In her 20s, Lori Popkewitz Alper loved the intense cardio workouts at her Boston gym. But, as her life and her body changed, so did her fitness repertoire.

During pregnancy, Alper found yoga. Soon she was pushing a jog stroller or hauling children in a double-wide bike trailer. Now 47, Alper has returned to some of the high-impact routines of her youth, but her approach has matured.

"I'm more aware of my body's needs, and I try not to be too hard on it," said Alper, who regularly incorporates strength training for bone health and yoga for her sanity. "(Exercise) is such an important piece of my existence - I hope it always will be."

Workout programs are like 401(k)s - they need to be re-balanced over the decades, said fitness expert Tom Holland. "As we age, we need to gradually take out the risk and put in more 'blue chip' elements," he said.

These four basic-yet-effective exercises - a squat, pushup, bicep curl and abdominal crunch - should remain in your program as long as you can perform them correctly, Holland said.

"When you're young, blue chips are often perceived as being too easy, yet they are the key to creating and maintaining a strong foundation," he said. "You may have to modify them slightly as you age - not going down as far on a squat, for example - but you keep them in as long as possible."

As the body ages, it naturally begins to fall apart, with some functions breaking down faster than others. After age 20, the maximal amount of oxygen your body can use - also known as VO2 max - decreases by 1 percent a year in healthy men and women.

By the time you've hit 30, muscular strength begins to head south. But the majority of the decrease occurs after age 50, when it falls at the rate of 15 percent per decade. Bone mineral density also decreases with age; in women the rate accelerates after menopause.

What to do ...

Experts say the ideal combination of exercise for healthy aging should include a combination of aerobic, strengthening and flexibility exercises.

Balance exercises are also vital in helping prevent falls, which can lead to fractures. And though higher-intensity training programs are effective, less rigorous works can be just as effective, as long as they are done consistently.

Kim Evans, 56, a fitness professional in Grand Haven, Mich., stresses functional fitness and de-emphasizes cardio as her clients age. "Older folks still need to get up and down off of the floor, to be able to chase after grandkids and play a round of golf or tennis without having to recover for several days," she said.

"Aging is not for sissies. You need to face it head on," Evans said. "Pay attention to your limitations, keep up your strength, keep trying new things and have a good attitude."

Tweaking your workout can keep you active well into your golden years. Here's how to reduce the risk in your exercise portfolio:

... if you're a runner

Train like a triathlete, Holland said, because if you only run, you'll be forced by injury to switch to swimming and biking to rehabilitate overuse injuries. Swimming is beneficial because "your posture and body weight is horizontal to gravity, so you work many muscles that receive little attention when running or can become weak and prone to injuries, such as the hamstrings, abdominals and low back," said Michele Olson, a professor of exercise physiology at Auburn University at Montgomery, Ala. "Swimming provides a top-notch cardio challenge for heart health; that's important since heart disease risk increases markedly as we age."

Runners don't necessarily need to drop their hard training days, said Amby Burfoot, 66, who plans to run the Boston Marathon in April on the 45th anniversary of his 1968 title. Burfoot isn't running as far or as fast, and he needs more recovery time. But he still runs vigorous hill repeats several times a week and alternates running with easy spinning on a recumbent bike. He eliminated his long runs - his longest is a 13-miler versus the 20-milers of his youth. "I still run marathons but don't race them," he said.

... if you're a swimmer

Add gravity. Be sure to incorporate strength training, walking or anything weight bearing to help prevent the loss of bone density, said Pete McCall, an exercise physiologist with the American Council on Exercise. Also spend an equal amount of time on your back to help balance out the curves of the spine, recommends Jill Murphy, a physical therapist, licensed athletic trainer and strength and conditioning specialist in Neenah, Wis. Adding some backstroke into the mix "will stretch your pectoral muscles and work the muscles between your shoulder blades that help stabilize your spine and maintain your posture all day long," she said.

... if you're a cyclist

Run. "Cycling mainly involves the quadriceps muscles while running is primarily a hamstring activity," Holland said. "When either of these muscles is too strong, injury occurs. Combining biking and running keep these muscle groups balanced, which keeps you injury-free." Also try the stationary rower, which doesn't put vertical pressure on the knees, McCall said.

... if you're a bodybuilder

Try yoga. "Improve your flexibility and provide a static challenge to the muscles versus the dynamic 'pump, pump, pump,' rep after rep you've experienced with a long-term routine of bodybuilding," Olson said.

... if you're a tennis player

Balance the other side. "Do resistance training and in the form of dumbbells, bands and tubing to balance the strength on each side of the body," Olson said. "If you are right-handed, most of the joints and muscles on the right side of the body will be better developed than those on the left side. With free weights, each arm has to independently hoist the weight such as shoulder presses with the left side versus the right side."

... if you don't work out

Start moving. "Don't worry about weights, just get up and walk or try something fun like Zumba," McCall said. Start with a form of cardio, such as walking, spinning or using a cardio machine. Adopt a good core-building activity, such a Pilates or use TRX to build a baseline of strength. Holland recommends exercise DVDs. "They're ridiculously inexpensive now, you don't have to leave home to exercise and you can find everything from tai chi to P90X," he said. "And, if you press play enough, they really work."

• Read more articles by JULIE DEARDORFF

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