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VO2 Max, Lactate Threshold, and the Body’s Energy Systems

Published March 27th, 2009 in BAMFing, Endurance, Training Science

For the beginner in multi-sport, or any endurance sport, the terms VO2 max and lactate threshold will inevitably become a part of his or her vocabulary. For most, it easy to be confused about what exactly each one is, and how they relate to training. Although it will involve a certain amount of scientific terminology, I will try to give you a better understanding of what these two terms mean, how they are important to you, and how the body creates energy for you to perform at your best.

Vo2 max is of particular importance for endurance athletes. It is defined as the maximum volume of oxygen the body can consume during intense exercise, or physical exertion. More to the point, it’s the maximum amount of oxygen from what you inhale, that the body is capable of converting to energy for use by the working muscles. (Each muscle cell uses oxygen to convert food energy into ATP for cellular work.)
It is measured either in liters per minute (L/min) or milliliters per kilogram of body weight per minute (ml/kg/min).The second method takes into account differences in body size and thus is more commonly used. If your body were a car, VO2 max would be described as the size of your engine. Based on genetics, or let’s say manufacturing, some people will have a higher VO2 max than others. The good news is that VO2 max can be trained and increased. (Think of it as upgrading to a higher performance engine.)The bad news is that there is a limit to the size of the engine that will ultimately fit.Genetics will limit maximum oxygen consumption. Whereas, you might come out of the factory with a VO2 max of 40 ml/kg/min, with proper training you could improve to 45-50 ml/kg/min. However, a champion endurance athlete can have a VO2 max of 70-85ml/kg/min!

So what does that mean for those of us who were not born a Ferrari?

Well, it means that we have to train to use as much of our maximum capacity as we can.Engine sizes being equal, the one who gets the most out of what they’ve got is the one that wins the race! Steve Prefontaine captured it best when he said, “Most people run a race to see who is the fastest. I run a race to see who has the most guts.” I know he wasn’t really referring to the same thing, but I think it works for what I’m about to say.

This brings us to what we call our lactate threshold (LT). Defined, lactate threshold is that speed of movement, intensity level, or percentage of VO2 max at which there is a specific blood lactate level, or where we see the onset of blood lactate accumulation (OBLA). Basically, the body is producing more lactic acid than it can get rid of in a timely fashion. In untrained individuals, LT can begin at 50-60% of VO2 max. In trained athletes, it can be from 60% to well above 70% of VO2 max. Many elite athletes are at 80% or more! Several studies have shown that lactate threshold is a better indicator of performance than VO2 max.
So, what exactly is lactate, or lactic acid, and why does it affect your performance? To answer this we must look at how the body creates energy for muscular activity.

I’m going to try not to be too scientific as I mentioned at the start, but I can’t make any promises…

The human body gets its energy from food, much the same way a car uses gas for fuel. The body breaks down carbohydrates, fats, and to a lesser extent, proteins into usable energy so it can perform work. In order to create this energy, food must be converted into molecules called adenosine triphosphate (ATP). Energy stored in ATP is used to power muscular activity. How the body creates ATP is determined by the energy needs or intensity level of the activity it’s performing.

There are 3 “energy systems” that the body uses to create ATP. 1) the Phosphagen system, 2) Glycolysis, and 3) the Oxidative system.

Since we are talking about energy production for endurance, and the phosphagen system is an anaerobic process (occurs without oxygen), and is used for very high intensity and very short durations, I’m going to skip it. There’ll be no drag racing in this article! I apologize to all of you “funny cars.”

Before I begin with the next two energy systems, it’s important to know that at no time during exercise or rest, does any one system provide the complete supply of energy. Depending on the intensity and duration of the activity, each system is contributing to ATP production.

The oxidative system (aerobic – occurs with oxygen) is the main source of ATP at rest and at low intensity activities.

It uses mostly carbohydrates and fats for energy production. At rest, it’s approximately 70% fats to 30% carbohydrates. As intensity levels increase, it changes to primarily carbohydrates, as long as there is enough supply. However, during long, submaximal, steady state exercise, it gradually shifts back to fats.

During fat oxidation, fat, or after being broken down by an enzyme, free fatty acids enter the mitochondria of the muscle cell where they undergo beta oxidation and are converted to acetyl CoA and eventually, a very high amount of ATP. Don’t worry too much about the scientific terms; just know that fat uses oxygen to create ATP (energy).

For carbohydrates, things work a little differently.

This is where it gets interesting. Remember what I said before beginning this paragraph, no system works alone.

When you eat carbohydrates, they are converted to glucose (sugar) for immediate use by the muscles, or glycogen. Glycogen is just a stored form of long glucose molecule chains. This process, called glycolysis, is where the oxidation of glucose and glycogen begins. That’s right, the other energy system! During glycolysis, glucose goes through a series of enzymatic reactions in the cytoplasm (cell fluid) of the muscle cells. Basically, it’s broken down from one glucose molecule into two pyruvate molecules which go on to produce ATP. Now, depending on the intensity level of the exercise, pyruvate can go one of two ways: 1) slow (aerobic) glycolysis or 2) fast (anaerobic) glycolysis.

By now, you know that aerobic means with oxygen, and anaerobic means without oxygen. I’m sure you know that at higher intensities, where energy demand is great, the body works anaerobically, and during lower intensities, where energy demand is smaller, the body works aerobically. If you were really paying attention, you will also know that the body is capable of doing both at the same time. So, let’s see where this plays out.

If there is enough oxygen present in the mitochondria of the muscle cells, the two pyruvate molecules created from the break down of glucose, are transported to the mitochondria. There they are converted to acetyl CoA and eventually ATP. This is slow glycolysis.

On the other hand, if there is reduced oxygen availability in the muscle cells, fast glycolysis occurs. During fast glycolysis, the pyruvate molecules are converted to lactic acid and eventually the salt lactate. Although this process creates energy fast, it leads to muscle fatigue due to high concentrations of lactic acid and a decreasing cellular pH which inhibits the enzymatic activity of the cell’s energy systems. Going back to the car reference, there’s too much viscosity in the engine oil, preventing it from running smoothly. Maybe you can also say that there’s too much junk in the trunk.

Okay, now you should have a basic understanding of the body’s energy systems and where lactic acid comes from. Let’s get back to lactate threshold and how it affects your performance.

As your body is performing exercise, the muscles involved are using carbohydrates and fats to create ATP/energy to keep you going. Some of the muscles involved are also creating lactic acid because they do not have enough oxygen present for the mitochondria to create ATP alone. At this point, the lactate is at a level that body can easily handle and it can be reconverted to pyruvate and used to create more ATP.

As you increase your intensity level, your energy demand also increases. Once the amount of lactate in your blood increases faster than it can be cleared, or reconverted to pyruvate, you have reached the onset of blood lactate accumulation (OBLA). This is what you know as your lactate threshold, or LT. This is the curse of the endurance athlete. Once you reach your LT, your intensity can only be sustained for a short duration. Depending on how high above your LT you are, will determine how long you will last.

Fatigue that occurs at exercise below LT is usually caused by low levels of carbohydrates or dehydration, but that may take hours to occur depending on the individual.

Fortunately today, you can find a number of places to be tested for your VO2 max and LT. Usually this will require a maximal effort test with breath analysis, and blood analysis, to determine at which heart rate you reach your LT and VO2 max. From there you can use the information to create a training program to improve your performance, or find a good coach to help you do so.

One thing is clear, however, that in order to improve your performance, regardless of what size engine you have (VO2 max), you will need to increase the level of your LT. This will require some training at elevated levels of blood and muscle lactate.

Oh, just one more thing before I go. For you multi-sport athletes, your LT will be different for each discipline due to the different muscles involved.

Good Luck! JB