[ Author's note: I wrote this for the First Strides group, and figured I could put it up here too in case anyone else is interested. It is geared toward beginning exercisers but the principles are smiliar for recreational to elite athletes.]

Aerobic Fitness

When we talk about people being “in shape,” or “fit,” the terms are vague. There are lots of different forms of exercise, and being fit for one does not mean that you are fit for another. For instance, an Olympic weight lifter might be just as fit as a marathon runner, but they are two different types of fitness. By running and walking, you are increasing your aerobic fitness.

Aerobic fitness depends on the body’s ability to use oxygen. You are aware that you breathe in air, use the oxygen, and exhale carbon dioxide. Whether you are at rest or running, your body is using oxygen. Oxygen powers the energy-producing reactions in the body. Running, you’ve probably found, requires a lot more energy than resting. Since you need more energy, you also need more oxygen.

How does the air that you breathe get to your muscles? It travels through your blood. Your blood has hemoglobin molecules in it - hemoglobin is a globular protein with four heme (iron) groups. Each of those iron groups can attach to one oxygen molecule (O2) and thus blood circulates from heart to muscles, filled with oxygen.

Blood is also responsible for taking waste products out of muscles. As the energy reactions happen, they produce lactic acid. You may have never heard of lactic acid, but you’ve certainly felt it: it is the nasty stuff that makes muscles “burn” during a lifting workout, or a sprint race. Lactic acid is always produced at low levels in the body. As intensity increases, so does the production of lactic acid. Since it’s an acid, it lowers the pH of blood. “Buffers” in the blood are responsible for keeping the pH steady, and they attempt to neutralize the acid. This process helps to make the carbon dioxide that you exhale. When you are breathing hard from exercise, it’s not so much that your body needs oxygen, as that it needs to get rid of carbon dioxide. That said, lactic acid isn’t totally bad. With enough oxygen present, it can actually be used for an energy source. It’s when you’re running really really fast and not getting the oxygen that the lactic acid starts to hurt. And contrary to popular belief, lactic acid isn’t what makes your muscles sore the day after a workout. Lactic acid concentrations return to normal within five minutes of exercise ending. (The soreness, if you’re curious, is because you’ve caused microtears in your muscles, and they have to be repaired).

Now you understand that you start breathing hard when you run because your body is trying to get more oxygen to your muscles, and trying to get rid of CO2. Aerobic fitness is determined by how much oxygen your body can use (transport large amounts of oxygen to muscles, and the muscles’ ability to use them (Pfitzinger 12)) at a moment in time. In a lab, it would be measured as VO2 – volume of oxygen consumption. The units are ml/kg/min. In other words, aerobic fitness is measured by how many milliliters of oxygen each kilogram of your body can use in one minute. Improving aerobic fitness, then, is all about improving oxygen consumption.

And you improve oxygen consumption by training. If you create a stimulus (running) such that muscles need more oxygen, adaptations in your body will occur so that you get that oxygen. To improve aerobic fitness, you need to spend at least 20 minutes, 3 times a week, exercising at a fairly hard level. Running should accomplish this goal.

There are four factors involved in oxygen consumption: maximum heart rate, stroke volume, hemoglobin concentration in blood, and the proportion of blood that is sent to muscles. Aerobic training increases stroke volume, or the maximum amount of blood your heart pumps with each beat. If you can pump more blood, you’re sending more to your tissues, and they get more oxygen. Training also affects the proportion of blood that goes to muscles – with training, your body gets used to diverting blood from
“temporarily unnecessary functions” (Pfitzinger 19) like digestion, and that blood can go to muscles instead. The other two factors – maximum heart rate and [Hg], you can’t change. Max heart rate is genetically determined, and the only way to increase [Hg] is to train at altitude (running up Cadillac doesn’t count).

So we know how to get more oxygen to the muscles, but more oxygen is useless unless we can, well, use it. We want to use it to release energy. The body’s form of energy is ATP, or adenosine triphosphate. This molecule is the “energy currency” of a cell – it is the usable form. We convert energy from food into glycogen and fat, both of which are stored in muscle and other body tissues. During exercise we want to get at that energy. Reactions occur in the mitochondria – the “energy producing factories” – little organelles in a cell. Training will result in larger mitochondria (bigger factories) and more mitochondria (more factories). It also increases the enzyme action in the mitochondria. Enzymes are catalysts for reactions. In other words, they make reactions happen faster. In other words, you’ve now got more little things inside your body to make energy, and they can make energy faster than they did before.

You’ll notice that as you run more, it gets easier. In the words of Greg Lemond, a professional cyclist, “it doesn’t get easier. You just get faster.” In other words, your body adapts to the workload (running) and develops mechanisms to handle it (increased oxygen consumption, increased stroke volume, increased mitochondrial density). It’s not easier. You are more aerobically fit.

Works Cited

McArdle, William D., Katch, Frank I., and Katch, Victor L. Exercise Physiology: Energy, Nutrition, & Human Performance. Sixth Edition. Lippincott Williams & Wilkins: Maryland, 2006.

Pfitzinger, Pete and Douglas, Scott. Advanced Marathoning. Human Kinetics: Illinois, 2001.