Evolution

While you might not have tried to put it into words, all of us have some sense of the meaning of endurance. As a species, we are evolutionarily and genetically predisposed toward the endurance end of the athletic spectrum. Among mammals, we are relatively weak and relatively slow. At one end of the spectrum are animals predisposed to speed and power, like the cheetah, vastly faster than the fastest human. But on the endurance end of that spectrum, very few animals can match the human’s ability to cover very long distances at moderate speeds.

A prevailing theory among evolutionary biologists is that early hominids’ endurance allowed them to hunt and run to exhaustion much larger, more powerful, and faster animals. Our lack of hair and the ability to cool ourselves by sweating kept those early proto-humans from overheating like their prey. The resultant high protein diet is thought likely to have contributed to the development of larger brains in homo sapiens and other early hominid species, allowing these species to become tool makers. If this theory is true, your ability to read these words may be due to our species’ natural endurance.

Psychology

Therefore endurance may be one of the most defining human traits. It may explain not only your ability to read these words but also why you are drawn to reading them. Modern humans have largely outgrown our need for endurance for survival. But you, dear reader, are attracted by something deeply encoded into your genome, a part of our genome shared with those early humans who had to roam the savannah, hunting to live and living to hunt. We moderns have created substitutes for this deep-seated need, whether climbing big mountains, running through the alpine meadows, or skiing deep into the wilderness. These things scratch an itch that you may not know you have.

Meaning

The Merriam-Webster definitions of endurance are: the ability to withstand hardship or adversity and the ability to sustain a prolonged, stressful effort or activity. The root word “endure” implies struggle, suffering, overcoming, and perseverance. If you’ve ever run a long distance or climbed a big mountain, these words will have a more visceral impact on your emotions than the dry definition of endurance. Besides the essential human-ness of these endurance endeavors, there is something heroic about those who endure. What else can explain that the longest-running theme in human literature is the story of struggling against the odds? From the Odyssey of Ulysses through Ernest Shackleton’s epic journey on the aptly named ship Endurance, chronicled in the book by the same name, heroic examples of endurance, struggle, and perseverance inspire us to find our own inner hero. The oldest books of the Bible used mountains as a metaphor for enlightenment and overcoming.

Something has drawn you to the mountains.  We go to the mountains to challenge ourselves, to endure. We are seeking something and hopefully, we come back as a better version of ourselves.

Physiology

Now it is time to mention the tiny unsung hero of this story. What powers the muscle contractions that propel you for those long hours of enduring? Why can some people run or climb faster for longer than others? The answer is encoded in three letters; ATP.

It is that simple. The molecule ATP is like the gasoline the muscles use for contraction, thereby creating movement that propels you. ATP (adenosine triphosphate) is the product of the metabolic process in every cell of your body.  We’ll stick to its role in the muscle cells. The more and faster your metabolism can crank out ATP, the faster you can move. The more reliant your metabolism is on oxygen (aerobic), the longer you can sustain that speed. Endurance training is simply an organized method to enhance your metabolism, or in other words, to increase your muscles’ rate of ATP production.

While the word metabolism gets bandied about, especially in the world of diets and nutrition, it might need a little explanation. It can be described simply as the process whereby the chemical energy stored in the food you eat is used to put assemble this wonder molecule, ATP, which, when broken down into its constituent parts, fuels muscle contractions.

Metabolism Acts as an ATP Recycling Plant

If any one molecule can be singled out for its life-giving force, ATP is it. It is what allows life to be. ATP is a short-term energy storage molecule used by every cell in your body. The energy released when ATP’s chemical bonds are broken powers the functions of all cellular life.

To avoid running out of ATP, you might expect that we need a massive reservoir of the stuff. Where is that ATP fuel tank in the body? Here’s a fun factoid for your next dinner party. How much ATP does it take to power a runner through a marathon? Around 75 kilograms. That would be an awful load to have to carry. But our cells have an ingenious recycling mechanism called metabolism. Because of this efficient recycling process, we neither need nor have much storage of ATP. Instead, after its bonds are broken, releasing energy for our muscles to use, ATP is resynthesized from its component parts.

This recycling of ATP requires energy. That is supplied by the chemical energy stored in the molecular bonds in the food you eat. Breaking those bonds releases this stored energy. 

Extending the gasoline analogy, the faster your muscles can resynthesize ATP and dump it into the muscle/engine, the more work you’ll be able to do in a given time.

Endurance is improved by manipulating your metabolism through training.

The two goals of endurance training:

1. To increase the rate of the metabolic recycling of ATP Speeding up the metabolic process.

2. To increase the amount of oxygen used in the metabolic ATP recycling plant.

It is that simple. Helping you understand this process and how to put these simple ideas into action is the goal of the rest of this article.

Here we go.

Fuels for Energy

The recycling of ATP is handled via two distinct metabolic pathways.

One is known as the anaerobic or glycolytic pathway because it does not require oxygen, and the energy to fuel it comes exclusively from glucose (carbohydrates). This process occurs inside the muscle cell and results in the byproduct lactate. The accumulation of lactate presents a problem, as discussed below.

The other and most important for our discussion is known as the aerobic or oxidative pathway because it requires oxygen to proceed. It can utilize fat, carbohydrate, and proteins as fuel sources. This life-giving process takes place deep inside the cell in tiny organelles called mitochondria in a many-step process most commonly referred to as the Krebs cycle. They are so important that every cell in your body contains mitochondria. We’ll come back to mitochondria in a bit.

While these two processes are distinct, your muscles usually get ATP from a mix of these two systems. At a lower intensity, the aerobic path dominates. As exercise intensity increases, a point commonly referred to as the aerobic threshold (AeT) is reached when the aerobic and anaerobic metabolic pathways’ ATP contributions are about equal or 50/50 when measured as calories used to produce this ATP. As intensity increases above this point, the anaerobic path dominates the ATP production.

Note that what follows is a significantly simplified explanation of a very complex series of processes and is not meant as a rigorous look at either ATP production or metabolism. It is intended to give you a very basic understanding. 

The Anaerobic Pathway

Anaerobic metabolism has the ability to recycle ATP rapidly because the breakdown of the sugars in carbohydrates is a simpler process than the breakdown of fats.  That’s why this is the dominant energy pathway for high-intensity, high-power output exercise.  

Unfortunately, the anaerobic pathway has a built-in negative feedback feature that severely limits the total output of ATP (known as the anaerobic capacity). Along with the production of ATP is the production of a molecule called lactate (I am skipping over the pyruvate step). 

The faster the rate of glucose metabolism, the greater the lactate production. The lactate level in the muscle cell continues to rise along with intensity. As that lactate accumulates in the cell, it begins to gum up the works. Its accumulation beyond a certain point coincides with a slowing and eventual stoppage of the powerful anaerobic pathway. This is why you can’t sprint for more than about 20 seconds before you are forced to slow down. Now you know the mechanism at play that is gumming up the works.

As the intensity of exercise increases and lactate levels in the muscle rise, you cross another identifiable metabolic frontier known confusingly by many names (listed in the footnotes) but which has traditionally been called the Anaerobic Threshold. Above this intensity, the exercise duration is limited to a few minutes before “fatigue” sets in, and you are forced to slow. The higher-powered Fast Twitch muscle fibers that propel you at these higher intensities are almost entirely dependent on that anaerobic metabolism and glucose for fuel, lactate accumulation becomes a real problem at intensities above the Aerobic Threshold. All endurance athletes have experienced fatigue when the pace is too fast for too long. The only option is to slow down.

Now you understand that this form of fatigue, the accumulation of lactate,  is due to a reduction in ATP output by the same anaerobic pathway that is fueling the higher-intensity exercise. The linkage between lactate accumulation and reduced ATP, causing a slowdown, is a negative feedback loop.

Fortunately, there is a remedy for this situation.  It turns out that lactate is itself a valuable, in fact, preferential fuel for the other (aerobic) metabolic process muscles use for energy.  If an athlete can improve his or her ability to utilize the lactate being produced during higher-intensity exercise aerobically, the result will be improved speeds for longer durations.  This is accomplished by increasing the aerobic capacity of the muscles.

The Aerobic Pathway

Unlike anaerobic metabolism, the aerobic pathway can use carbohydrate, fat, protein, and after a conversion process, even the lactate mentioned above.  This metabolic flexibility is key not only to our endurance but to our dietary flexibility.  The greater the aerobic capacity, the more ATP we can produce using this pathway.  Unlike the anaerobic pathway, there is no negative feedback mechanism involved in aerobic metabolism.  The more an athlete can rely on the aerobic system to produce the energy needed for propulsion, the greater will be that person’s endurance.

Increased aerobic capacity is the foundation for increased endurance. Luckily aerobic capacity is one of the most trainable physical attributes contributing to endurance.  We’ll discuss how this is accomplished later.  Unfortunately, in the minds of many athletes, it is one of the most misunderstood concepts about endurance training.  As a result, many endurance athletes suffer from a condition known as Aerobic Deficiency Syndrome.  This deficiency will be the defining limit of those athletes’ endurance until that condition is remedied.

There are terms used to describe the aerobic system’s maximum capacity to provide most of the ATP needed. When you hear the term Aerobic Base used as in “She’s got a huge aerobic base,” that’s a not very scientific way of describing that aerobic capacity. Likewise, “He’s got a big motor” is another reference, often referring to aerobic capacity. Exercise scientists call the upper limit of aerobic capacity the Aerobic Threshold (AeT). When the upper limit of the oxidative pathway is exceeded, the anaerobic pathway must make up the difference between the ATP needed to power the exercise and the ATP supply available from the aerobic pathway.

The body’s store of fat calories is around 50 times that of glycogen calories (the stored form of carbohydrates).   Aerobic fat metabolism must dominate as the fuel of choice for long-duration exercise.

Besides the advantage that our fat gas tank holds nearly inexhaustible reserves of calories, there is another significant benefit of using mainly fat for fuel. It produces no lactate. That pesky lactate negative feedback loop disappears.

Adapting your muscles to use fat for fuel has two important benefits.

A big fuel tank

No lactate production

The Holy Grail of Endurance Training

For short-to-medium-length events (2 minutes to 2 hours), athletes are primarily speed limited by lactate accumulation and fuel stores. If there were a way to reduce the accumulation of lactate, you would be able to sustain a higher speed for longer.

Anti-glycolytic Training

Research has shown that you can’t do anything to lessen lactate production. That’s because glucose use and the accumulation of lactate go hand in hand with intensities above the aerobic threshold (AeT).

Read the following paragraphs carefully Twice!

The lactate removal rate from working muscles is highly trainable and is the main benefit of proper endurance training. Removing this lactate as fast as it is being produced will keep it from accumulating and causing fatigue. In 1984 George Brooks published a paper explaining the lactate shuttle, whereby lactate is removed from the working muscles where it is being produced and moved to other skeletal muscles, the heart, or the liver, where it is used for fuel.

So, how do you improve endurance? Counterintuitively, as it turns out: By increasing the aerobic capacity of those lowly, aerobically powered, mitochondria-rich Slow Twitch muscle fibers. For these fibers, lactate is a preferred fuel. The greater the aerobic capacity of your slow-twitch muscle fibers, the more lactate they can take up and use for energy. Using a vacuum cleaner analogy here might help. The slow twitch fibers are the vacuum sucking up the lactate. The bigger the vacuum cleaner (the higher the aerobic capacity), the faster and longer you can run, ski, or climb.

Here is the counterintuitive rub: To improve your endurance, the highest speed you can sustain for the time of your desired goal, be it a 5km road race or a multi-day climb, you must first maximize the aerobic capacity of those unsexy low-intensity, low-power slow twitch fibers. Without this base, no amount of strength training, high-intensity intervals, or Cross Fit will get you where you want to be with your endurance.

How is that done? THERE IS ONLY ONE WAY

The only way to increase aerobic capacity is with a high volume of training below your personal AeT. While there are other important components of endurance training, this is the heart of it and will make up the vast majority (around (80-90%) of your training time. You read that correctly. Top endurance athletes do as much as 90% of their training at this lower intensity.

Those with an underdeveloped aerobic base can find this intensity agonizingly, embarrassingly, frustratingly slow. It won’t always be that way. With proper training, as your aerobic capacity increases, so will your speed at your aerobic threshold and above. This process does not rely on a secret training method.  That’s why it is not hyped like so many other get-fit-quick schemes.  All it takes is time and patience to consistently train this way for weeks and months at a time.  

If it is now occurring to you that you have just stumbled on the Holy Grail of endurance training, you have understood. If not, re-read this physiology section S-L-O-W-L-Y.

– Scott Johnston

*Lactate Threshold, Anaerobic Threshold, Maximum Lactate Steady State, Onset of Blood Lactate Accumulation, and Functional Threshold are all names for essentially the same metabolic event, where lactate production exceeds its rate of removal. Physiologists love to split hairs of the various definitions, but for our purposes, these arguments are like debating how many angels can dance on the head of a pin.