The FitStar app uses data about your body to help you keep your New Year's resolutions. Geoffrey Fowler

Living in an era of phones that monitor our location and watches that record our pulse can make us feel like we're swept along in an information tsunami. Technology should give us more control of our lives, not less.

The gym is one place where data really has the potential to work on our behalf and a new app called FitStar exemplifies where that technology is heading. The hard part about exercise is all the failure. Who wants to be that guy at the health club who can't even do one pull-up?

Introducing WSJD, the Journal's new home for tech news, analysis and product reviews.

Personal Technology columnist Geoffrey A. Fowler joined The Wall Street Journal in 2001. He has covered technology, media and marketing, U.S. politics and culture, China and the Olympics.

FitStar uses data about your body to eliminate intimidating workouts that lead to broken resolutions. The app creates customized exercise routines, presented in slick videos featuring telegenic NFL star Tony Gonzalez, and tailored to what you can honestly accomplish. Then it keeps adjusting future workouts based on how you actually perform.

Go wobbly after doing just a few push-ups? FitStar takes notice and will limit how many it asks you to do until you're ready for more.

FitStar isn't the first fitness app, but it is one of the first to replace static workout videos with exercises tailored on the fly based your age, weight, your performance with each exercise and the activity of people like you, based on demographics and performance. Its customization software also can draw on data collected by wearable fitness trackers made by Jawbone and Fitbit—devices that on their own haven't been able to motivate me to exercise.

The app is free to download and costs $5 per month to unlock all its features. It is available only on iPads and iPhones, and hasn't set a date for when it will be available on Android devices.

OK, doing push-ups while staring at your phone is a little geeky. And while FitStar isn't a perfect substitute for a non-virtual trainer, its algorithm works remarkably well. Sweating with FitStar for half an hour in the mornings helped me atone for gorging on holiday gravy and left me with more energy. I plan to keep subscribing to the app.

"It is an attainable workout," says Nick Price, a 27-year-old student from Portland, Ore., who's been tweeting about his experience with FitStar on his iPad over the past six months. Unable to even get started with other workouts, he says he's lost 50 pounds by using FitStar about four times per week and making major adjustments to his diet.

A FitStar workout begins with a tiny version of Mr. Gonzalez on the screen to introduce the first exercise. "We've been doing it since we were little kids—old-school jumping jacks working the whole body," he says.

FitStar's peppy soundtrack (or your own Abba playlist) kicks in, and you're off and jumping. Hopefully you remembered to move the coffee table out of the way.

A human trainer, of course, can help you correct your form and provides peer pressure to perform. (If you have any injuries, a trainer would modify your routines, which FitStar can't do.)

FitStar approximates the guidance of a trainer with gobs of helpful video demonstrations and tips on form—running commentary that also serves to keep you from feeling lonely. "Light on your feet," chirps tiny Tony during a round of jumping jacks.

The magic comes at the end of each assignment, when the app asks for how long you kept up and to rate whether the exercise was "Too Easy," "Just Right," or "Brutal." That data, which you enter with two clicks after each exercise, feeds FitStar's training algorithm, which then slowly amps up the intensity of your workout to meet your goals, which you select during the app's setup.

To keep you coming back, there are prizes. FitStar offers rewards for completing workouts, such as $2 coupons for's MP3 store.

Shrinking your personal trainer to pocket size has its pluses and minuses. Tiny Tony goes everywhere your phone travels. Most of FitStar's exercises use your own body weight, so you don't have to worry about equipment.

Figuring out where to put your phone during a workout is a challenge. At my gym in San Francisco, which bans cellphones, nobody complained that I was using the app, but I felt self-conscious. Hiding the phone in my shorts pocket didn't work, because I kept accidentally pausing the app. So I set the phone on the floor, against a wall. In the park, I assembled a phone stand from loose rocks. At home, I used the AirPlay feature with my Apple TV to beam the app's videos from my phone onto my TV.

A word about honesty: You shouldn't lie to FitStar. If you report a fantasy tally for how long you can hold a plank, the app will only punish you by amping up your next workout. (Right now, the wearable fitness trackers feed FitStar just data about your overall activity level and sleep habits. FitStar plans to integrate with gear that measures real-time workout data like heart-rate monitors.)

Putting so much data about your body and fitness in the hands of an app would make anyone nervous. The company pledges it won't share data with others without your permission. That could someday include an insurer that offers customers discounts for proving they workout, says FitStar CEO Mike Maser.

The magic comes when FitStar asks you to rate your performance and whether the exercise was 'Too Easy,' 'Just Right' or 'Brutal.' Geoffrey Fowler

The app still has some small bugs and the company is in the process of adding needed features like warm-up and cool-down routines. FitStar plans to add the ability to schedule workouts—appointments that the virtual Mr. Gonzalez will pressure you to keep through app alerts and possibly pre-recorded calls. The company also says adding the ability to customize routines for injuries is "high on the list" for their upcoming features.

After two weeks of using FitStar, it has mercifully lowered its expectations on how many push-ups I can do and yet still offers a workout that makes me sweat. Even if I don't have the brawn of a personal trainer, now my phone helps me think like one.

Here's how the FitStar app generates workouts tailored for a user:

  • First, the app asks for basic information, including fitness level, gender, age, height and weight.
  • User does a basic fitness test, above, and assesses his performance.
  • A fitness goal is selected and the app begins creating custom workouts.
  • After a workout task, the app asks if it was too easy or too hard and how much was done. If a move is too difficult, the user may see a shorter version of it, or a new activity next time.
  • The app gently pushes the user to do more over time and looks for big performance changes that may signal the need for a workout adjustment.
  • App uses data from Jawbone or Fitbit exercise trackers, such as overall activity level and how much a user sleeps, to tailor workouts.

—Have a favorite fitness app—or one you hate? Share your experience in the comments section. Contact me at and on Twitter @geoffreyfowler.