The pounding and jarring from years of gymnastics, running or contact sports can create biomechanical problems, as well as contribute to stiff, achy joints along with tight, inflexible muscles.

This wear-and-tear forces some people to abandon their favorite exercise routine and look for other options.

Fred Hammond, 67, was forced to stop running long distances after he developed a bad case of sciatica and circulation problems in his lower extremities.

Patty Barth, in her early 40s, hung up her running shoes and opted for the bike and inline-skating path after she could not overcome several problems with her knees and feet.

A herniated disk caused Megan Strzelec, 52, to abandon the stair climbing and racquetball she had done for 25 years.

"Most people don't want to stop working out because the benefits of exercise -- from lower blood pressure to improved mood -- are just too great to pass up," wrote Christine Gorman in her article "Getting Fit: How To Age Gracefully," published in the June 6 edition of Time magazine.

Gorman's piece was part of a 21-page special fitness section with a comprehensive guide for young and old, couch potato and exercise nut.

"Most people who develop orthopedic or medical problems want to remain active," Gorman wrote, "so they eventually learn to accommodate their aging bodies by changing sport activities or exercise routines."

Continuing to function at optimal levels requires making smart activity choices throughout the decades. Recent studies have taught exercise physiologists a lot about what combinations of physical activities and exercises work best at different ages.

Gorman reinforces the idea of staying fit for a lifetime by providing her readers with a decade-by-decade breakdown of what components of fitness are most important.

Here is a synopsis.

20s -- You should do moderate and vigorous cardiovascular exercise, as well as resistance training.

In terms of cardiovascular exercise, you want to break a sweat at least three times a week for at least 20 to 30 minutes. Strength training should also be done two to three times per week.

Balance exercises also are a good idea. A person's natural sense of balance begins to fade around age 16.

30s and 40s -- Start getting more consistent with your fitness program. Make aerobic activity a daily routine. Also include stretching, flexibility and balance exercises.

Women, in particular, should strength-train two to three times per week to build muscle mass and slow down or halt bone loss. If your muscles and joints cannot take the physical stress of past decades, it's time to investigate cross-training.

50s and 60s -- What you do now starts to depend more on your medical and orthopedic background and your fitness level. There's an additional concern; it takes more work to maintain a healthy body weight than it did in younger decades.

A multidimensional fitness program, with cardiovascular activities, strength training, flexibility exercises and balance work becomes important to perform daily activities.

To save joints from wear and tear, reduce the number of days you perform exercise that involves pounding. If you used to run five days per week, cut back to three days and cross- train in between.

How well you function and the activities you do in your 50s and 60s set the table for what you can tolerate in your 70s and 80s.

70s and beyond -- Strength, flexibility and balance take center stage.

Getting up from a chair, carrying groceries, etc. become almost impossible without being in good physical shape.

However, "The 70s are different than they used to be," Gorman wrote. "It appears that seniors are starting to heed the advice they're getting to keep moving.

"According to surveys conducted by the Centers for Disease Control and Prevention, the number of sedentary men age 70 and older has dropped over the past 15 years from 40 percent to 30 percent, while the number of sedentary women 70 years and older has decreased from 50 percent to 40 percent."