Cardio commonly refers to any activity that uses your cardiorespiratory system (heart, lungs, arteries, veins, and blood) to produce and deliver the energy for movement. In general, any activity you sustain beyond three minutes must be primarily aerobic in nature. Aerobic training is the term commonly used to describe activity that uses oxygen as the primary source of fuel. A broader definition of aerobic training is activity that improves the body’s oxygen consumption, essentially making the cardiorespiratory system better at its job. Conversely, anaerobic training is the term commonly used to describe activity that does not use oxygen as the primary source of energy. In this case, the primary fuel source is carbohydrate.
Although the distinction between these two terms seems simple enough, nothing in them implies exclusivity. Aerobic training does not mean only with oxygen, just as anaerobic training does not mean only with carbohydrate. With a few extreme exceptions, you always use a mix of both. The terms simply define which source of energy provides the majority (51 percent or more) of fuel for activity. Too often, a given form of activity is presented as either aerobic or anaerobic. However, this either-or categorization of exercise may not accurately reflect what is happening in your body.
Most problems with our understanding of cardio, and exercise in general, stem from our curious tendency to oversimplify concepts with extreme terms. Given the wonder and complexity of the human body, just how simple can we really expect things to be? The two main misunderstandings are the fat-burning zone and intensity for effective workouts.
When you step onto any common piece of cardio equipment, you’re also stepping into a misleading concept—the existence of the fat-burning zone. The charts adorning the consoles of cardio equipment exist to provide some guidance on intensity during exercise. They typically show a graph with age labeled along the bottom and percent of max heart rate running up the vertical axis, and two colored bands. One is labeled the fat-burning zone and the other is called the cardio zone.
The design of these charts implies that you will only burn fat while working in the fat-burning zone. Likewise, you will only make your heart and lungs stronger while working in the cardio zone. As strange as it may seem, this ridiculous premise is based on truth. At lower intensities, you use more fat than carbohydrate for fuel. As intensity increases, your body continually adjusts the fuel mixture. Eventually, if intensity continues to rise, you will reach a point where your body uses more carbohydrate than fat for fuel. But what exactly does more mean?
If the premise that low-intensity activity burns more fat is carried out to its logical conclusion, the problems with this concept become glaring. According to this idea, you can burn a lot of fat by sitting on the couch all day. After all, sitting on the couch is the lowest form of activity. Your body uses calories even at rest, and in this case, most of the calories burned will come from fat. Of course, the problem is that you burn very few calories while at rest.
Here’s another way to look at this problem. Suppose you received a gift of 80 percent of a friend’s savings account. You’d be excited, right? If you then found out that your friend has only $100 in the bank, you might be disappointed. On the other hand, suppose you received a gift of 1 percent of a friend’s savings account. You’d hardly be impressed. However, if you found out a million dollars are stored in the account, you’d be thrilled with your cut of $10,000! The same logic applies to your body. If most of your calories come from fat and you only burn a small number of them, you won’t get anywhere in terms of fitness. To shore up your defenses against the folly of the fat-burning zone, remember that a big percentage of small number is still a small number.
A kernel of truth does exist in the myth of the fat-burning zone, and it involves how your body uses the energy it has. Energy use in your body occurs on a spectrum. When you are less active, your body burns more fuel from fat than from carbohydrate, but it burns less fuel overall. As you increase the intensity of your activity, your body adjusts the fuel mixture, first by upping the fat use, and then by lowering it. This process gives rise to the misconception that activity at lower intensities burns more fat.
Instead of doing more cardio, do better cardio. If your cardio workout is more challenging, your results will be better. The solution for stripping away unwanted body fat lies in interval training, which has gained popularity in recent years because it works. When you alternate bouts of higher and lower effort, you can continuously work harder within a given span of time. You can work out harder for shorter periods of time and get better results than you would with long workouts.
Every move you make draws energy from one of three sources: the phosphagen system (think of this as your rocket fuel), the fast glycolytic system (think of this as automobile fuel), and the oxidative system (similar to battery power). The first two systems are anaerobic in nature and the third is aerobic. Let’s take a closer look at each of these systems to understand how they provide the fuel for various types of activity.
- Rocket fuel (phosphagen system). This system dominates when you perform a short-duration, high-intensity effort. It produces maximum power (90 to 100 percent of your ability), but burns out quickly, lasting only between 10 and 15 seconds. If you keep going when this fuel source runs out, your body will forcibly decrease intensity and access the next energy system, essentially slowing down the rocket.
- Automobile fuel (fast glycolytic system). This is the dominant energy system for efforts of higher intensity (75 to 90 percent of your ability). It kicks in after 15 seconds of activity (after the rocket fuel is exhausted) and lasts at least two minutes, but can go as long as four minutes in elite athletes.
- Battery power (oxidative or aerobic system). This is the main source of energy when you perform at 75 percent or less of your ability. It dominates anytime sustained activity lasts longer than the three-minute limit on automobile fuel.
This information should demystify the idea of the exclusive fat-burning zone. Now you know what’s really going on. Your body uses carbohydrate and fat (with oxygen) through three energy systems to supply the fuel for activity. It simply adjusts the mixture, moment to moment, based on the physical demands of your activity. Your body is very smart.
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