A colleague of mine has a high-school-age daughter who was recently working on a project for PE, having to do with types of cardio training. This generated some family debate about what constituted cardio (running, biking, swimming, etc.) and then the question arose: is cardio a muscle?
After we all got done laughing about this, I thought, what a great thing to write about. Because "cardio" is one of the most over-used and ill-defined words in the fitness universe, and it is hardly surprising that someone new to fitness concepts would need some clarification.
"Cardio" comes from the Greek word kardia meaning "heart." Anatomically, our hearts are the pumps that send oxygenated blood throughout the muscles, organs, and other tissues of our bodies, and send deoxygenated blood to the lungs. "Cardiac arrest" is what we call it when the heart stops. So technically, yeah - cardio is a muscle. But that's not what we really mean when we use this term in fitness.
Two essential systems of the body use the term "cardio." First is the cardiovascular system, which is the system comprising the heart and the blood vessels. Second is the cardiorespiratory system, which refers to the system comprising the heart and the lungs. The cardiorespiratory system is a sub-set of the cardiovascular system, because blood vessels permeate the spongy tissue of the lungs (which are nothing like balloons). Both systems are trained by what is commonly called "cardio" exercise, but the system receiving the most direct benefit is the cardiorespiratory system.
Muscles get oxygen from blood, which is delivered through the blood vessels from the heart. The heart gets oxygenated blood from the lungs. It is a heart-lung-heart-muscle-heart continuum. It is almost completely involuntary: the heart muscle complex beats without any conscious input from us, and we cannot stop it; and the lungs, while we can stop them temporarily, will start up again the second we pass out from lack of oxygen to the brain. The system never stops until you die.
By placing a load on the cardiorespiratory system, we stimulate healing, growth, and greater efficiency in that system. We place such loads by engaging in exercises which cause the body's muscles to demand more oxygen. In response, our heart rate increases and with it our rate of blood flow. Blood vessels expand to deliver the greater volume (and increasing the demand on them over time increases their fitness, too). The breathing rate also increases, because the amount of oxygen provided to the blood in order to feed the demand of the muscles fairly quickly outstrips the amount the lungs can supply just through ordinary respiration.
The goal of "cardio" exercise is to train the heart and lungs over time to deliver a higher volume of oxygen with greater stamina - meaning, you can exercise longer before fatigue. Athletes in the distance running and biking sports tend to be wiry in build, light-framed, and don't carry a great deal of muscle weight in proportion to their total weight. They also tend to carry very low levels of body fat. Their training is designed to deliver performance at a certain level - say, running at a pace of twelve miles per hour - for longer and longer periods of time.
Your average sedentary American has such poor cardiorespiratory fitness that s/he is substantially fatigued by walking a mile or climbing five flights of stairs. Forget about running a marathon: these people are subject to dropping dead of a heart attack by running to catch a bus.
Just for comparison, the averagely-fit person can walk a mile in twenty minutes; that's a pace of three miles per hour. So a top marathon runner is moving four times that fast. For a little over two hours.
I don't aspire to be a distance runner, but to me it is important to be able to keep up a pace of about twelve miles per hour for at least three minutes. Why? Because that's about how fast my feet are moving in a Viennese Waltz. It is also important to me to be able to hike or walk for at least three hours at a pace of three miles per hour. Because the best hikes are the ones that the average sedentary American drops out of at the one-mile mark.
These are two different fitness challenges, but they can be met the same way: by designing a program that requires short bursts of high demand with much longer periods of sustained, lower-level effort. It's called interval training, and is one of the most effective methods known in the fitness world.
You can do cardio intervals in a gym by working on the treadmill, with (for example) ten-minute walks at 3 to 4 mph alternating with two-minute sprints at 6, 8, or 10 mph. You can do strength-training intervals, by doing a set of reps at 50% single-rep max and then a single rep at 90% single-rep max. You can do swim or bike intervals by doing ten laps at an easy pace, and one lap at a full-out sprint. You can do barre intervals by doing ten demi-plies and then a grand plie.
Or you can do it on the dance floor by staying out there for a solid hour and doing every single dance - not just the slow ones. Obviously, I think that option is MUCH more fun!
When you more than double the demand you place on your muscles, within the framework of a given movement pattern, you create the tiny breakdowns that permit your muscles (and other tissues) to heal, grow, and increase efficiency during recovery.
That's an important point, by the way. Muscles don't grow or get stronger just because we want them to. If we do an exercise that is easy, our muscles may maintain, but will not increase, their strength and stamina. It is only when we increase the demand that they will respond by demanding healing/growth cells, getting tougher, and getting stronger. When we increase the demand, we are creating microscopic injuries. It is the mechanism of repair that produces growth.
Nutrition is also an essential part of cardiorespiratory fitness. Some people have a tendency to have inflammatory responses - they have many allergies, for example, a tendency toward headaches, arthritis, or carry a lot of belly fat. Feeding the healthy systems of the body with whole fruits & vegetables, healthy fats, and lean protein provides the nutrition needed for the repair of stressed tissues. A diet heavy in grains, starch, saturated fats and sugar, on the other hand, will contribute to inflammatory response and slow down the healing process.
So: people in training need to spend a little more time, money, and attention on their diet - and so do people who would like to be healthy enough to survive some training. Get used to putting the right fuel in, and then go burn some of it off. You'd be amazed how quickly you can gain fitness.
But you won't get fitter unless you increase the demand on your system. It's gotten too good at doing the bare minimum. Efficiency is important, but what will really get you healthier is shooting for maximum capacity. Jogging a mile is great, but being able to sprint that block to catch a bus - without dying - is even better.