The human body derives energy from food in order to fuel exercise movement as well as every essential bodily function. As ingeniously created machines, our bodies can manufacture energy in a variety of ways. Read on to learn about different sources of energy utilized for everything from sleeping to cycling to strength training, as well as the pathways required to create the fuel for all of our activities.
First Fuel: Nutrient Intake
Upon digestion and entrance into the bloodstream, carbohydrates, protein, and fat break down into simple compounds: glucose, amino acids and fatty acids, which get shuttled to various cells throughout the body. Within these cells, and from these energy sources, the body can generate adenosine triphosphate (ATP), used as a primary source of fuel. Our bodies utilize ATP for all body processes, including muscle contraction. Unfortunately, at any given time, the body houses a very limited quantity of readily available ATP; thus, it must constantly produce the substance. If not, all muscle contraction would cease entirely. Humans accomplish this re-synthesis of ATP via three energy systems.
Energy Explained
The body utilizes 3 unique systems to supply cells with the ATP necessary to fuel energy needs:
- creatine phosphate (ATP-PC), also called the phosphagen pathway
- anaerobic lactate (Glycolysis), also called the glycolytic pathway
- aerobic systems, also called the oxidative system
Most of the body’s activities use a continuum of all three energy systems working together to ensure a constant supply of energy. Although the body calls upon all three simultaneously, the intensity and duration of the chosen exercise dictates from which pathway it will draw the greatest percentage of its energy. We can likewise look at this a different way; the type of metabolism predominately used during physical activity gets determined by the availability of oxygen as well as the body’s need to utilize carbohydrates, fat, and protein. Therefore, the vast majority of athletes must train all three systems. We will delve more deeply into the functions of each process.
Anaerobic and Aerobic Metabolism
The human body draws not only from the carbohydrates, fat and protein in food; it also uses energy stored in the body to power through physical activity. These 3 key nutrients play an important role regardless of the intensity of the activity. Whether lying down and reading a book or running a marathon, the body will always require these macronutrients.
Since the nutrients themselves cannot produce energy, the calories contained within need to undergo processes and pathways in order to generate ATP. The body has the capacity to accomplish this with or without the presence of oxygen.
The Aerobic Energy Pathway
The aerobic energy system provides the foundation for the majority of the body’s ATP production. In the presence of oxygen, ATP gets manufactured through glycolysis. This system also involves the Krebs cycle and the Electron Transport Chain, a series of chemical reactions designed to generate energy in the mitochondria, known as the “powerhouse” of the body’s cells.
The most complicated of all of the body’s pathways, the aerobic energy system relies heavily on the circulatory system to supply oxygen. As such, this proceeds at a much slower rate as compared to the other two aforementioned pathways, and functions when one exercises at a very low intensity. The slow rate of production of energy relies on oxygen consumed/utilized with each breath. The aerobic pathway supplies energy for body movement lasting more than just a few minutes, such as long periods of work or endurance activities.
This system also serves to recover other energy systems during times of low intensity (rest periods between intervals). The body’s ability to recover between intervals of high intensity work reflects the functioning of the aerobic system.
Absence of Air: The Anaerobic Energy System
“Anaerobic vs Aerobic Metabolism” by Allison Calabrese is licensed under CC BY 4.0
Anaerobic metabolism occurs within the cytosol of muscle cells. As seen in the diagram above, cytosol can generate a small amount of ATP even without the presence of oxygen. Glucose, the sole source of fuel utilized during anaerobic metabolism, goes on to produce pyruvate and lactic acid. Pyruvate can also serve as fuel for aerobic metabolism.
As mentioned above, the fuel sources for both anaerobic and aerobic metabolism can change as a function of the amount of nutrients available as well as the type of metabolism ~
- Glucose: originates from blood glucose (dietary carbohydrates, liver glycogen, and glucose synthesis) or muscle glycogen; the primary energy source for both anaerobic and aerobic metabolism.
- Fatty acids: stored as triglycerides in muscles, but about 90% of stored energy resides in adipose tissue; As low- to moderate-intensity exercise continues using aerobic metabolism, fatty acids will emerge as the predominant fuel source for working muscles.
- Protein: while not considered a major energy source, the body can utilize small amounts of amino acids while at rest; the quantity utilized for energy metabolism increases if the total energy intake from the diet does not meet current nutrient needs, or if engaging in an endurance exercise. When amino acids break down, releasing the nitrogen-containing amine group, the remaining carbon molecule can get broken down into ATP via aerobic metabolism, or to make glucose.
How to Effectively Train Each of These Systems
When designing a client’s workout, keep in mind the sport in which he engages, and train the energy system along with the body. We will look at each energy system, what exercise utilizes which one primarily, and how to maximize the client’s potential.
- ATP-PC Energy System
The length of recovery between repetitions plays an important role in the recovery of one’s power output via the resynthesis of creatine phosphate. Studies indicate an 80% recovery in peak power output with a 1-minute recovery, and a 92% recovery after 3 minutes. Upon depleting the available ATP (through a 1-rep max squat attempt, for example), the body requires at least 3 minutes of rest for muscles to recover the maximum amount possible of ATP and creatine phosphate. After this prolonged rest interval, the ATP-PC system can once again perform explosive movements. To fully develop this energy system, suggest your client engage in 3 to 10 seconds of high-intensity work at near peak velocity. Lifting the heaviest weight possible for 1 or 2 repetitions will also work well.
- Lactic Acid (Glycolytic) System
The lactic acid system remains the most predominant energy system for events lasting anywhere from 10 seconds to a few minutes. It produces energy from muscle glycogen, the body ‘s storage form of glucose. Glycolysis can occur with or without oxygen. During a lack of sufficient oxygen, the series of reactions that transform glucose into ATP lead to the painful and familiar lactic acid “stitch in the side”, produced in the body’s attempt to generate additional ATP. The accumulation of lactic acid may induce fatigue as well as a burning sensation in the muscles.
- Improving the Lactate Threshold
We can define the lactate threshold as the intensity at which the blood concentration of lactic acid begins to increase exponentially. In training to improve the lactate threshold, particularly for track athletes, one may discover that muscles run out of endurance sooner than expected; this comes about as the muscles produce lactic acid very rapidly, in contrast to running at a slower and steady pace. Therefore, in order to build the ability to run faster without the early onset of lactic acid, one must train at a faster speed for longer duration, continuing until the aerobic and anaerobic endurance improve to a point whereby the body ‘s efficiency at producing the necessary energy adapts likewise.
If a client trains slowly, he will show an anaerobic threshold lower than if he trains at faster speeds. Below we outline a sample protocol that personal trainers may wish to use with their clients who run track, in an attempt to develop this energy system:
- 10 X 300m intervals – 1 minute recovery;
- 8 X 400m intervals – 2 minutes recovery; or
- 6 X 600m intervals – 3 minutes recovery.
- Developing the Aerobic Energy System
Continuous exercise at an intensity just below lactate threshold (for most individuals, typically 70%-75% maximum heart rate) trains this system quite efficiently. Remember, the body relies upon the aerobic energy system during medium to low intensity activity.
Key Take-Home Points
The energy system utilized by the body at any given point in time primarily reflects the type of activities performs, as well as the intensity and duration of exercise and the fitness level of the athlete. The more training one does in that particular type of exercise, the better the body adapts efficiency at using that energy system.
The energy continuum, or the interchanging of the energy systems during exercise, will proceed fluidly when an athlete succeeds in his appropriate training. For example, individuals trained in powerlifting can store more phosphocreatine and ATP than either an endurance runner or a sedentary individual. Conversely, endurance-trained individuals boast better ventilation ability, maximizing oxygen availability for the oxidative pathway.
Regardless of one’s chosen sport, consistency in training tops the list, along with proper nutrient intake and rest periods. To excel at a particular type of exercise, keep doing it; over time, the beautifully intricate machine we call the human body will adapt.
References:
https://www.army.mil/article/254967/understanding_the_three_energy_systems_used_during_exercise
https://openoregon.pressbooks.pub/nutritionscience/chapter/10b-fuel-sources-exercise/lE
Cathleen Kronemer is an NFPT CEC writer and a member of the NFPT Certification Council Board. Cathleen is an AFAA-Certified Group Exercise Instructor, NSCA-Certified Personal Trainer, ACE-Certified Health Coach, former competitive bodybuilder and freelance writer. She is employed at the Jewish Community Center in St. Louis, MO. Cathleen has been involved in the fitness industry for over three decades. Feel free to contact her at trainhard@kronemer.com. She welcomes your feedback and your comments!
