Energy Systems
Energy Systems of the human body
The human body is a remarkable machine, capable of converting the food we eat and the air we breathe into the energy that fuels our daily activities. This process is managed by a series of complex, interconnected energy systems that work to ensure our cells, tissues, and organs have the necessary energy available to function optimally.
Imagine this:
You find yourself on the ground floor of a multi-storage building. Suddenly, you hear someone screaming in agony! You’re no hero - or maybe you are -, but without a second thought, you speed up the stairs to see what’s going on.
The scream came from the first floor; you’re in luck. You take 3, 4 steps and in no time, you’re on the first floor. You notice how at first your breathing pattern hasn’t changed at all. That makes sense, because your quick, high-energy need was fueled by the phosphagen system. After a few seconds, you notice how you do start breathing heavier, deeper and faster. Your body is signaling the need for oxygen, in order to regain homeostasis (EPOC).
The scream came from the third floor. You start off at a sprinting pace, but by the time you’ve reached the first floor, you have to slow down. As the phosphagen system reaches its limits, your body starts metabolizing blood glucose and muscle glycogen to fuel your way up to the third floor. It’s not like you’re gasping for air just yet, but you do have to slow your pace down. Having reached the floor, now too, breathing becomes heavier because of the demand for oxygen.
The scream came from way up in the building. As you speed off, your climbing speed gradually decreases. You can feel your lungs and heart working to process all the oxygen your body needs. You still have to go up a few floors, but you cannot go any faster. You have maxed out the quick energy production pathways and are now limited by the oxidative capacity of your cells.
The seamless interaction between energy systems ensures that the body can maintain energy production across various intensities and durations of exercise, demonstrating the complementary nature of the energy systems in supporting performance and overall physical activity.
Figure 1: The relative contribution of the three energy systems to the total energy supply during 90 seconds of all-out cycle exercise. Adapted from Gastin, 2001
Understanding these energy systems is crucial for appreciating how our bodies adapt to different physical demands, from periods of rest, even when you’re asleep, to intense workouts. This article delves into the mechanics of these systems, exploring how they interact and contribute to our performance.
The Energy System and the role of ATP
The human body relies on three primary energy systems to fuel physical activity: the phosphagen system, anaerobic glycolysis, and the aerobic system. These systems rely on a common key concept: ATP hydrolysis, or the chemical reaction that releases energy into our body.
The ATP molecule, or adenosine triphospate, is often referred to as the energy currency of the cell because this molecule powers almost all cellular functions, including muscle contractions. However, the muscles store only a small amount of ATP, sufficient for about 2-3 seconds of maximal effort. To sustain activity beyond this brief window, the body must rapidly regenerate ATP.
In order to better understand the process of how adenosine triphosphate contributes to the energy production process, we should take a brief look at its molecular structure and functioning. We speak of ATP when one adenosine (which is a combination of adenine and a simple sugar called ribose) and three phosphate groups are bonded.
Due to their nature, these bonds are strong and contain “potential energy”. The energy stored in ATP is released when the bond between one phosphate group and the rest of the molecule is broken down, converting adenosine triphospate (three phosphate groups) into adenosine diphosphate (ADP, two phosphate groups) and an inorganic phosphate (Pi).
Figure 2: simplified schematic of ATP hydrolysis, the process which creates energy from ATP.
This process is known as ATP hydrolysis and releases the potential energy stored within the phosphate bonds. This energy acts upon the muscle fibers which allows them to contract. However, the muscles' ATP stores are limited, providing sufficient energy for only a few seconds of maximal effort.
You could compare this process to an elastic band holding together three crayons. When the elastic band snaps it very quickly releases its energy. You’ll be sure to feel the effect of that energy when the band pinches your hand, and you might clench your fist as a reaction. Because the elastic band is broken, it will no longer bind the three crayons together. It will be maxed out at two crayons, leaving the third one unbonded.
The Phosphagen System: The Body's Immediate Energy Source
Our muscles contain only very small amounts of ATP, but our body is able to regenerate ATP. This is where the phosphagen system comes into play. The phosphagen system, also known as the ATP-creatine phosphate (ATP-PCr) system, is the quickest and most immediate source of energy, crucial for high-intensity, short-duration activities.
The phosphagen system is a highly efficient energy pathway that provides an immediate supply of energy through the breakdown of ATP and the rapid regeneration facilitated by creatine phosphate. While its capacity is limited and quickly depleted, its role is critical in the initial stages of high-intensity efforts, bridging the gap until other, slower-acting energy systems can take over.
In the first stage of the energy production process, ATP-PCr uses creatine to break the bond between the phosphate groups of ATP. The result of the process is threefold: a creatine phosphate molecule consisting of the creatine bonding with the broken-off phosphate group, the remaining adenosine diphosphate, and of course energy.
During the second stage, when ATP levels drop during intense activity, an enzyme called creatine kinase facilitates the transfer of a phosphate group from creatine phosphate to ADP, thereby regenerating ATP (the ADP regains one phosphate group) and releasing creatine. This process allows for a rapid replenishment of ATP, providing a quick, albeit short-lived, burst of energy.
Figure 3: simplified schematic of ATP-PCr, the process which creates energy from ATP and creatine.
Due to its ability to deliver immediate energy, the phosphagen system is predominantly utilized during short-duration, high-intensity activities. Sports and exercises that require explosive power and speed, such as sprints at maximal power, high jumps, and Olympic weightlifting, heavily depend on this system. These activities demand quick bursts of energy, and the phosphagen system efficiently meets this requirement, albeit for a brief period. The system's rapid energy output is crucial for the performance of these tasks, enabling athletes to execute movements with maximum force, power and speed.
The phosphagen system and the concept of oxygen debt
The concept of oxygen debt, also known as excess post-exercise oxygen consumption (EPOC), is closely related to the activity of the phosphagen system. Oxygen debt refers to the increased rate of oxygen intake following intense exercise, during which the body works to restore itself to a resting state. This process involves several physiological activities, including the replenishment of depleted ATP and PCr stores, the removal of hydrogen ions, and the restoration of muscle oxygen levels.
During activities fueled by the phosphagen system, the body rapidly consumes ATP and PCr without requiring oxygen. This anaerobic process leads to a temporary oxygen deficit, as the demand for energy outpaces the supply of oxygen. Once the activity ceases, the body experiences an elevated need for oxygen to recover. This is reflected in increased breathing and heart rate as the body works to restore homeostasis.
This post-exercise oxygen consumption serves several vital functions. Firstly, it helps to replenish the depleted ATP and PCr stores in the muscles, a process that occurs more efficiently in the presence of oxygen. Secondly, EPOC facilitates the removal of metabolic byproducts, such as hydrogen ions, that accumulate during anaerobic glycolysis. Finally, the oxygen consumed during this recovery phase is used to re-oxygenate myoglobin, a protein that transports oxygen within muscle cells.
The magnitude and duration of EPOC are influenced by several factors, including the intensity and duration of the exercise performed. High-intensity, short-duration activities that significantly engage the phosphagen system generally result in a more pronounced EPOC compared to lower-intensity, longer-duration efforts. Understanding EPOC and its underlying mechanisms provides valuable insights into the recovery process and can inform training strategies to optimize performance and recovery.
Phosphagen system interaction
The phosphagen system does not operate in isolation; it interacts dynamically with other energy systems in the body. During a typical bout of exercise (say, a 1500 meter race), the transition from the phosphagen system to anaerobic glycolysis and eventually to the aerobic system is fluid and continuous. While the phosphagen system dominates the initial phase of high-intensity exercise, providing immediate energy, its rapid depletion necessitates a swift shift to anaerobic glycolysis. This system takes over as the primary source of ATP, particularly as exercise duration extends beyond 10 seconds. As exercise continues, especially in endurance activities, the aerobic system becomes increasingly important, providing sustained energy through oxidative phosphorylation.
This seamless interaction ensures that the body can maintain energy production across various intensities and durations of exercise, demonstrating the complementary nature of the energy systems in supporting athletic performance and overall physical activity.
Anaerobic glycolysis: Sustaining High-Intensity
Anaerobic glycolysis takes over as the dominant energy source when the phosphagen system's capacity is exhausted. This process primarily involves the breakdown of simple sugars, such as glucose and glycogen, into pyruvate, yielding energy in the form of adenosine triphosphate (ATP). This anaerobic process operates without the presence of oxygen and produces ATP at a faster rate than aerobic pathways but slower than the phosphagen system. The energy produced through anaerobic glycolysis is sufficient to fuel intense activities lasting from approximately 10 seconds to 2 minutes, making it essential for efforts like mid-distance sprints and certain team sports where bursts of speed are critical. However, this system is predominantly limited by the accumulation of hydrogen ions, a byproduct that contributes to muscle fatigue.
Glucose and glycogen: what’s the difference?
Glucose and glycogen are both essential carbohydrates in the body, but they serve different roles and have distinct characteristics. Glucose is the primary source of energy for the body's cells. It is a monosaccharide, meaning it is a single sugar unit. This simplicity allows it to be easily transported in the bloodstream and utilized by cells for energy. Glucose can be rapidly taken up from the bloodstream and metabolized to produce energy, making it vital during high-intensity activities.
Glycogen is a polysaccharide, which means it is composed of many glucose molecules linked together. It serves as the body's primary storage form of carbohydrates. Glycogen is stored mainly in the liver and muscles. When energy is needed, the liver can break glycogen down into glucose through a process called glycogenolysis, making it easy to transport via the bloodstream.
In short: when we say glucose, we mean sugars in the blood. When we talk about glycogen, we refer to sugars that are stored in the muscles and liver.
Anaerobic glycolysis metabolism
Metabolism is everything that happens within every cell and throughout our body to provide us with the energy to live. As we continue our high-intensity exercise and the phosphagen system reaches its limits, our body still is in dire need of ATP in the production of energy. What’s interesting with glycolysis, is the two-step process of energy production. The first step is the energy investment phase, which prepares glucose for breakdown into intermediates. During the second phase, these intermediates are converted in pyruvate. Since this is the step that generates energy, we call this the energy pay-off phase. Both of these phases are anaerobic, which means that they do not require oxygen to happen (but oxygen is still present in our cells).
Step one begins with the conversion of a glucose molecule into two glyceraldehyde molecules. This conversion comes at a cost of 1 ATP for each glyceraldehyde molecule, totaling an energy cost of 2 ATP for each glucose molecule. This investment pays off, however, during phase two. Here, each glyceraldehyde molecule is converted into one pyruvate molecule, an organic compound produced primarily through glycolysis. This process delivers energy, at a rate of 2 ATP per pyruvate molecule, totaling 4 ATP molecules.
Figure 4: a very simplified schematic showing the energy cost and return of glycolysis.
The complete glycolysis process for one glucose molecule costs 2 ATP (= 2 x 1 ATP), but returns 4 ATP (= 2 x 2 ATP), therefore delivering a net gain of 2 ATP molecules. This is significantly less than the amount produced by aerobic metabolism, but the rapid availability of ATP makes it an invaluable source of energy during short bursts of moderate-to-high-intensity exercise.
Glycolysis and lactate
In the absence of sufficient oxygen, the pyruvate produced at the end of glycolysis cannot enter the mitochondria for further oxidation. Instead, it is converted into lactate through the action of the enzyme lactate dehydrogenase. This process is called lactate shuttling and serves a dual purpose: it regenerates a coenzyme (NAD+) essential for the continuation of glycolysis, and allows glycolysis to proceed in the absence of oxygen. Said differently: glucose becomes pyruvate, which becomes ATP, which provides our muscles with the energy to contract. When glycolysis produces more pyruvate that can be converted to ATP, our body would be obliged to slow down speed at which glucose is converted into pyruvate. However, by using part of the pyruvate to make lactate, we don’t have to slow down glycolysis (too much).
The accumulation of lactate in the muscles is often associated with the sensation of muscle fatigue and burning during intense efforts. However, the more recent thinking is that lactate serves as an important energy source during anaerobic conditions. Lactate shuttling would allow for metabolic flexibility (the ability to utilize both carbohydrates and fats effectively), enabling different tissues to communicate and share metabolic intermediates. After intense exercise, lactate is transported to the liver, where it can be converted back to glucose via gluconeogenesis. This process is vital for replenishing glycogen stores and facilitating recovery. Lactate is not thought of anymore as the culprit of muscle fatigue, but rather as a facilitator in maintaining energy production.
The association between lactate and muscle fatigue did not come out of the blue, though. Recent thinking however relates that feeling of muscle burn to hydrogen ions (H+) instead of lactate accumulation. The relationship between lactate, lactate production, and hydrogen ions is a key aspect of cellular metabolism, particularly during anaerobic conditions.
As discussed before, during high-intensity exercise, ATP is rapidly broken down to ADP and inorganic phosphate (Pi). The accumulation of H+ ions contributes to a decrease in pH, leading to acidosis in the muscle cells. This drop in pH can impair muscle function and contribute to fatigue. While lactate is often associated with acidosis, lactate can actually act as a buffer for excess hydrogen ions, helping to mitigate the drop in pH. As lactate accumulates, it can bind to hydrogen ions, but this process does not significantly increase the total hydrogen ion concentration in the muscle.
The production of lactate allows for the temporary storage of excess H+ ions. By converting pyruvate to lactate, the body effectively "removes" hydrogen ions, which helps maintain a more favorable pH balance during intense exercise. This buffering capacity is crucial for sustaining performance, as it delays the onset of fatigue associated with acidosis. After its production, lactate can be transported to other tissues, such as the liver, where it can be converted back to pyruvate for further metabolism.
The Aerobic System: Don’t Hold Your Breath
As exercise extends beyond the brief window where the phosphagen and anaerobic glycolysis systems dominate, the body gradually shifts towards aerobic energy production. This transition becomes more pronounced as the duration of the activity increases and the intensity decreases, allowing the cardiovascular and respiratory systems to deliver sufficient oxygen to working muscles. The aerobic system, which relies on oxidative phosphorylation, becomes the primary source of ATP for sustained, lower-intensity efforts.
Oxidative phosphorylation
The aerobic system is slower in producing ATP compared to the phosphagen and anaerobic glycolysis systems, but it can do so indefinitely, provided there is a steady supply of oxygen and fuel substrates. Indeed, at the heart of the aerobic system is a metabolic process known as oxidative phosphorylation. This process involves the breakdown of macronutrients — carbohydrates, fats, and to a minor extent proteins — to generate ATP. This occurs in the mitochondria, organelles present in every cell, that generate most of the chemical energy needed to power the cell's biochemical reactions. Oxidative phosphorylation is highly efficient process. Its dependency on oxygen and complex regulation underscores its role in maintaining cellular energy homeostasis.
Glycolysis: Krebs cycle
For carbohydrates, the breakdown starts with glycolysis, which converts glucose into pyruvate (see above). Under aerobic conditions, pyruvate is transported into the mitochondria and converted into acetyl-CoA, entering the Krebs cycle. Simply put, the Krebs cycle is a chain of reactions which uses oxygen (which we inhale) and gives out water and carbon dioxide (which we exhale). In the process, ADP is transformed into ATP.
Acetyl-CoA undergoes a series of chemical transformations. These transformations result in the production of high-energy electron carriers, NADH and FADH2, and a small amount of ATP. The high-energy electrons carried by NADH and FADH2 are then transferred to the electron transport chain, a series of protein complexes located in the inner mitochondrial membrane. As electrons pass through these complexes, they release energy. This drives ATP synthase, an enzyme that synthesizes ATP from ADP and inorganic phosphate. The total energy yield from one molecule of glucose (including the yields of glycolysis, pyruvate to acetyl-CoA conversion, and the Krebs cycle) is 36 ATP.
Lipolysis: beta-oxidation
For fats, the process begins with lipolysis, where triglycerides are broken down into free fatty acids and glycerol. The fatty acids are then transported into the mitochondria, where they undergo beta-oxidation to produce acetyl-CoA. Each cycle of beta-oxidation shortens the fatty acid chain by two carbon atoms, producing acetyl-CoA, NADH, and FADH2. The acetyl-CoA enters the Krebs cycle, while NADH and FADH2 feed into the electron transport chain, leading to further ATP production. The ATP yield from fat oxidation is substantial; a single molecule of palmitic acid (a common fatty acid) can produce up to 129 ATP molecules.
Proteins, though less commonly used, can also be broken down into amino acids, which can enter the Krebs cycle at various points depending on their structure.
ATP yield
The aerobic system's ATP yield is markedly higher than that of anaerobic pathways. Anaerobic glycolysis, which operates without oxygen, produces only 2 ATP molecules per glucose molecule. In contrast, the complete oxidation of glucose through the aerobic pathway produces up to 38 ATP molecules per glucose molecule. The integration of these processes — glycolysis, the Krebs cycle, the electron transport chain, lipolysis, and beta-oxidation — enables the aerobic system to support prolonged physical activity efficiently. Its ability to produce a high yield of ATP ensures that athletes maintain performance over extended periods.
Carbs vs fats
Ever heard people say something like “I had a fat-burning run” or “At that pace, she really switched to carbohydrates”? Well, let me tell you: our bodies do not come with a built-in fat-to-carbs switch.
During almost the entire range of intensities, the energy we need to perform and to survive, comes from a mixture of fat and carbohydrates. Whether you are sleeping or running a 5k race, both fats and carbs are used as fuel substrates for energy production. What does change however, is the ratio by which each of the substrates makes up your energy production. It can be estimated that during sleep about 20-30% of energy comes from glycolysis, while 70-80% comes from fat oxidation. At intensities of around VO2Max (an intensity at which our oxygen uptake is maximal) we also reach the maximum ratio of glycolysis to fat oxidation.
While there is no “switch”, no singular point or limit where we go from fat oxidation to glycolysis, there a region in which fat oxidation reaches it’s maximum. This happens at intensities of around 50-65% of VO2Max, which represents an activity that should feel comfortable, with controlled breathing and minimal muscle fatigue (e.g. a jog or relatively easy bike ride). While fat oxidation is at its highest peak, it only provides in about half of required energy, with the other half coming from glycolysis.
