Aerobic exercise helps maintain a healthy heart and lungs, and encompasses a broad range of activities.  Such activities include long-distance running, bicycling, aerobics, and various sports such as tennis.  What is common in proper aerobic exercise is a period of at least 20 minutes of significant activity.

Proper warm-up and stretching before aerobic exercise is important, and a gradual increase in intensity ensures that the activity can be sustained for a long period of time.  Equally important is a cool down period of 5-10 minutes to allow the body to adapt to a resting state.  Warm-up and cool down involve the aerobic activity performed at a lower intensity.

A convenient measure of how intensity is % of maximum heart rate.  This can be calculated with the formula HRmax = 220-age.  During warm-up and cool down your heart rate should be around half the maximum heart rate, and during aerobic exercise it should be around three quarters of the maximum heart rate.  Heart rate can be measured by taking pulse and measuring the number of beats per minute, or using a heart rate monitoring device.

It is important to remain hydrated during exercise, as the body will produce a lot of sweat.  If a large quantity of water is consumed, some salt is necessary to help maintain your body’s chemical balance.  Carbohydrates are also useful to provide the body with fuel, and replenish it after exercise. 

The key to good aerobic exercise is sustainability: start off slow, especially beginners, and increase intensity gradually each week as your body adapts.  This is to ensure that at least 20 minutes of sustained activity is possible, and also to avoid injury.  Building up intensity and duration each week encourages your body to improve, but it is important never to exceed what you can currently handle.