A cellphone with a bit of smart software could help people to track their physical activity levels.

A U.S. researcher says a program designed to take advantage of the motion sensors in devices such as Apple's iPhone and iPod Touch was able to detect small changes in movement that could be missed by commercially available accelerometers. Those lower-tech devices typically measure exercise such as jogging or walking but not slower day-to-day movements such as climbing stairs.

The Walk n' Play app was able to detect movements as slow as 0.8 kilometre per hour, Chinmay Manohar of the Mayo Clinic in Rochester, Minn., told the Federation of American Societies for Experimental Biology annual meeting Monday in Anaheim, Calif.

The iPhone app, devised by the computational physiology lab at the University of Houston, was tested on 31 volunteers who sat, stood, reclined and used a treadmill at seven speeds. The researchers assessed the accelerometers in the mobile devices and converted the motion detected into activity units to measure physical activity.

The idea to track everyday movements came out of a previous study by Manohar's team indicating that lean people tend to stand for 2½ more hours a day than heavy people. Manohar was looking for a way to encourage people to get out of their chairs with incremental increases in activity.

"You have to put an element of fun into the whole thing to encourage people to be more active," Manohar said. With a gaming mindset, people unknowingly do exercise, he said.

Walk n' Play users enter their height and weight, and their activity score is monitored throughout the day. The program tells users whether they are winning or losing against the computer but there is no pressure to do a specific exercise.

Since fitness and weight loss in general is easier with a buddy, new features will enable users to play against their friends or compete against others using basic social networking tools, such as posting performance as a Twitter status.

Walk n' Play can be downloaded free from the iTunes store, and the researchers expect it won't be difficult to make the program compatible with other smartphones.