4.1 What is aerobic exercise?

The word aerobic literally means "with oxygen" or "in the presence of oxygen." Aerobic exercise is any activity that uses large muscle groups, can be maintained continuously for a long period of time and is rhythmic in nature. Aerobic activity trains the heart, lungs and cardiovascular system to process and deliver oxygen more quickly and efficiently to every part of the body. As the heart muscle becomes stronger and more efficient, a larger amount of blood can be pumped with each stroke. Fewer strokes are then required to rapidly transport oxygen to all parts of the body. An aerobically fit individual can work longer, more vigorously and achieve a quicker recovery at the end of the aerobic session.

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4.2 What are some examples of aerobic activity?

(from Ron Hogan )

Aerobic dance, aerobic machines, backpacking, ballroom dance, basketball, belly dancing, boxing, broomball, calisthenics, canoeing, cycling, fencing, Frisbee, golf, gymnastics, handball, hiking, hockey, ice skating, jazzercise, jogging, judo, jumping rope, karate, kayaking, mountaineering, racquetball, rock climbing, roller skating, rope climbing, rowing, running, skateboarding, skiing, skin diving, spelunking, square dancing, squash, step aerobics, swimming, walking, water skiing or any other activity that meets the criteria in section 4.1.

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8.1 How do I know when I'm exercising hard enough to burn fat?

Actually, you're *almost* always burning fat at one rate or another, but you burn most when your body is in its aerobic range. A good rule of thumb is that after 20 minutes in your aerobic zone, you will be burning more fat than carbos. Covert Bailey, in "Smart Exercise", states that you will be burning fat after only twelve minutes of aerobic exercise. If you can increase your aerobic activity to 30 minutes or longer, you will be burning a larger percentage of calories from fat. There is still some disagreement as to which is better - longer duration at lower intensity, or shorter duration at higher intensity. If you are limited in time, then the higher intensity will maximize your aerobic benefits in a shorter amount of time. If you can work for a longer duration at a lower intensity, you will decrease your chance of injury. If you are interested in decreasing the amount of fat on your body, the idea is to use more calories than you take in. Your muscles will continue to burn fat after both aerobic and anaerobic (muscle training) exercise.

(from Michael G. Kurilla [mgk2r@uva.pcmail. virginia.edu])

This is perhaps the most common question raised by individuals exercising for the purpose of either weight loss or simply weight control. This stems from the recognition that aerobic exercise is a significant adjunct to any weight loss program, that is diet plus aerobic exercise produces more weight loss than diet alone. In addition, the weight lost with exercise tends to be a higher percentage of fat.

Exercise can be grouped into three broad levels of intensity, mild, moderate, and high. Mild intensity is a comfortable walking pace and can be sustained almost indefinitely, moderate intensity is equal to an average cardiovascular conditioning workout (able to talk, but not sing) and can be sustained (in a trained individual) for upwards of 3 - 4 hours, and high intensity is not able to talk and can only be sustained for 30 - 45 minutes.

Based on recent and very detailed research studies, in terms of absolute fat burning, a moderate intensity workout burns the most fat. At a heart rate equal to about 75% of max, fat burning will approach 0.5 grams - 1.0 grams of fat per minute. There is a weight dependence with the lower end referring to a 100 pound individual and the upper end to a 200 pound person. As the duration continues (greater than 1 hour), fat burning can increase slightly (another 10%).

At a mild intensity, the majority of calories expended (85 - 90%) are fat calories, but the absolute level is only about 60% of the moderate intensity. At high intensity levels, fat burning declines to a level of about 65% of the moderate pace, as sugar burning supplies the rest. The high rate of sugar burning exhausts the limited sugar supply in muscles and causes muscular failure.

The only caveats for the above burn rates are that these numbers are derived from individuals who were already aerobically trained and were conducted in the AM before breakfast. Less fit individuals are known to burn less fat and more sugar (part of aerobic conditioning is greater reliance on fat burning for energy). Exercising after a meal will tend to promote more sugar burning. Consumption of sugar during an exercise session will also tend to retard fat burning in favor of the sugar. These numbers were derived from cycling and so the absolute numbers can be increased if exercises that involve more muscle groups are utilized (running, rowing, etc.). From peak energy production rates for various exercises, rowers might reach about 40% higher.

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8.2 How long do I have to work out before I burn fat? and If I workout before eating, will I burn 100% fat?

You are _always_ burning fat. There is no magic on/off switch for "fat burning", or any other system in the body. Your body gets its energy from several sources all the time; the proportions change depending on the intensity and duration of the activity, but stored body fat is always one of them.

Stored body fat is utilized more for low- to moderate-intensity, long-duration activity; this could be where the confusion about needing to exercise for x number of minutes arises. During the first several minutes of exercise your body gets started by tapping primarily (not exclusively!) its more immediate energy sources, like glycogen in your muscles. These sources cannot keep up with the continued demand for energy, so your body gradually taps into stored body fat as well to continue at that intensity.

As an analogy, think of your body as always carrying around a cord of firewood and a small bottle of jet fuel. The firewood is your stored body fat, the jet fuel is the glycogen in your muscles. When you need to suddenly dash for the bus, you use the jet fuel. It won't get you far because you don't have much, but you can get there very fast. When you need to go on a long day hike, you use the firewood--a long, slow burn that can last for hours, and you have plenty of it. But you always have to use a few drops of jet fuel to get going while you kindle the firewood, and to keep the flame bright. And you always have to use the firewood, even if only as a pilot light.

From this analogy it should be easy to see that it's not possible to work out ensuring that 100% of your energy is coming from fat-- you couldn't get started, or once started couldn't perform at more than a "slow smolder" intensity. In fact, many people report feeling very sluggish if they try to work out in the morning on an empty stomach, while simply eating a simple high-carbohydrate snack an hour before yields a much better performance. It takes energy to make energy!

It may at first come as a surprise to learn that the time when your body is getting its highest _percentage_ of energy from fat is when you're asleep! But consider that when you're asleep, your body has no need for bursts of high intensity activity, so those energy systems are quiet. Obviously, sleeping is not a good activity for losing weight, because your total energy requirements are quite low then. This should show you that the _rate_ at which you burn fat/calories is not as important as the _total_ that you burn on a daily basis. In other words, the minute-to-minute fluctuations in the proportions of fat vs. carbohydrate used by your body are not linked to long-term weight managment.

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8.4 What exercise should I do to burn the most fat?

If your goal is fat loss, then try to achieve a maximal _calorie_ burn, and don't worry about a maximal _fat_ burn. As long as you are expending more calories than you are consuming -- on a regular and consistent basis -- then the fat/weight will come off. Any aerobic activity which you enjoy doing enough to do 3-5 times a week at a moderate intensity for at least 20 continuous minutes at a time will help you burn lots of calories; dance/step aerobics, bicycling, swimming, basketball, soccer, running, skating, hiking, and walking are all good examples.

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9.1 Is it better to break my exercise sessions up, or exercise for a longer period?

In general, for the average aerobicizer, it doesn't matter whether you exercise for 2 shorter sessions or 1 longer session. Keep in mind that your body requires a "warm-up" period of 5 to 10 minutes and a "cool-down" period of similar length (cf. section 12). So if you exercise for one 60 minutes period, 40 to 50 minutes of that time would be for aerobic training with the remainder for warm-up and cool-down. If you exercise for two 30 minute periods, 10 to 20 minutes of each period or 20 to 40 minutes total, would be aerobic training.

So if you break up your workout but using the same amount of time, you might be training aerobically for a shorter amount of time. Does this really matter? It depends on why you are aerobically training. If you are interested in training for an activity like soccar/football or water polo, where you are actively working aerobically for extended periods of time, then, yes, it will make a difference. The duration of the training periods very directly affect the aerobic capacity of an athlete. For example, if you don't have the aerobic capacity for swim constantly for the duration of a water polor game, you aren't going to be able to complete an entire game.

However, if you are exercising for general fitness and health, then other issues are important. If it works for you to exercise in 2 shorter periods, and that is what you are able to do ,then that is what is best for you. For most of us, it is more practical to exercise in 1 period, be it long or short. A very short period of exercise, like a 10 minute session will have limited aerobic benefits for all but the most deconditioned because of the warm-up/cool-down necessities. That doesn't mean that it doesn't have benefit; it's not aerobic training.

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The previous Questions and Answers are excerpted from the
misc.fitness.aerobics Frequently Asked Questions page (FAQ).
http://www.oaktrees.org/fitness/misc.fitness.aerobic.faq.txt