- Aerobic exercise is sometimes known as “cardio” — exercise that requires pumping of oxygenated blood by the heart to deliver oxygen to working muscles.
- Aerobic exercise stimulates the heart rate and breathing rate to increase in a way that can be sustained for the exercise session. In contrast, anaerobic (“without oxygen”) exercise is activity that causes you to be quickly out of breath, like sprinting or lifting a heavy weight.
- Examples of aerobic exercises include cardio machines, spinning, running, swimming, walking, hiking, aerobics classes, dancing, cross country skiing, and kickboxing. There are many other types.
- Aerobic exercises can become anaerobic exercises if performed at a level of intensity that is too high.
- Aerobic exercise not only improves fitness; it also has known benefits for both physical and emotional health.
- Aerobic exercise can help prevent or reduce the chance of developing some cancers, diabetes, depression, cardiovascular disease, and osteoporosis.
- An aerobic exercise plan should be simple, practical, and realistic. Specific equipment (such as cardio machines) may be used but is not necessary for successful aerobic exercise.
At lower intensities of exercise, muscles burn a higher percentage of fat than carbohydrate, but not necessarily more total fat, or more total calories, than at higher intensities. This is a subtle distinction, but it’s an important one. Here’s some background to help understand why.
What is aerobic exercise?
Imagine that you’re exercising. You’re working up a sweat, you’re breathing hard, your heart is thumping, blood is coursing through your vessels to deliver oxygen to the muscles to keep you moving, and you sustain the activity for more than just a few minutes. That’s aerobic exercise (also known as “cardio” in gym lingo), which is any activity that you can sustain for more than just a few minutes while your heart, lungs, and muscles work overtime. In this article, I’ll discuss the mechanisms of aerobic exercise: oxygen transport and consumption, the role of the heart and the muscles, the proven benefits of aerobic exercise, how much you need to do to reap the benefits, and more.
It all starts with breathing. The average healthy adult inhales and exhales about 7 to 8 liters of air per minute. Once you fill your lungs, the oxygen in the air (air contains approximately 20% oxygen) is filtered through small branches of tubes (called bronchioles) until it reaches the alveoli. The alveoli are microscopic sacs where oxygen diffuses (enters) into the blood. From there, it’s a beeline direct to the heart.
Getting to the heart of it
The heart has four chambers that fill with blood and pump blood (two atria and two ventricles) and some very active coronary arteries. Because of all this action, the heart needs a fresh supply of oxygen, and as you just learned, the lungs provide it. Once the heart uses what it needs, it pumps the blood, the oxygen, and other nutrients out through the large left ventricle and through the circulatory system (cardiovascular system) to all the organs, muscles, and tissues that need it.
A whole lot of pumping going on
Your heart beats approximately 60-80 times per minute at rest, 100,000 times a day, more than 30 million times per year, and about 2.5 billion times in a 70-year lifetime! Every beat of your heart sends a volume of blood (called stroke volume — more about that later), along with oxygen and many other life-sustaining nutrients, circulating through your body. The average healthy adult heart pumps about 5 liters of blood per minute.
Oxygen consumption and muscles
All that oxygen being pumped by the blood is important. You may be familiar with the term “oxygen consumption.” In science, it’s labeled VO2, or volume of oxygen consumed. It’s the amount of oxygen the muscles extract, or consume from the blood, and it’s expressed as ml/kg/minute (milliliters per kilogram of body weight). Muscles are like engines that run on fuel (just like an automobile that runs on fuel); only our muscles use fat and carbohydrates instead of gasoline. Oxygen is a key player because, once inside the muscle, it’s used to burn fat and carbohydrate for fuel to keep our engines running. The more efficient our muscles are at consuming oxygen, the more fuel we can burn, the more fit we are, and the longer we can exercise.
Why do you need aerobic exercise?
The average sedentary adult will reach a level of oxygen consumption close to 35 ml/kg/minute during a maximal treadmill test (where you’re asked to walk as hard as you can). Translated, that means the person is consuming 35 milliliters of oxygen for every kilogram of body weight per minute. That’ll get you through the day, but elite athletes can reach values as high as 90 ml/kg/minute! How do they do it? They may have good genes for one, but they also train hard. And when they do, their bodies adapt. The good news is that the bodies of mere mortals like the rest of us adapt to training too. Here’s how.
What are the benefits of aerobic exercise?
Here’s what happens inside your body when you do aerobic exercise regularly:
- Your heart gets stronger and pumps more blood with each beat (larger stroke volume). Elite athletes, as I just mentioned, can have stroke volumes more than twice as high as average individuals. But it’s not just that. Conditioned hearts also have greater diameter and mass (the heart’s a muscle too and gets bigger when you train it), and they pump efficiently enough to allow for greater filling time, which is a good thing because it means that more blood fills the chambers of the heart before they pump so that more blood gets pumped with each beat.
- Greater stroke volume means the heart doesn’t have to pump as fast to meet the demands of exercise. Fewer beats and more stroke volume mean greater efficiency. Think about a pump emptying water out of a flooded basement. The pump works better and lasts longer if it can pump larger volumes of water with each cycle than if it has to pump faster and strain to get rid of the water. High stroke volume is why athletes’ hearts don’t pump as fast during exercise and why they have such low resting heart rates; sometimes as low as 40 beats per minute, whereas the average is 60-80 beats per minutes.
- Downstream from the heart are your muscles, which get more efficient at consuming oxygen when you do regular aerobic exercise (remember, “consuming” oxygen means that the muscles are taking the oxygen out of the blood). This happens because of an increase in the activity and number of enzymes that transport oxygen out of the bloodstream and into the muscle. Imagine 100 oxygen molecules circulating past a muscle. You’re twice as fit if the muscle can consume all 100 molecules than if it can only consume 50. Another way of saying it is that you’re twice as fit as someone if your VO2 max is 60ml/kg/min. and theirs is 30ml/kg/min. In terms of performance in this scenario, you’ll have more endurance because your muscles won’t run out of oxygen as quickly.
- Mitochondria inside the muscle increase in number and activity. Mitochondria are the powerhouses of your cells. They do all the heavy-duty work to keep you moving. They use the oxygen to burn the fat and carbohydrate that makes you go. The good news is that they increase in number and activity, by as much as 50%, in just a matter of days to weeks in response to regular aerobic exercise in adults of all ages.
Burn, baby, burn
I mentioned that fat and carbohydrate are the fuels our muscles burn. The difference between them is that fat is high-test; it contains 9 calories per gram whereas carbohydrate has only 4, and so you get more energy and can go farther on a gram of fat than on a gram of carbohydrate. You want to burn fat because it’s such an efficient fuel, plus it’s nice to lose some of your excess fat! The catch is that you need more oxygen to burn fat because it’s denser than carbohydrate. The good news is that your body gets better at using oxygen and burning fat when you do regular aerobic exercise; like I described, your heart pumps more blood, your muscles consume more oxygen, and you have more mitochondria.
How much aerobic exercise do you need to gain the benefits?
There are two physical activity guidelines in the Unites States. The first, the Surgeon General’s Report on Physical Activity and Health, is a lifestyle recommendation. That is, you can modify it to fit into your daily routine and activities of daily living. The recommendation is that all adults should accumulate 30 minutes of moderate-intensity activity on most, if not all days of the week. The key words are “accumulate” and “moderate-intensity.” Accumulate means that you can do 10-15 minutes at a time and repeat that a couple of times throughout the day; for example, 10 minutes in the morning, 10 minutes at lunch, and 10 minutes around dinner. Moderate intensity is equivalent to feeling “warm and slightly out of breath” when you do it. Recently there has been some controversy about the effectiveness of this guideline and its benefits. At the moment the recommendation stands, but we may hear more about it in the not-too-distant future.
The second recommendation is from the American College of Sports Medicine. The ACSM recommends 20-60 minutes of continuous aerobic activity (biking, walking, jogging, dancing, swimming, etc.) three to five times a week, at 60%-90% of maximum heart rate, and two to three days of resistance training. This is a more formal, “workout” recommendation, although you can also accumulate the more intense workout in bouts of 10-15 minutes throughout the day if you like. Follow this recommendation and your aerobic fitness and your health will improve.
Which one you choose is a personal choice. They are not intended to compete with each other but rather to provide options and maybe even complement each other. For instance, the Surgeon General’s recommendation may be more practical for individuals who are unwilling, or unable, to adopt the more formal ACSM recommendation. Of course, there’s no downside to working out regularly with aerobic exercise and also becoming more physically active as per the Surgeon General (take more stairs, mow the lawn by hand, park far away from the store and walk), so combining them might be a good decision.