FAYETTEVILLE, Ark. - May represents National Senior Citizens’ Month, and according to a University of Arkansas researcher, Americans have more reasons to celebrate it this year than ever before.

"For the first time ever, the U.S. now has more senior citizens than teenagers," said Ro DiBrezzo, co-director of the UA Center for Aging Studies.

As the Baby Boomers enter their retirement years, the number of senior citizens in America will continue to grow. And they will not be alone. Better nutrition and health care have resulted in longer life expectancies across the globe.

"There are currently 72,000 people worldwide who are over one hundred. By 2030, we anticipate that number will jump to 300,000," DiBrezzo added. "We’re reaching a point where people don’t just live ten or fifteen years after retirement but another whole lifetime."

However, DiBrezzo believes that extra lifetime is wasted unless people remain physically active and able. In fact, the most common fear among aging Americans is that they will lose their independence and mobility - proving that quality of life matters just as much as quantity.

As a researcher in exercise science, DiBrezzo has devoted years of study to the effects of exercise on older subjects. Her work not only has provided evidence about the benefits of physical exertion, but it has offered insight into the changing needs and abilities of the human body as it enters older age.

And these changes begin sooner than you might think, warned DiBrezzo.

Around age 40, the body experiences a metabolic shift. It begins to store more fat. Lean muscle mass gradually begins to deteriorate, and cardiovascular capacity lessens. As a natural part of the aging process, these physical changes affect even the healthiest individuals, but they are particularly dramatic in women.

As the body’s composition changes, its needs and abilities also change, said DiBrezzo. Therefore, people should reassess their lifestyle habits at this point and consider modifying their exercise routines to accommodate a mature body.

The first step in making such a transition is to consult a physician. This is particularly important for people who have led a sedentary lifestyle in their youth. A routine physical will help to identify potential health risks and will enable doctors to ease individuals into an exercise regimen that will gradually increase strength and endurance.

Older individuals should not jump into the same routine they performed as twenty-year-olds, said DiBrezzo. While younger people can obtain general health benefits from any type of regular exercise, mature people need a more varied routine.

"One day you might walk, the next day ride a stationary bicycle or go swimming," said DiBrezzo. "By alternating activities, you use different muscles to perform different motions. That way, you’re not putting repetitive strain on any one area of the body. It preserves your joints and prevents orthopedic injuries."

In addition, DiBrezzo emphasizes the importance of challenging yourself. Don’t accept a slow pace simply because it’s comfortable. Rather, once you’ve reached a comfortable level of exertion, push your body to perform at a slightly faster pace or to exercise for a slightly longer period of time. By alternately increasing the duration and intensity of your workout, you ensure that your body continues to build muscular strength and cardiovascular endurance.

If you feel your workout has reached a plateau and no longer challenges you, DiBrezzo recommends changing activities or consulting with a professional trainer, who can offer advice on how to boost your routine to the next level.

DiBrezzo also gives a simple suggestion that can make a significant difference:

"The best investment you can make is to buy two different kinds of walking or jogging shoes, then periodically alternate wearing them," she said. "Different shoes translate forces differently, and it can make a big impact on which muscles you work."

In addition to these general health tips, DiBrezzo recognizes some of the common pitfalls of exercising in your retirement years.

On the most fundamental level, many people neglect to drink enough water, she said. Thebody does not always use thirst to signal dehydration, so older individuals often forget to replenish fluids after exercising. DiBrezzo recommends people drink water before and after each workout. And some may want to invest in commercial sports drinks to help replace nutrients and minerals.

Another common problem is that people confuse discomfort and pain. This mistake often causes older individuals to abandon exercise in fear of injury.

"These people are right in one sense. The body has a sophisticated alarm system, and pain often indicates that injury will follow. You should always stop at the first sign of pain," said DiBrezzo. "But if you haven’t exercised for some time, you need to learn how to distinguish reasonable aches and strain from actual pain."

She suggests people start exercise slowly, gradually testing the limits of their flexibility and strength. Once individuals know their limits, they can recognize when they’ve exceeded them.

Finally, DiBrezzo says that older people often find it difficult to stick with an exercise routine, especially when it’s new. She recommends the buddy system. Whether that means exercising at a gym or taking a walk with your next door neighbor, the companionship can help speed the time and can provide valuable motivation.

"It only takes 21 days to form a habit," she said. "Once you’ve toughed out those 21 days, you’ll start to feel strange when you don’t exercise."

DiBrezzo also claims that, just as you’re never too young to start a health regimen, you’re never too old either.

"We have all sorts of documentation of people starting fitness programs as late as 75 years old and reaping definite health benefits from it," she said. "That’s very encouraging."