The Science of Endurance: Unlock Your Full Potential

“A runner is a miser, spending the pennies of his energy with great stinginess, constantly wanting to know how much he has spent and how much longer he will be expected to pay. He wants to be broke at precisely the moment he no longer needs his coin” (Alex Hutchinson, Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance, 2018). Ever wonder why you hit a ‘wall’ during a long ride or run? Or how endurance athletes seemingly go forever? Endurance athletes have a deep understanding of how to fuel the body and manage their brain’s perceptions. Every athlete can benefit from understanding the energy systems of the body and how they connect to our brain. A proper fueling strategy that is uniquely tailored to you will help to optimize your training, races, and recovery.

We have three energy systems. These systems break down adenosine triphosphate (ATP). ATP is the energy currency of your cells. The first, the ATP-PC system, is the phosphagen system. This system uses stored ATP and creatine phosphate in your muscles for quick bursts of energy. Think: short-explosive efforts lasting up to 15 seconds. It is an instant production of energy. The second is the anaerobic glycolytic system (glycolysis). This system breaks down glucose from carbohydrates (carbs) without using oxygen to produce ATP. Think of efforts lasting 15 seconds to 2 minutes, such as a fast 400m run. Again, this production is quick but produces lactic acid as a byproduct, which can cause fatigue. The third system is the aerobic system (oxidative pathway). This system uses oxygen to break down carbs and fats to produce large amounts of ATP. It is a slow production of energy but efficient for long-duration efforts lasting greater than 3 minutes, such as steady-state runs or long bike rides. These three systems never work in isolation. Your body transitions between them depending on the intensity and duration of your workout. For example, at the start of a bike race, you may be pushing very hard and relying on ATP-PC to sprint off the start line. For the bulk of the race, your aerobic system is in charge, but during a short climb or surge in the peloton, the anaerobic glycolytic system may kick in. 

Above, I mentioned carbs and fats, the two key fuel sources for the energy systems in our bodies. Carbs are the most efficient fuel source for high-intensity endurance. Taking in carbs during exercise is important to keep glycogen storage high and avoid depletion (bonking). A good number to aim for is 90 grams of carbs per hour as you train and adjust accordingly. Glycogen is primarily found in your muscles and liver. Muscle glycogen fuels your working muscles and liver glycogen helps maintain blood glucose levels to keep your brain and body functioning. Proper fueling during prolonged bouts of exercise allows the brain and body to function optimally. It is also important to maintain hydration, as staying hydrated helps transport glucose through the body. If you are dehydrated, it will impair glycogen breakdown and reduce your ability to sustain effort. Use electrolyte mixes or sports drinks with sodium and potassium to ensure efficient carb absorption. Fats are the go-to source for long, low-intensity sessions. Fats are essentially unlimited, but they are not the first choice for fuel because they are energy dense, meaning it takes more time and oxygen to convert fats to ATP. That is why we need to incorporate training for fat adaptation and explore strategies to optimize energy use.

It is important to train all the energy systems. Incorporate endurance base training to strengthen the aerobic system and interval training for anaerobic performance. Oxygen is the game-changer. In the aerobic system, oxygen allows for the complete breakdown of glucose (from carbs) and fat into ATP. When you improve your aerobic fitness, your body becomes better at delivering oxygen to your muscles and using it efficiently to produce ATP. Zone 2 training, such as aerobic base building, comprises long, low-intensity efforts to train your body to utilize fat as a fuel source. You can also occasionally train fasted, in a glycogen-depleted state, to encourage fat oxidation. Fat adaptation allows you to preserve glycogen for critical moments in races and training efforts by improving your body’s ability to utilize fat at lower intensities. The anaerobic system helps you to surge, but you need to train it with intervals to manage lactic acid buildup in the muscles. Look at all aspects of your fueling pre-workout, intra-workout, and post workout. How do you fuel your endurance training? 

How does the brain play into all of this, is it the final barrier to endurance? While the energy systems fuel your body, Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance by Alex Hutchinson explores the crucial role your mind plays in determining how far and hard you can push yourself. Energy systems tell us how our muscles work, but Hutchinson argues that mental limits are often the true reasons we stop during long bouts of exercise. The central governor concept was first proposed by exercise physiologist Tim Noakes. This theory suggests that your brain acts like a safety regulator. It slows you down before you reach true physical exhaustion. Your brain monitors signals like muscle fatigue, glycogen depletion, core temperature, and heart rate. Rather than letting you push to the point of collapse, it reduces your perceived ability to continue as a form of self-protection. This theory is based more on perception than on actual energy depletion. This explains why you can always sprint at the end of a race. Your body still has resources left, your brain just has to let go of the brakes. Looking beyond this concept, the book explores various studies into all the vectors of what stops us from continuing on during exercise.

Glycogen depletion, lactic acid buildup, and ATP supply set real constraints on how long your muscles can sustain effort. Hutchinson argues that perception of effort, not just physical exhaustion, is what makes you stop. It would behoove us to understand fueling in regard to our perception of effort. Mental fatigue often coincides with low blood sugar and dehydration. Ensuring proper nutrition supports both energy systems and cognitive endurance.  If we perfect our fueling, we will perceive less effort, allowing ourselves to achieve true physical exhaustion, or close to it.

So, what are the practical takeaways from fueling and our brain's role in endurance? We can practice surpassing our perceived limits. This involves high-intensity interval training, pushing through the pain and building a higher pain tolerance to find a sense of calm during intense efforts. We can incorporate visualization techniques such as breaking down larger efforts into smaller chunks. Lastly, we can balance our training programs to include all zones of training (aerobic and anaerobic) and train the brain to tolerate monotony and discomfort. “...achievement is not possible without discomfort” (Hutchinson 2018).

In summary, your muscles may run out of ATP or your brain might convince you to stop long before that. The true art of endurance performance comes from maximizing energy systems efficiently through training and fueling as well as training the mind to push through perceived barriers. Overcoming mental barriers will allow you to push farther than you thought possible. The only limits we have are the ones we put on ourselves. Endurance isn’t just a test of the body; it’s a test of the mind. And understanding the relationship between the two is the key to achieving more.