Lost your motivation for working out? A crop of smartphone apps aims to turn exercise into a game, complete with a virtual cheering squad in some cases.

Traditional fitness apps can help people develop an exercise routine and track their progress. But the newer game apps offer social encouragement for exercise and some entertainment too, said Steve Portenga, chief executive of Denver-based iPerformance Psychology, which works with professional and amateur athletes. It's a good way 'to shake things up because some people find working out incredibly boring.'

Games can motivate people to exercise if they use the right game mechanics, said Debra Lieberman, director of Health Games Research, a program at the University of California, Santa Barbara, that studies ways digital games can improve health. Simply bribing a person with cash and points doesn't lead to long-term behavior change after the bribe goes away, Dr. Lieberman says. 'The best apps motivate players to fulfill intrinsic desires such as to be fit and healthy, feel good, have fun, and be socially connected,' she says.

Here are some fitness apps that aim to keep things interesting.

Platform: iPhone and Android, free

Motivational hook: Users hit next game level by logging workouts

Fitocracy uses a form of social network to motivate people. For every workout they log, users get points, which generally are awarded based on specific exercises and a workout's intensity, says Brian Wang, New York-based Fitocracy co-founder. A whole-body barbell movement like a dead lift will earn more points than a slow walk in the park. As users build up points, they acquire virtual badges and medals