Aerobic Exercise: Benefits
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, 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.
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 minute.
- 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 carbohydrates. 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.
What is an example of a daily home aerobic exercise program?
My suggestion for getting started with an exercise program is almost always the same. Keep it simple, keep it practical, keep it convenient, keep it realistic, keep it specific, and don’t try to make up for years of inactivity all at once. Select any activity and amount of time where the probability of sticking with it is high. You may not love walking, but if you can do it right outside your door, and it requires no special equipment, and you already know how to do it (you’ve been walking your entire life!), then walking might be your best bet for getting started because it’s so convenient.
What I mean by “be specific” pertains to setting exercise plans. Planning is helpful for behavior change, and I suggest that you set goals each week. I suggest writing down what day(s) of the week you’ll exercise, what time of day, minutes of activity, location, and the activity that you’ll do. Be as specific and realistic as possible, and remember that it’s not how much you do when you get started but that you simply get started (getting started is usually the hardest part).
A sample plan might look like this:
- Monday: Take a 20-minute brisk walk at 7 a.m. around the block four times.
- Tuesday: Do the same as on Monday.
- Wednesday: Take a 20-minute walk at 6:30 p.m. (right before dinner) around the block.
- Thursday: Take the day off.
- Friday: Take a 20-minute walk at 7 a.m. around the block.
- Saturday: Walk with your family at 10 a.m. for 45 minutes in park.
- Sunday: Bike ride with your spouse for 60 minutes in park at noon. Mom will baby-sit.
Another plan I like is the five-minute out, five-minute back plan. Just like it sounds, you walk for five minutes from your starting point, turn around, and walk back. It’s simple and doable for almost everyone. It’s a change in your activity behavior even though it’s not all that much, and you can increase as you get more used to it. From five minutes you could go to seven and a half out, seven and a half back, a total of 15 minutes just like that. And you can keep your eye on 15 out, 15 back, and there you go meeting the Surgeon General’s recommendation of 30 minutes. If you’re feeling ambitious, you can add some abdominal crunches and push-ups once you get back. For push-ups, if you can’t do a standard one on the floor, modify them by leaning against a wall, leaning against a table, or on your knees on the floor. The lower you go the harder they are. Start with two to three sets of crunches and push-ups, 12–15 repetitions, three to four days a week. As they get easier, you can increase the intensity of crunches by going slower or putting your legs in the air with your knees bent. As push-ups get easier, you can go to the next lower level (for example, from wall to table to on your knees on the floor).
I suggest keeping records of your weekly progress by writing down what happens, or at least checking off that you followed through, and then setting your weekly plan every week for at least three months. Then at three months, you can evaluate your progress and see if any changes need to be made. How will you know if you’re ready to stop setting weekly goals each week? Ask yourself if you believe you will be exercising regularly in six months. If the answer is “I’m not sure,” or “no,” then you ought to continue to set weekly goals. If you are confident that you can maintain the behavior and will be exercising in six months, then you may not need to set weekly goals, but at the first sign of slipping, you ought to go back to it.
Most of us who exercise regularly understand that exercise can elevate our mood. There have been a number of studies investigating the effects of exercise on depression. In one of the most recent studies, it was shown that three to five days per week for 12 weeks of biking or treadmill for approximately 30 minutes per workout reduced scores on a depression questionnaire by 47%. It’s not a substitute for therapy in a depression that causes someone to be unable to function (in which case medication and/or psychotherapy may be necessary), but for milder forms of depression, the evidence is persuasive that it can help.