Instead of bouncing to the beat on hard floors, more athletes and exercise enthusiasts are taking their aerobics classes in a swimming pool.

Exercising in the water is nothing new. Physical therapists often use water workouts to rehabilitate injured limbs. Research has shown that when people jump up and down in water, the impact on the feet is less jarring than performing the same activity on land.

That makes water aerobics an ideal form of exercise for people who are overweight, out of condition or have a history of ankle and knee injuries, according to Peter Francis, associate professor of physical education at San Diego State University.

Fitness experts also recommend water aerobics to the general population to complement other exercise programs.

"If you are already involved in a fitness activity, try to add one water workout a week to your training," said Joseph A. Krasevec, associate director of recreational services at Georgia State University. "It provides more of a cross-training approach."

The idea behind water aerobics is to perform vigorous movements with the arms and legs while standing in water that is chest or shoulder deep. Classes consist of a general warm-up, followed by 30 to 45 minutes of aerobic activity that includes under water jumping jacks, running in place with the knees and legs raised, taking giant leaps forward and backwards and simulating cross-country and downhill skiing motions. People can burn between 300 and 500 calories per half hour, said Krasevec.

Some experts debate whether people can expend the same amount of energy exercising in water as they would on land. "Most of the energy in aerobic exercise is used to keep the body upright and to prevent it from crumbling," said Francis. "Because water partially supports the body, we greatly reduce the energy demand." Further research is also needed to determine how much cardiovascular benefit people achieve in water workouts compared to traditional aerobics classes.

One benefit water aerobics can provide is variety. Many exercise enthusiasts grow bored with doing the same workout each week.

"More than 40 percent of the population move in and out of exercise programs and don't stay with it," said Francis. "The more choices people have, the more likely they are to continue exercising."