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We all know that the human body needs energy to function, but where does this energy come from? Ultimately, the energy that keeps us moving comes from the food we eat. However, we cannot use energy directly from food—it must first be converted into adenosine triphosphate, or ATP, the immediate useable form of chemical energy utilized for all cellular function. The body does store a minimal amount of ATP within the muscles, but the majority is synthesized from the foods we eat.
Food is made up of carbohydrates, fats and proteins, and these nutrients are broken down into their simplest forms (glucose, fatty acids and amino acids) during digestion. Once these nutrients are broken down, they are transported through the blood to either be used in a metabolic pathway or stored for later use.
Because we do not store a significant amount of ATP and need a continuous supply, it must be constantly resynthesized. This occurs in several ways using one of three energy systems:
- Phosphagen (immediate source)
- Anaerobic (somewhat slow, uses carbohydrates)
- Aerobic (slow, uses either carbohydrate or fat)
Phosphagen
This system uses creatine phosphate (CP) and has a very rapid rate of ATP production. The creatine phosphate is used to reconstitute ATP after it’s broken down to release its energy. The total amount of CP and ATP stored in muscles is small, so there is limited energy available for muscular contraction. It is, however, instantaneously available and is essential at the onset of activity, as well as during short-term high-intensity activities lasting about 1 to 30 seconds in duration, such as sprinting, weight-lifting or throwing a ball.
Anaerobic Glycolysis
Anaerobic glycolysis does not require oxygen and uses the energy contained in glucose for the formation of ATP. This pathway occurs within the cytoplasm and breaks glucose down into a simpler component called pyruvate. As an intermediate pathway between the phosphagen and aerobic system, anaerobic glycolysis can produce ATP quite rapidly for use during activities requiring large bursts of energy over somewhat longer periods of time (30 seconds to three minutes max, or during endurance activities prior to steady state being achieved).
Aerobic Glycolysis
This pathway requires oxygen to produce ATP, because carbohydrates and fats are only burned in the presence of oxygen. This pathway occurs in the mitochondria of the cell and is used for activities requiring sustained energy production. Aerobic glycolysis has a slow rate of ATP production and is predominantly utilized during longer-duration, lower-intensity activities after the phosphagen and anaerobic systems have fatigued.
It is important to remember that all three of these systems contribute to the energy needs of the body during physical activity. These systems do not work independently of each other, but rather dominate at different times, depending on the duration and the intensity of the activity.
He holds a bachelor's degree in kinesiology from San Diego State University, a master’s degree in kinesiology from
A.T. Still University, and a certificate in orthotics from Northwestern University Fienberg School of Medicine.
ENERGY SYSTEMS AREN’T JUST FOR ATHLETES
You may have heard the term “energy systems” mentioned in the gym, at practice–or perhaps your children’s practices–in fitness articles, or other health outlets. It’s a complex term that’s often referred to, and is perhaps one of the most confusing and misunderstood things about human performance. And if you haven’t heard the term “energy systems,” then you at least most certainly have heard the term “lactic acid.” If someone has ever told you that “your muscles are sore from the lactic acid you produced during your workout,” then, you’ve dealt with the myths firsthand (more on that later).
Energy systems may sound like something only serious athletes need to think about–because they’re performance-related. But if you are a human being, then the performance of your cells does matter, because it affects your quality of life, health, and longevity. The energy systems influence the health of your mitochondrion, respiratory system, circulatory system, and muscle growth–all of which improve overall wellness. Mitochondria, the cellular generators responsible for synthesizing the body’s energy, are critical to longevity. In fact, deteriorating mitochondrial health speeds aging and increases mortality. Mitochondrial dysfunction has been linked to an array of degenerative illnesses, ranging from diabetes to neurological disorders and even heart disease. Essentially, we can’t afford to not care about our energy systems, cellular health and production, muscle mass, and robustness. And the way to care for these systems is to engage in regular exercise and conditioning so that you put these systems to work. Repeatedly providing stimulus to these pathways will force them to adapt positively. Lifting weights, interval training, cardiac output training, and pushing yourself to places that are physically uncomfortable will stimulate new growth and capability in your body. And that matters to everyone–whether you are a division one hockey player or the parent of a division one hockey player.
Energy systems are the chemical pathways that cope with energy production and the products of physical work. I use the words “cope with” for a reason. I don’t want to say that they “create energy,” because energy is never created or destroyed–it is transferred. I also don’t want to suggest that these energy systems exist solely to give us the ability to move–their existence is multipurpose. They must release stored energy from molecules in order to power cellular work, but they also must deal with the byproducts from those chemical reactions. So overall, they’re coping with movement demands and byproducts (like heat) to provide power and threat deterrence strategies (overheating is dangerous).
Energy for cellular work comes from the molecule ATP, or adenosine triphosphate. The three phosphates attached to the sugar on the molecule can be thought of as springs that get released in order to provide free energy. The cleavage of those bonds creates byproducts–such as water, hydrogen, and heat–as well as available energy to drive more reactions. In this regard, ATP serves as both an energy receiver and donor because it can be degraded and resynthesized. All of the energy systems work to generate ATP, or generate molecules that will further drive ATP production, and also deal with the hydrogen and heat that surfaces from such mechanisms.
There are three energy systems: the immediate energy system, the glycolytic system, and the oxidative system. All three systems work simultaneously to a degree, but parts of the system will become predominant depending on what the needs of the body are.
The immediate energy system copes with demands that require an explosive, rapid response–such as a one-rep max of a fast and heavy weight lift.
The glycolytic system copes with demands that require a relatively high energy output for a relatively short amount of time–such as a sprint down the ice in a hockey game.
The oxidative system copes with lower output work for longer durations of time–such as a road race.
The Immediate Energy System
The Immediate Energy System in skeletal muscle utilizes several integrated chemical reactions to liberate energy for cellular work in an explosive, rapid sequence, but then quickly put the ATP back together again. It does not require oxygen (anaerobic) and it does not produce lactate (as with glycolysis). Instead, this system involves ATP and creatine phosphate that are stored within the muscle fibers. Through several enzymatic steps, the system will liberate energy from ATP and then resynthesize it using creatine phosphate to produce ATP and creatine. The overall capacity of this one pathway is quite limited, such that during explosive exercise, the energy yield from this system can continue until the creatine phosphate stores are mostly depleted, which may occur in approximately ten seconds. The rate-limiting factor for this system is partially creatine phosphate-dependent, which is why athletes often supplement with creatine.
The Glycolytic System
Glycolysis is the pathway that splits carbohydrate (glucose or stored glycogen) in order to generate ATP to power cellular work. Only carbohydrate can be used as substrate for this pathway. This system functions during short-duration, high-intensity exercise. You’ve probably heard the term “lactic acid” in regard to muscle soreness or fatigue–however, both of those common remarks are inaccurate. Lactic acid does not exist inside the human body, lactate does. And lactate does not cause muscle soreness. In fact, it is shuttled back to the liver quite efficiently.
The product of glycolysis is pyruvate, and this is where the glycolytic system can be alactic, or lactic. That is, in situations where the products of glycolysis (pyruvate molecules) are exceeding the rate at which they can be shuttled into the citric acid cycle (the next phase of the energy systems), the body will bind a hydrogen to each pyruvate molecule to form lactate, which will then be shuttled back to the beginning of glycolysis to be reused. The lactate production, therefore, is both a coping mechanism (handle the excess hydrogen), and a way to create ATP in situations where the slower, more efficient system can’t run its course but the demands of the body are too intense.
The Oxidative System
The Oxidative System comes to prominence during lower intensity, sustained exercise wherein ATP needs can be met almost indefinitely, but the production rates are not as rapid as glycolysis. Unlike glycolysis, this system is aerobic, and can be powered not only by glucose and glycogen, but by fatty acids.
This energy system is rather profound, and given that adequate substrate is available–as in, you’ve eaten enough–the production of ATP can last for long durations. The Oxidative System is powered by what are referred to as “high energy electron carriers,” which are molecules that bond with hydrogen (threat reduction) and then create a hydrogen gradient inside mitochondrial inner membranes to power the electron transport chain–which ultimately provides the energy to resynthesize a large amount of ATP. Of all the systems, this one is most efficient at coping with hydrogen and regenerating ATP.
Energy Systems and Your Body
These pathways for coping with and powering work have incredible impacts on human performance, health, and longevity. Muscle fibers change and adapt as new stimulus is repeatedly placed upon them, and some of their characteristics will change depending on what that stimulus is. If you’re training for a long distance run, your Type I muscle fibers will develop more mitochondrial density to power a stronger oxidative system. If you’re doing heavy, explosive lifting, your muscles will store more glycogen and creatine (if you’re eating properly) to power the new demands that must be met. And if you’re doing short sprints to work on your glycolytic adaptations, you may increase your contraction rate in your Type IIa muscle fibers to power your change of speed in your hockey game.
Mitochondrial health, cellular health, muscle health, and so many other facets of our well-being depend on the use of our energy systems. By understanding what they are, we can work in the gym and the kitchen to optimally train and fuel these ancient pathways that make our bodies so adaptive and plastic. By doing so, we increase the ability of our bodies to perform and thrive for longer periods of time.
ENERGY SYSTEMS AREN’T JUST FOR ATHLETES
You may have heard the term “energy systems” mentioned in the gym, at practice–or perhaps your children’s practices–in fitness articles, or other health outlets. It’s a complex term that’s often referred to, and is perhaps one of the most confusing and misunderstood things about human performance. And if you haven’t heard the term “energy systems,” then you at least most certainly have heard the term “lactic acid.” If someone has ever told you that “your muscles are sore from the lactic acid you produced during your workout,” then, you’ve dealt with the myths firsthand (more on that later).
Energy systems may sound like something only serious athletes need to think about–because they’re performance-related. But if you are a human being, then the performance of your cells does matter, because it affects your quality of life, health, and longevity. The energy systems influence the health of your mitochondrion, respiratory system, circulatory system, and muscle growth–all of which improve overall wellness. Mitochondria, the cellular generators responsible for synthesizing the body’s energy, are critical to longevity. In fact, deteriorating mitochondrial health speeds aging and increases mortality. Mitochondrial dysfunction has been linked to an array of degenerative illnesses, ranging from diabetes to neurological disorders and even heart disease. Essentially, we can’t afford to not care about our energy systems, cellular health and production, muscle mass, and robustness. And the way to care for these systems is to engage in regular exercise and conditioning so that you put these systems to work. Repeatedly providing stimulus to these pathways will force them to adapt positively. Lifting weights, interval training, cardiac output training, and pushing yourself to places that are physically uncomfortable will stimulate new growth and capability in your body. And that matters to everyone–whether you are a division one hockey player or the parent of a division one hockey player.
Energy systems are the chemical pathways that cope with energy production and the products of physical work. I use the words “cope with” for a reason. I don’t want to say that they “create energy,” because energy is never created or destroyed–it is transferred. I also don’t want to suggest that these energy systems exist solely to give us the ability to move–their existence is multipurpose. They must release stored energy from molecules in order to power cellular work, but they also must deal with the byproducts from those chemical reactions. So overall, they’re coping with movement demands and byproducts (like heat) to provide power and threat deterrence strategies (overheating is dangerous).
Energy for cellular work comes from the molecule ATP, or adenosine triphosphate. The three phosphates attached to the sugar on the molecule can be thought of as springs that get released in order to provide free energy. The cleavage of those bonds creates byproducts–such as water, hydrogen, and heat–as well as available energy to drive more reactions. In this regard, ATP serves as both an energy receiver and donor because it can be degraded and resynthesized. All of the energy systems work to generate ATP, or generate molecules that will further drive ATP production, and also deal with the hydrogen and heat that surfaces from such mechanisms.
There are three energy systems: the immediate energy system, the glycolytic system, and the oxidative system. All three systems work simultaneously to a degree, but parts of the system will become predominant depending on what the needs of the body are.
The immediate energy system copes with demands that require an explosive, rapid response–such as a one-rep max of a fast and heavy weight lift.
The glycolytic system copes with demands that require a relatively high energy output for a relatively short amount of time–such as a sprint down the ice in a hockey game.
The oxidative system copes with lower output work for longer durations of time–such as a road race.
The Immediate Energy System
The Immediate Energy System in skeletal muscle utilizes several integrated chemical reactions to liberate energy for cellular work in an explosive, rapid sequence, but then quickly put the ATP back together again. It does not require oxygen (anaerobic) and it does not produce lactate (as with glycolysis). Instead, this system involves ATP and creatine phosphate that are stored within the muscle fibers. Through several enzymatic steps, the system will liberate energy from ATP and then resynthesize it using creatine phosphate to produce ATP and creatine. The overall capacity of this one pathway is quite limited, such that during explosive exercise, the energy yield from this system can continue until the creatine phosphate stores are mostly depleted, which may occur in approximately ten seconds. The rate-limiting factor for this system is partially creatine phosphate-dependent, which is why athletes often supplement with creatine.
The Glycolytic System
Glycolysis is the pathway that splits carbohydrate (glucose or stored glycogen) in order to generate ATP to power cellular work. Only carbohydrate can be used as substrate for this pathway. This system functions during short-duration, high-intensity exercise. You’ve probably heard the term “lactic acid” in regard to muscle soreness or fatigue–however, both of those common remarks are inaccurate. Lactic acid does not exist inside the human body, lactate does. And lactate does not cause muscle soreness. In fact, it is shuttled back to the liver quite efficiently.
The product of glycolysis is pyruvate, and this is where the glycolytic system can be alactic, or lactic. That is, in situations where the products of glycolysis (pyruvate molecules) are exceeding the rate at which they can be shuttled into the citric acid cycle (the next phase of the energy systems), the body will bind a hydrogen to each pyruvate molecule to form lactate, which will then be shuttled back to the beginning of glycolysis to be reused. The lactate production, therefore, is both a coping mechanism (handle the excess hydrogen), and a way to create ATP in situations where the slower, more efficient system can’t run its course but the demands of the body are too intense.
The Oxidative System
The Oxidative System comes to prominence during lower intensity, sustained exercise wherein ATP needs can be met almost indefinitely, but the production rates are not as rapid as glycolysis. Unlike glycolysis, this system is aerobic, and can be powered not only by glucose and glycogen, but by fatty acids.
This energy system is rather profound, and given that adequate substrate is available–as in, you’ve eaten enough–the production of ATP can last for long durations. The Oxidative System is powered by what are referred to as “high energy electron carriers,” which are molecules that bond with hydrogen (threat reduction) and then create a hydrogen gradient inside mitochondrial inner membranes to power the electron transport chain–which ultimately provides the energy to resynthesize a large amount of ATP. Of all the systems, this one is most efficient at coping with hydrogen and regenerating ATP.
Energy Systems and Your Body
These pathways for coping with and powering work have incredible impacts on human performance, health, and longevity. Muscle fibers change and adapt as new stimulus is repeatedly placed upon them, and some of their characteristics will change depending on what that stimulus is. If you’re training for a long distance run, your Type I muscle fibers will develop more mitochondrial density to power a stronger oxidative system. If you’re doing heavy, explosive lifting, your muscles will store more glycogen and creatine (if you’re eating properly) to power the new demands that must be met. And if you’re doing short sprints to work on your glycolytic adaptations, you may increase your contraction rate in your Type IIa muscle fibers to power your change of speed in your hockey game.
Mitochondrial health, cellular health, muscle health, and so many other facets of our well-being depend on the use of our energy systems. By understanding what they are, we can work in the gym and the kitchen to optimally train and fuel these ancient pathways that make our bodies so adaptive and plastic. By doing so, we increase the ability of our bodies to perform and thrive for longer periods of time.