If there are two groups of people who can attest to just how hard it is to motivate healthy behavior change, it's health app developers and trainers -- especially trainers who work at gyms during the first few weeks of the year. Scores of people resolve to go to the gym more in the New Year, manage to sign up for memberships, and maybe end up going a few times. But for many it stops there.

GymPact, a mobile app that celebrates its first birthday today with the launch of its first Android app, is trying to change that reality for its users. When users sign up for GymPact, they give the company their credit card information and choose an amount of money that they'll have to pay if they miss their workout goal for the week. If the user achieves their goal instead, GymPact pays them -- divvying up the money made off of the slackers. The company tracks whether the user really went by requiring them to check in with their phone's GPS.

In August the company partnered with RunKeeper to integrate outdoor workouts into the system as an option for meeting goals. Just a few weeks ago at the mHealth Summit, Aetna CEO Mark Bertolini named GymPact as one of the apps that would integrate with Aetna's CarePass platform.

But does it work? Has GymPact helped users keep their resolution to go to the gym? CEO and Co-Founder Yifan Zhang said that the data the company has accumulated after a year of operation is promising.

"There's some things that went really well, some things we definitely had to tweak," she said. "We still have over 50 percent of the users who signed up January 12th [of 2011]. About 500 people having been with us for an entire year." Of those, Zhang said 82 percent have largely stuck to their workout goals.

One of the biggest challenges, Zhang said, was dealing with cheaters -- people who, because the possibility exists to get real money, will do everything they can to get it without doing the work. The company has decreased the GPS radius that the software counts as being "at the gym" and introduced verification protocols for gyms.

The Android version of GymPact was widely requested from day one, Zhang said. Since there is a social networking component of the software, it's especially important to expand to other platforms. The product will be functionally the same, she said, although new features might be tested in the Apple version first as they're released.

Among the new features the company plans to release is support for at-home workouts, which Zhang described as a big hole in their current functionality. Such a feature obviously presents unprecedented challenges when it comes to verification.

"I can't say much right now," she said. "But we will be launching that feature pretty soon, and I think we've done it in a way that's in line with our philosophy."