Researchers recently put 30 of the most popular free iPhone fitness apps to the test to determine which are providing users with the most well-rounded workout. The results may surprise you.

Each app was tested for a variety of factors including warm-ups, stretching, and cool-downs, ultimately grading them in three categories: aerobic exercise, strength / resistance and flexibility. The results were then compared against guidelines for physical activity from the American College of Sports Medicine (ACSM).

As The Washington Post notes, 90 percent of the apps met some of the criteria for strength / resistance. More than half met some of the criteria for aerobic exercise but many floundered in the flexibility category. Specifically, more than 66 percent didn't meet any flexibility criteria.

All data considered, only a single app - the Sworkit Lite Personal Workout Trainer - met more than half of the guidelines.

Francois Modave, the study's lead author, said the problem with the apps put users at risk for injury because they fail to prepare people for the exercise, use proper technique and address safety concerns. If nothing else, Modave said he hopes the study will encourage discussion among developers as it relates to proper techniques and the overall safety of its users.

Richard Cotton, national director of certification for the ACSM, told the Post via e-mail that all three categories are necessary en route to a comprehensive exercise program. He added that even apps that address only one category provide potential value for users based on their individual health and fitness needs.

In the absence of fit friends or other support groups, fitness apps provide many with the motivation they need to get in shape and stay that way. As Cotton alludes to, using such an app is better than not doing anything at all.

According to the Centers for Disease Control and Prevention, more than a third of US adults are obese. Being overweight can lead to all sorts of health concerns including heart disease, stroke, type two diabetes and even certain types of cancer.

Above all else, the best piece of advice would be to simply listen to your body as it relates to safety.

Lead image courtesy GaborfromHungary via