Warm up
Activity sessions should always start with warm-up exercises. Take five to 10 minutes to do the activity at a slower-than-normal pace. This will gradually elevate your heart rate and get blood pumping into your muscles. Once you've warmed up, do some light stretches to loosen your muscles and lessen the chances of tearing unprepared muscles and tendons. Make sure the stretches apply to muscles that you will use in the activity. (If you're going to play tennis, stretch your arm, shoulder, and elbow as well as your lower body and back.)

Progression
Aerobic activity should be progressive: It must gradually place more and more demands on the body to condition it. Like other muscles, the heart grows stronger when it's challenged by increasing workloads.

Intensity
To build optimum fitness, activity should be brisk so that it raises your heart and breathing rate. But if you're inactive and/or overweight, be careful not to overdo it at first. It's best to aim for low-to-moderate levels of exertion when you're starting out. As you become more fit, you can boost your exercise time or pace to make it more vigorous.

Target heart range. You can keep track of how hard you're exercising by monitoring your heart rate during exercise. Your target heart range is the range of heartbeats per minute that is ideal for you during workouts, and it is based on lower and upper percentages of your maximum heart rate.

There are several ways to determine your maximum heart rate, including a stress test, highest heart rate measured during lengthy and strenuous exercise, or a calculated estimate based on age. Here's how to calculate your maximum heart rate (MHR):

For men, MHR = 210- (half your age)-(0.05 x your weight) + 4
For women, MHR = 210- (half your age)-(0.05 x your weight)

If you've been inactive, or your goal is to stay generally fit, you should have a target heart range with a lower value of 50% of your maximum heart rate and an upper value of 70% of your maximum heart rate. For fitness improvement, use a target heart range of 70% to 90% of your maximum heart rate.

Taking your pulse. To check if you are within your target zone, take your pulse during your workouts. Immediately after stopping exercise, gently press down on your pulse inside your wrist or at the side of your neck near your Adam's apple. Count the number of beats in 10 seconds and multiply by 6 to calculate your target heart rate. Adjust your level of exertion to match your target heart rate.

Important note: Target heart rate is a useful guide, but it's not always reliable. Heart and blood pressure medications, for instance, can affect heart rate and may change a person's target heart rate. If you're using such medications, check with your doctor to determine an appropriate target heart rate for you.

Talk test. Taking your pulse may not always be convenient or desirable. Instead, you can monitor exercise intensity by using the simple "talk test." If you're breathing harder than normal but still can carry on a conversation while you're exercising, then you're in the appropriate range. If you're huffing and puffing too hard to talk, slow down the pace.

Duration
To improve fitness, exercise must be sustained—done for at least 15–30 minutes a session without interruption. Again, if you've been inactive for a long time, don't get hung up on these numbers. Any increase in activity will contribute to your health and longevity. Start by doing whatever you can—a walk around the block, an easy bike ride, a few laps back and forth across the pool. Then build up to at least a half-hour, and aim for even longer if you're doing low-intensity exercise, like easy walking.

Frequency
Aerobic exercise should be done at least three or four times a week to maintain fitness gains. If you're doing vigorous activity, experts generally recommend that you take a few days off during the week, or at least alternate types of exercise day to day. This gives your body time to recover and lessens the chance of injury. If you're doing moderate exercise, aim for more frequent sessions—five or six times a week.

It's easier to maintain fitness than to build it. If you miss a few days of workouts, you won't slide back to square one. But if you miss a few weeks, you'll lose some fitness gains and will have to work your way back. Also, the more fit you are in the first place, the easier it is to hold on to gains when you have lapses.

Types of aerobic activities
Certain forms of exercise, which use large muscles with moderate-to-vigorous effort, are more likely than others to offer aerobic-conditioning benefits. These include running, jogging, stationary cycling, rowing, cross-country skiing, and jumping rope.

Other activities tend to be moderately vigorous and can be excellent conditioners if they're done briskly for at least 30 minutes, three times a week. These include swimming, biking (constant pedaling), fast-paced walking, aerobic dance, tennis, racquetball, soccer, and basketball. They are more likely to deliver benefits if they're done steadily without too many breaks in the action.

Finally, a third group of activities tend to have long pauses and only occasional bursts of vigorous activity, so they aren't considered ideal aerobic exercises. This group includes baseball, softball, golf, bowling, housework, and gardening. Still, don't forget the new research: Even these activities can make a significant contribution to your daily activity quota and promise a host of benefits.

Weight-supported activities. Activities also can be classified as weight-bearing or weight-supported. Running, jogging, and various sport activities force you to carry your body weight. They tend to burn a lot of calories and may help ward off osteoporosis, so they have advantages. But they also place a great deal of stress on your bones and joints. Such pounding can cause injury, especially if you're overweight or have been inactive.

It's probably best to alternate weight-bearing exercise with types of exercise that support your weight and place less stress on joints. These include swimming, biking, stationary cycling, and rowing. Fitness walking is often an ideal choice. It's vigorous enough to burn up calories but won't cause undue strain. It consists of walking at a brisk pace (about 15 minutes per mile) and adopting a no-nonsense, arms-swinging gait.

Cool down
Allow time following exercise to let your heart rate return gradually to its resting rate. Slow your pace and walk around for five to 10 minutes. Don't stop vigorous exercise suddenly. Never go into a hot tub, sauna, or steam room immediately after a hard workout or you can put too much stress on your heart and circulatory system.