Depending on how much they jostle you, aerobic activities can be high impact (such as jogging or jumping jacks), which involve heavy contact with a surface; low impact (walking, cycling), which involve moderate impact levels; or no impact (rowing or swimming), which involve fluid, con¬tinuous motion.

The greater the impact, the greater the stress on your body, which older or more frail exercisers should note.

Walking
On the treadmill or on the road, walking is Amer¬ica’s most popular form of exercise. A continuing national survey sponsored in part by the Fitness Products Council found it the top choice of fre¬quent exercisers, with 14.5 million of them putting on their walking shoes a hundred times or more in 1996.

In the survey’s top-ten list of fitness activities, the combined categories of people who fitness-walked and walked on treadmills totaled 65.4 mil¬lion (with 31.5 million joggers fast on their heels). And in U.S. shopping malls, 3 million people walk for exercise, double the number five years ago, says the National Organization of Mall Walkers. Other than a good pair of shoes, walking requires no spe¬cial equipment or attire.

To get a better workout, add movements to incorporate resistance training and maintain flexibility. Swing your arms, walk uphill, shadowbox, lengthen strides, or carry light weights (in your hands, not around your ankles). Among die features offered by treadmills are vary¬ing degrees of incline and movable arm poles, plus all manner of monitors, from speed to mileage.

Cycling
On a stationary cycle or outdoors, cycling provides one of the best cardiovascular workouts, while at the same time strengthening the large muscles of the lower body—including thighs, hips, and buttocks— without putting a lot of stress on the joints.

Indoors, some stationary cycles feature movable handles to give your arms a good workout as well. Outdoors, sturdy mountain bikes help navigate rougher roads should reach about chest high. To add to the workout, try jumping backward, elevate your knees, or get creative with foot patterns.

Swimming and Aquatics
It’s up to fifteen times harder to walk through water than through air, and yet its buoyancy "reduces" your weight by about 90 percent. That translates into more exercise value per movement, and less stress on muscles, bones, and joints.

Offered in many class formats, low-impact aquat¬ics, also known as aquasize or aqua aerobics, is a great alternative for those who find the same movements on land too jarring. And exercising in water can great improve muscular strength and endurance (the water acts as the resistance) and flexibility (the buoyancy helps joints through a wider range of motion), plus it has cardiovascular and fat-reducing benefits.

To increase intensity, enlarge the size of your movement, or increase the speed. For an on-land, water-sport-related workout, try using rowing machines, which work the back, arms, and legs.

Stair Climbing
Taking to the steps instead of the elevator is one of the most readily available exercise options for many city dwellers, and even suburban types can get in on the action on stair-climber machines— continually revolving staircases on which you never quite reach your floor.

Good for lower-body toning, with cardiovascular and respiratory bene¬fits, the workout can be performed on StairMaster or other pedal-stepping machines, some of which offer an upper-body workout as well—a sort of pull-up climbing motion to the arms.