’s up?’ ” said Zhanna Muchnik, who trains with Ms. Harrigan in Commerce, Mich., a Detroit suburb. “And if she can’t see what I’m doing, she’ll text me to sync it.”

Ms. Harrigan said, “I know if they’re keeping me from seeing the data, they’re probably up to no good.”

It’s the gym as Big Brother. Besides Life Time Fitness, which now employs a “national program manager of devices,” Equinox made its first appearance at the South by Southwest festival in Austin, Tex., teaming up with the popular Up by Jawbone and presenting a panel on how to use all the data (“data is only as good as the person interpreting it,” said a slick video advertising the event). The chain recently began selling the Up24, the newer model, in its shops, and it is testing the device with personal and small-group training clients. Crunch encourages clients to buy the fittingly named Exerspy, while Sports Club/LA and Reebok Sports Club/NY sell the Polar Loop.

Other gym chains are not far behind. Anytime Fitness, for example, started testing the Fitbit versus the Up in March to decide which to recommend to franchises at the annual meeting in September. Eventually the chain, which has more than 2,400 clubs, will bundle the devices with training or membership and charge for “virtual coaching,” said Chuck Runyon, the company’s chief executive.

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