Injuries were attributed to shoes, floors and teaching techniques. But as refinements in each area were made, injuries persisted, making it apparent that there was something wrong with the activity itself. Aerobic exercise aims to improve cardiovascular fitness (the heart and circulatory system) by raising and sustaining the heartbeat. The target pulse rate is achieved in high-impact aerobics by jogging, jumping or vigorous dance moves that are sometimes too much for the average American. Enter low-impact aerobics, in which one foot is always kept on the ground, the arms are almost constantly in motion, and most of the pounding of high-impact aerobics is eliminated.

The out-of-shape, the elderly, the obese and arthritics who want an effective alternative to aerobic dancing, running or cycling may find what they're looking for in water workouts. Introduced years ago by swim teachers who recognized the cardiovascular benefits of a wet workout, aquatic aerobics are only now attracting a wide following. Kim Pye, an aquatic fitness specialist at the Structure House Center for Diet and Life Style Change in Durham, N.C., has been teaching water aerobics for seven years but says the biggest surge in enthusiasm has been in the last two-"just about the time stress injuries became increasingly evident from land aerobics," she says.

For generations, mothers have been telling children to walk, not run. Now fitness experts and the medical community are giving the same advice. Dr. Ralph Paffenbarger of Stanford University School of Medicine, who studied the exercise patterns of 17,000 middle-age and older Harvard alumni, reported last year that those who walked and climbed stairs regularly outlived their sedentary counterparts by two years. More surprising, Paffenbarger's study found that people on a vigorous fitness regime, such as running, were not in significantly better health than those whose exercise consisted only of regular, moderate activity, such as walking. The study went on to say that major health benefits came from only 2,000 calories burned off by exercise per week-which for the average person translates to five 30-minute, purposeful walks a week. In other words, a walk that doesn't even cause you to break a sweat can improve your fitness without the risk to knees, shins, calves and ankles posed by running or aerobic dance.