Energy Pathways, A Brief Introduction

I find this topic to be very interesting. If you have never heard of an energy pathway, it is simply the term to describe how your body produces energy. In other words, how does your body take food and turn it into energy? There are three ways the body does this, each is known as a “pathway”. Most people know that our body needs food, or calories, in order to function, but the discussion about energy pathways gets a little more into the weeds. This article will explain in more detail, the three ways in which the human body converts foods, or substrates (think “fuel”) into usable energy for the muscles. If you think about all human movement as a degree of muscle contraction, then this article is about the fuel that feeds those contractions. Remember the first law of conservation of energy? Energy is neither created nor destroyed. So let’s take a look at how the body finds the energy to move.

The human body is a highly complex organism. It is so complex, that in order for human science to attempt an understanding of its many functions and interdependencies, the body’s internal behavior and processes are separated into systems. It should be noted that the systems are not entirely independent of each other, and although we often study them in exclusion, they each operate simultaneously in response to stimulus. Regarding metabolism, there are three “pathways” in which energy is generated in the human body. The three pathways are different in the substrates used and the processes by which these fuels are converted into muscle contractions. Although some resources may use slightly different terminology, the substrates used, duration and intensities associated are standard. The three pathways are the ATP-CP, glycolytic, and oxidative.

Rowing is a great example of an exercise that can utilize all of the energy pathways when performing high intensity intervals separated by low intensity “rest” periods.

The first pathway is commonly known as the ATP-CP pathway, which is short for adenosine triphosphate (ATP) creatine phosphate (CP). This pathway is also known as the phosphagen system (UESCA, 2016). This pathway provides immediate energy for a very short duration in response to high intensity demands by “splitting” CP bonds through a process called hydrolysis to create ATP. ATP-CP provides energy anaerobically, meaning it does not utilize oxygen to produce ATP for muscle contraction. Example exercises that work the ATP-CP energy pathway are a back squat “one rep max” effort or a max effort vertical jump.

The second metabolic pathway is the glycolytic pathway. This pathway is also anaerobic, and as the name implies, uses glucose in the blood or glycogen stored in the muscles and liver to convert adenosine diphosphate (ADP) into ATP for energy production. As glucose and glycogen are used, pyruvate and hydrogen ions accumulate in the muscles, creating an acidic environment. Lactate is produced and absorbs some of these hydrogen ions. The presence of lactate is the body’s effort to clear hydrogen ions, but as exercise intensity and/or duration increases and hydrogen ions can no longer be cleared properly, this causes the sensation of muscle “burn” (Fitzgerald, 2011; Yu, 2016). Examples exercises that stress the glycolytic pathway include 100m & 200m sprints, or a high-rep set of barbell thrusters.

The third and final pathway is called oxidative. This pathway is aerobic as it utilizes oxygen to produce ATP. More ATP (energy) is produced in the oxidative pathway but it takes much longer to generate. This pathway is also unique because it uses fatty acids to produce energy, but as intensity increases, the percentage of fat utilization decreases as the body prefers carbohydrates. The oxidative pathway is the primary working pathway for long duration, low intensity exercises such as endurance running. Example exercise to stress the oxidative pathway include extended endurance events such as swimming, cycling, and running.

The oxidative pathway would be the focal point for marathon training and beyond. Due to the relative low intensity and long duration of a marathon, the oxidative pathway is the most efficient producer of energy. However, this does not mean that an athlete can neglect the other two metabolic pathways in training for a marathon. A successful marathon training program uses periodization to ensure that proper stresses and recovery times are optimized for proper adaptation. This means conditioning each of the energy pathways so that when they are called upon during the race event, they are in prime condition and ready to fire. The ATP-CP pathway is crucial for that final kick to the finish line, while the glycolytic system must be developed to deal with hills or terrain that might mean an excursion into a higher intensity during the marathon. The athlete’s aerobic (oxidative) base capacity must be highly developed and efficient as a foundation upon which each pathway can play its unique role.

There are several ways to approach a marathon training program, and there is no “right answer”. Training plans must be tailored to fit the unique needs of the individual athlete. With that said, there are some fundamental principles that should be adhered to when designing a marathon training program. In general, a large amount of time (roughly 80 percent) will be spent building the athlete’s aerobic base by prescribing long slow distance runs. These are low intensity and will provide the many benefits of aerobic conditioning. The other 20 percent of training will be dedicated to higher intensity running, such as tempo runs, lactate threshold runs, fartleks, and strides. These runs incorporate more of the glycolytic and ATP-CP pathways and can help the runner gain speed and strength to endure hours of running. A good marathon training plan must also include some strength work, mobility exercises, and dedicated recovery days. By incorporating a mix of volume, intensity, and frequency of workouts, an athlete will be prepared to deal with the demands of a marathon.

Understanding energy pathways is crucial to being a good coach and trainer. Anyone can find workouts online or from a book, but those who have a foundational knowledge and understanding of how the body metabolizes energy will be able to tailor a training program to an individual, prescribe workouts that allow for effective stress and adaptation, and will be able to explain “the why” behind the program when a client asks for more information.

 

References 

Fitzgerald, Matt (2011). The Lactic Acid Myths. Running Competitor. 27 Jan. Retrieved from http://running.competitor.com/2010/01/training/the-lactic-acid-myths_7938

United Endurance Sports Coaching Academy (UESCA, 2016). Running Coach Certification. Course Content. https://coachendurancesports.com/certifications/running

Yu, Christine (2016). Fuel, Not Foe? The Truth about Lactic Acid. The Daily Burn, 18 July. Retrieved from http://dailyburn.com/life/fitness/truth-about-lactic-acid-lactate/

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