Going the Distance

It’s true. Some people can go into an endurance event completely untrained and make it through without dying. Just ask my friend Wes who was introduced to running by being thrown, untrained, into a half-marathon race because he lost a bet. Wes was a pack-a-day smoker, fast-food aficionado, and sloth. It wasn’t pretty, but he finished that half marathon, promptly lighting a cigarette after crossing the finish line. (Tsk, tsk.) Let’s be real though, that is not the norm and it most certainly is not sustainable. Wes quit smoking, changed his diet (a little), and started training. Today, Wes is an ultramarathoner currently training for his fourth 100-mile race.

To complete an endurance event with a low risk of injury and a high chance of completion, you better train. The body adapts to physiological stresses, so it is better prepared to take on the load again and again, becoming more economical and proficient and less prone to injury. As the body becomes more efficient, performance increases, and after a while you are able to bump up training to further improve performance. But you must give the body time to adapt!

Let’s start from the bottom: the body’s reaction to exercise. There must first be a stimulus before there can be a change. The body is a network of systems that keep you functioning, and they adapt to outside stimuli. Exercise is a physical stressor on the body. This stressor can cause many adaptions within the body’s systems from increased strength and reduced body weight to increased coordination and improved aerobic efficiency. Check Sidebar I (bottom of page) for a simplified scientific breakdown of how the body adapts to exercise!(1)

All the body’s systems share the function of producing energy. Adenosine triphosphate (ATP) is the chemical energy your body uses to fuel physical activity. Your body breaks down fats, carbohydrates, and sometimes proteins to create ATP. Your body produces this ATP through three different metabolic systems: lipolysis, gluconeogenesis(2), and glycolysis (aerobic and anaerobic IIa/IIb). A brief explanation of these systems is provided in Sidebar II (bottom of page).

These three energy systems work together to make the energy required for exercise. The involvement of each energy system depends on the intensity of the exercise.(3) Lipolysis can sustain physical activity for 10—90 seconds. Anaerobic glycolysis IIb can sustain high-intense physical activity for 10—120 seconds. But aerobic glycolysis, anaerobic glycolysis IIa, and gluconeogenesis can sustain long bouts of exercise. Their ability to sustain it and how long they can sustain it depends on you. The efficiency of your movement, your lactate threshold, and your muscles’ max capacity for consuming oxygen all play a role in how much energy you expend during activity. Luckily, they can all be trained. Training movement efficiency, lactate threshold, and VO2max can help you get the most out of your metabolic energy systems, ultimately improving your endurance and performance.

Movement efficiency

When a movement is repeatedly patterned by the central nervous system, the energy it requires to recruit the necessary muscles to complete that pattern lessens. Why? Because now that movement is habitual—no thought needs to go into it. But you need to make sure you’re patterning the movement correctly. Did you know there is a ‘correct’ way to run? Books have been written on it! In fact, there is a most efficient way to run, bike, swim, walk, and to do all other things. This most efficient way to move is where the body expends the least amount of energy with the least amount of injury risk. So, if you want to improve your race times, get your form checked and corrected! Once you have the form down and sufficiently patterned by the central nervous system, you can further train it.

When training movement efficiency, think volume. Build volume by increasing it about 10% each week. Building volume at an easy pace--whether it’s biking, running, swimming, or something else requiring endurance--will increase the number of mitochondria within your muscles, the number of slow-twitch fibers being recruited within your muscles, and how well your body utilizes fat for fuel.

Going an easy pace will keep you below the lactate threshold (which we’ll talk about more in a moment), so you will still be utilizing aerobic metabolic systems which use oxygen for ATP conversion. This is important because the mitochondria are what make that conversion happen: They use oxygen to convert the nutrients from what you eat into ATP. This is why they are called the powerhouses of the cell. The more mitochondria in your muscles, the more energy your muscles can store and, therefore, have to pull from. So, building volume below the lactate threshold is the stimulus your body needs to signal the production of more mitochondria to keep up with the increasing demand.

Slow-twitch muscle fibers are important because they are what are most utilized in endurance exercise. These fibers have a greater oxygen capacity and so can support your movement for the long haul. Therefore, increasing the health of these fibers will further increase your movement efficiency, because the more oxygen they can store, the longer they’ll be able to support you. Another way to train slow-twitch muscle fibers, in addition to building volume, is through weight training to increase the max strength of your muscles. This will decrease the percentage of max strength required for each contraction, which will stave off the recruitment of fast-twitch muscle fibers and the associated fatigue.(4)

Glucose is stored within your muscles as fuel. However, your body can store only enough glucose to fuel about 100 minutes of marathon running.(5) Therefore, increasing volume will help your body become more efficient in using fat as a fuel source in an effort to preserve some of the muscles’ glucose stores. The body uses fat as fuel through lipolysis, breaking down chains of fatty acids to convert into ATP. Lipolysis is your main energy producer while at rest. Over time, as your training volume increases and your movement efficiency increases, your body will be able to tap into this “at-rest” metabolic system more. No matter how lean, every person has almost unlimited fat stores. So, train your body to use them!

Lactate threshold

The ATP conversion process results in the byproduct of pyruvate which, if not used by the mitochondria, is immediately converted to lactate. In low-intensity exercise, lactate is removed at the same rate at which it is produced. Lactate build up leads to a buildup of hydrogen ions in the muscles that results in fatigue. This lactate/hydrogen build up occurs when you exceed the lactate threshold, which is the point of highest exercise intensity you can sustain before lactate production exceeds its rate of removal.(5) In other words, this threshold is the top most bar of aerobic metabolism. Beyond this bar, the body moves into anaerobic metabolism, ATP production without the use of oxygen, which is not able to sustain exercise for very long. To move the lactate threshold bar higher, resulting in higher exercise intensities within the aerobic energy system, you must increase the mitochondrial density and health within your muscles.

Increasing the health and number of mitochondria within your muscles will increase the ability and duration of mitochondrial respiration (aerobic metabolism), which will lower the amount of lactate production at a given intensity.(3) This means that you will be able to sustain a greater intensity of exercise while remaining in the aerobic metabolic systems. Like with training movement efficiency, increasing mitochondrial density within your muscles is done through volume.

Once you have a strong base, you can begin to incorporate long training sessions at lactate threshold. However, it is important to not go beyond your lactate threshold. So, utilize the intensity measurement of rate of perceived exertion (RPE) scale. Studies have found that RPE is related to the lactate response to exercise and that the lactate threshold occurs between 13 and 15 on the RPE scale, which is when the exercise feels “somewhat hard” or “hard.”(3) Do long intervals at your lactate threshold, and over time the RPE will lessen and so the intensity can be bumped up a little to your new, higher lactate threshold!


VO2max is the max volume of oxygen your muscles can consume per minute. The more oxygen your muscles can consume, the longer it takes for lactate to build up and the associated fatigue to set in. VO2max is largely determined through genetics, but it can be improved between 5-20% with training.(4) Increasing VO2max can be done with long intervals at high intensity (~90% VO2max). It can also be done with short intervals at high intensity but with shorter rest periods between repeats. Training VO2max has also shown to increase movement efficiency and the lactate threshold.(6) Win-win-win!

This was a lot to take in. But before you run off and start doing all the volume, all the strength training, and all the high-intensity intervals, take a deep breath. Improving your endurance and your performance takes time. So be patient! Do not rush the process! Rushing the process will lead to injury, overtraining, and/or missed personal records. Start from the beginning. Gain a strong base with increasing volume, then start slowly incorporating lactate threshold training and strength training, and then slowly add in those high-intensity interval training days. It will take years of dedication and hard work, but I promise you, it’s worth it.


The Systems of the Body and Their Adaptations to Exercise(1)

The body is a network of systems that keep you alive and functioning. These systems adapt to outside stimuli. Exercise is a stimulus that can create many changes within the body’s systems, leading to a stronger, healthier version.

Lifting weights creates stress within the body which then adapts by becoming stronger to handle the added stress. The mechanical stress of weightlifting causes microtrauma to the muscle which will then begin the process of repair, resulting in stronger muscle fibers. Progressively increasing the load will make the body continue to grow stronger. Failure to maintain a consistent strength program will cause the muscles to begin to atrophy, losing their ability to generate the force they were once able to. Lifting weights will also increase bone and joint strength, because the skeletal system will also adapt to the physical forces applied to it. Bodyweight exercises, or the use of lighter loads, will build strength but will also allow you to safely perform multidirectional movements that will strengthen the body’s fascia and connective tissue. As your body adapts to an exercise program, the exercises must also adapt to the body’s growing strength by becoming more intense as you move through the program. Intensity can increase by increasing load, repetitions, and/or sets; and/or by increasing speed and/or complexity. Remember though, you shouldn’t change more than one variable at a time to reduce the risk of injury and overtraining.

All this movement can’t be done without your central nervous system (CNS). The CNS identifies a movement pattern and then recruits the necessary muscles to complete the pattern. Exercise programs focusing on movement patterns help develop and fine tune the connection between your CNS and your muscles, allowing the patterns, over time, to occur naturally with little to no effort. (Ever hear of “muscle memory” and “the mind-body connection”? Yea, that’s that.) The CNS also reacts to the environment. It’s what tells your stabilizers to be on high alert when you walk on sand and then it’s what lets your stabilizers chill out when you’re back on solid ground. This is what they call proprioception. Progressively increasing the intensity and complexity of the trained movement patterns will continue to strengthen the connection between your CNS and your muscular system, which will, over time, increase the speed at which muscles are recruited, synchronized, and activated. Done properly, your coordination and power will increase.

But before you get carried away, remember that you must breathe, eat, and sleep too! In addition to the structural network of bone, muscle, fascia, and connective tissue; and the electrical network of the CNS, you have the cardiorespiratory and endocrine systems that help produce movement and influence change in the body. The cardiorespiratory system moves oxygen throughout the body, aiding in energy production. The endocrine system regulates production of hormones which are directly responsible for many of the physiological changes that exercise promotes. Hormones, among other things, build and degrade muscle, metabolize and store fat, and regulate hydration and mood. The hormones listed below directly relate to exercise:

  • Insulin is used to promote the storage or absorption of carbohydrates, sugars, and fats. It also helps reduce blood sugar levels.

  • Glucagon helps elevate low blood sugar by releasing stored carbohydrates from the liver.

  • Cortisol helps metabolize fats and proteins and is responsible for the utilization of muscle proteins for fuel instead of damage repair if the physical stress has been too great or recovery has been insufficient. (Yes, you can cause more harm than good by exercising too much and/or recovering too little!)

  • Epinephrine and norepinephrine are separate but closely related. They both elevate the amount of blood pumped per minute, elevate blood sugar levels to fuel exercise, and aid in carbohydrate and fat metabolism. Norepinephrine also constricts blood vessels in areas of the body not being used in exercise so more oxygenated blood can reach those areas that are being utilized for physical activity.

  • Testosterone is responsible for muscle repair and growth in response to exercise.

  • Human growth hormone is responsible for, among other things, muscle growth, increase in bone density, proper immune system functioning, and promoting fat metabolism. This hormone is produced during your dream (or REM) cycle while sleeping and is released in response to highly intensive exercise.

  • Insulin-like growth factor promotes muscle growth by aiding the human growth hormone in the repair of muscle damage caused by exercise.

  • Brain-derived neurotrophic factor, in response to highly intensive exercise, helps initiate the production of new brain cells.

ALL of the body’s systems mentioned above share the function of producing energy. The body’s production of energy is commonly known as metabolism, which allows for sustained physical activity. Your body’s three metabolic energy systems are described in Sidebar II of this article, “Going the Distance.”


Your Body’s Three Metabolic Energy Systems

Your body requires energy, even at rest, to function. Adenosine triphosphate (ATP) is the chemical energy your body uses as fuel. There are three metabolic energy systems that produce ATP: lipolysis, gluconeogenesis, and glycolysis.

Lipolysis (also known as the oxidative system) is the main energy producer when the body is at rest. It converts fats and carbohydrates (but mostly fats) into ATP to keep the other systems of the body working while you’re sitting on the couch catching up on Game of Thrones. If your activity were to increase, this system would be able to supply energy for only 10—90 seconds, depending on intensity.

Gluconeogenesis helps maintain blood glucose levels during times of fasting, long stretches between meals, or during long bouts of exercise. This process occurs in the liver and kidneys, creating glucose from non-carbohydrate sources (such as lactate). It is a very expensive system because it takes 6 units of ATP to convert lactate to glucose, which, when converted to ATP through glycolysis, will produce only 2 units of ATP.(2) (Talk about taking 4 steps back for each step forward!)

Glycolysis is the main energy producer during exercise. It can break down glucose to produce energy either with the use of oxygen (aerobic) or without the use of oxygen (anaerobic IIa & IIb). Aerobic glycolysis produces ATP through mitochondrial respiration. Mitochondria are the powerhouses of the body’s cells. They use oxygen to convert the nutrients from what you eat into ATP. The more mitochondria in your muscles, the more energy your muscles have to pull from. Anaerobic glycolysis produces ATP without utilizing oxygen by converting glucose to pyruvate which then almost immediately turns into lactate. Anaerobic glycolysis IIb is the main energy producer of high-intense activity lasting 10—120 seconds. And anaerobic glycolysis IIa can be used either to fuel a sudden burst of intensity or to aide in fueling longer steady-state activity.


  1. Pete McCall, Smarter Workouts: The Science of Exercise Made Simple (Champaign, IL: Human Kinetics, 2018), 3-25.

  2. Chris Masterjohn, “Gluconeogenesis: Expensive, but Essential,” Chris Masterjohn PhD (August 19, 2017). https://chrismasterjohnphd.com/2017/08/19/29-gluconeogenesis-expensive-essential-mwm-2-29/

  3. Lance C. Dalleck & Len Kravitz, “Optimize Endurance Training,” IDEA Personal Trainer 14, no. 1 (2003): 36-42. (Accessed through The University of New Mexico.) https://www.unm.edu/~lkravitz/Article%20folder/optimizeendurance.html

  4. Jason Karp, “Endurance Training Research,” Personal Training on the Net (November 13, 2015). https://www.ptonthenet.com/articles/endurance-training-research-4024

  5. Jason Karp, “Chasing Pheidippides: The Science of Endurance,” IDEA Health & Fitness Association (September 30, 2008). https://www.ideafit.com/fitness-library/chasing-pheidippides-the-science-of-endurance

  6. “Exercise Economy / Economy of Motion,” Training 4 Endurance (2019). https://training4endurance.co.uk/physiology-of-endurance/exercise-economy/

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