In a few short years, fitness has become so much more than just recording how many calories you burn or how many miles you run. There are mobile apps and standalone gadgets that'll monitor your heart rate, how many hours you slept, the number of steps you take at work and so on.

Now RunKeeper, the company behind one of the original health and fitness apps for the iPhone, has revealed an ambitious new plan that it hopes will make it the Facebook of fitness, a one-stop location for all of your important health information. Imagine having data on your blood pressure, cholesterol, diet, cycling output, heart rate, REM sleep and BMI, all continuously updated from a slew of third-party gadgets and services.

If Facebook tells you everything about what's happening with your friends, RunKeeper wants you to know everything that's going on in your body.

>'Aggregating the world's health information is where we ultimately want to head.'

Tucked away in Boston's South End neighborhood, the 11-person, 3-year-old startup announced this morning that users may now see all their health and fitness data points aggregated together into a Health Graph, an interactive graphical representation of their workouts over time and how they compare to friends in their FitnessFeed, akin to Facebook's News Feed or your standard Twitter stream.

More over, RunKeeper has also released an open API for outside developers to plug into RunKeeper users' feeds, like so many various Twitter clients and Facebook partners. FourSquare, Zeo and Polar are just a few of the launch partners being announced today.

But with an open API setup, RunKeeper cofounder Jason Jacobs expects a steady stream of new additions to the FitnessFeed every week. And with more partners feeding health data into RunKeeper, that means more useful information for the site's 6 million-plus users.

"It became clear that no one was pushing us harder than our community to enable them to get a holistic view of their health in one place," RunKeeper cofounder Jason Jacobs told Wired.com. "If someone gains 10 pounds, it's not just the food they eat or how much exercise. There are all these different drivers. Who's peeled back the onion to start to understand what those factors are for you as an individual?

"Nobody has ever had all this information in one place before to be able to make sense of it."

The Health Graph and its corresponding API is the result of a 15-month development overseen by Mike Sheeley, RunKeeper's other cofounder. It's an idea that sprung up after RunKeeper partnered up with Withings, which manufactures a $160 Wi-Fi-connected body scale.

"We had started to collect weight and body fat percentage, and as soon as we integrated with Withings, all of our users were saying, 'Well, there's this other scale and there's one for heart rate and so on,'" Sheeley told Wired.com. "From a technical standpoint, we looked at our database where we were storing all of our data and we knew that if this was going to continue, we had to think about how we should store it, collect it and actually structure it so that it makes sense."

Jacobs sees the Health Graph – and its inevitable growth from the open API – as a significant step toward connecting all the relevant health points in your life, from working out to your regular physician checkups.

"You have one Health Graph and when you go to the gym, your program is based on the graph that the trainer can know about you, and then the exercise you do will feed back into the graph," Jacobs said. "When you run a race or when you go to a doctor, same thing."

Since its earliest days, RunKeeper has battled fitness giants like Nike for positioning among consumers looking to shed pounds and lead a healthier life. RunKeeper was actually one of the first iPhone apps available in Apple's App Store, beating even Nike+ to release. (At the point, Nike+ was still a web-based service that required a dongle on your shoe and specialized sensor plugged into your iPod.) Since then, RunKeeper has formed partnerships, one at a time, with various companies, including Garmin and Fitbit.

RunKeeper has seen two rounds of funding to date, totaling some $1.5 million in seed money, but Jacobs says they "haven't touched a dime," even while developing the Health Graph and scaling up operations to handle more than 6 million users with no service outages, although "there have been scaling issues and scaling isn't something that you ever really solve," Sheeley said.

>'Nobody has ever had all this information in one place before to be able to make sense of it.'

Still, the upwards curve can't be denied, as RunKeeper has more than doubled its user base since last November (when it had 2.5 million) without making a single withdrawal from its nest egg. That's owed to the success of its Elite subscriptions, which cost $20 a year and offer users the ability to broadcast their runs live and get access to more detailed fitness reports. Jacobs actually receives an email every time a RunKeeper user decides to go Elite, so we can guess that he's been getting a lot of emails over the past year, keeping both his investors happy and the RunKeeper operation lean and efficient.

Another move, which could've easily backfired, was when Jacobs announced last December that the company's flagship app, the $9.99 RunKeeper Pro and one of the highest-grossing apps in App Store history, would be made free for a month, a seemingly perfect promotional move to capitalize on people's New Year's resolutions to get more fit.

But as the window was set to close, Jacobs dropped another bombshell: RunKeeper Pro would continue to be free permanently. At $10 a pop down suddenly going for nothing, it's understandable that RunKeeper's investors, who had just ponied up $1.1 in seed money, would be a tad nervous, but Jacobs now says the move was liberating for RunKeeper's dev team: "We saw that setting it free would be a catalyst to making everything that we wanted to happen, happen faster."

While the RunKeeper Health Graph fulfills a significant part of the company's long-term plans, users and investors will be watching closely as the company moves to announce more initiatives later this year.

"Aggregating the world's health information is where we ultimately want to head," Jacobs said. "We looked at the landscape of everything we don't yet integrate with and realized that the more inputs that come into the system, the more powerful the system becomes."

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