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This is the third and final part of the article series describing the body’s energy systems. The final system is the Aerobic system. This is the most complex and comprehensive energy system of the body.
A brief overview of the three different energy systems:
1.
ATP-CP System / Phosphagen System
2.
The Anaerobic System / Lactic Acid System
3. The Aerobic System
The ATP-CP system is the easiest and quickest way to generate ATP. This happens in the absence of oxygen (anaerobically). CP stands for creatine phosphate, which is stored in the skeletal muscles. Creatine phosphate allows for the creation of ATP. This energy system is especially active during very intensive forms of training. Think of short sprint activities (100m), powerlifting, but also high jump and long jump. In this way, energy can only be provided to muscles for 10 to 15 seconds before exhaustion occurs.
Through the Anaerobic / lactic acid system, ATP is produced by breaking down glucose in the absence of oxygen. Before glucose or glycogen (stored form of glucose in muscle and liver cells) can provide energy, it must first be broken down into glucose-6-phosphate. The end product of this reaction is lactic acid. This system is capable of delivering more energy than the phosphagen system (ATP-CP system), but is still limited to 30-50 seconds of energy. The typical 8-12 repetitions that many people choose during strength training primarily use this energy system. Another example is a 400m sprint.
The Aerobic System
The Aerobic system, as the name suggests, is the only system that depends on oxygen (aerobically). ATP is produced more slowly than with the other two systems. Therefore, it cannot provide the same explosive power as the ATP-CP and lactic acid systems. However, it can provide energy for a much longer period.
The Aerobic system itself can also be divided into three different oxygen systems:
1. Aerobic glycolysis
2. The Krebs Cycle / Citric Acid Cycle
3. Oxidative Phosphorylation (oxphos) or Electron Transport Chain
The Aerobic system uses both glucose, glycogen, and fat as fuel to generate ATP in the Mitochondria (“powerhouse”) of muscle cells.
In the Anaerobic energy system, glucose and glycogen are broken down in the absence of oxygen, the end product of that reaction is lactic acid. In the aerobic energy system, the process is the same, only oxygen is present, leading to a different end product called pyruvic acid. This end product is important for the second process of the aerobic system called the Krebs Cycle.
Pyruvic acid is converted into the molecule
acetyl coenzyme A. The complete oxidation (burning with the use of oxygen) of this molecule produces two units of ATP along with the by-products carbon dioxide and hydrogen. These hydrogen ions then bind with other enzymes and eventually provide energy for the
electron transport chain.
The complete metabolism of 1 glucose molecule can generate about 35 to 40 ATP in this way. That’s 18 times more than through the Anaerobic system.
Fat can also be burned in the presence of oxygen. Triglycerides must first be converted into free fatty acids. These are then converted into acyl-CoA molecules which in turn enter the Krebs Cycle. Triglycerides can generate up to 129 ATP molecules in this way, which is more than carbohydrates. On the other hand, burning fat requires more oxygen to generate ATP. Therefore, the energy delivery from fats is less efficient (it requires more energy to generate energy). As a result, carbohydrates remain the preferred and designated fuel for ATP production.
The Aerobic system is not quick in producing ATP, but it has the capacity to produce energy for a much longer period. There can be a larger store of fat than glycogen from carbohydrates. The latter can only provide energy for 30 to 40 minutes of maximum effort. An “excess” of carbohydrates in the bloodstream is also converted into body fat with the help of insulin as reserve fuel.
Proteins, and the amino acids from which they are made, are not the preferred source of energy for the body. Yet, there are situations in which amino acids are used to help in energy production. Especially when there is a negative energy balance. This means consuming fewer calories (food) than you expend (metabolism at rest and consumption through activity), resulting in weight loss. In a process called “gluconeogenesis,” glucose is made from non-carbohydrates but other sources such as amino acids, glycerol (fats), and pyruvic acid. Dietary amino acids can be used, but in the absence of sufficient food, it can also be broken down from muscle tissue. This means that muscle mass decreases. Unfortunately, this situation occurs when people use strict diets. These cause a large negative
energy balance. Herein lies the biggest challenge for bodybuilders who want both a lot of muscle mass and a low body fat percentage. They need a calorie surplus to gain muscle mass (“bulking phase”) and then a calorie deficit to reduce their body fat percentage, trying to lose as little muscle mass as possible (“cutting phase”). Hence the relatively high proportion of protein in the diet of bodybuilders to ensure that the body continuously has enough amino acids to stimulate muscle growth and limit muscle breakdown.
Training and the Aerobic System
The Aerobic system is mainly active during long efforts with moderate intensity. Jogging, cycling, walking, swimming laps, etc. Team sports such as football, hockey, and basketball also fall under this category.
The body’s highest energy consumption, in which the aerobic system is mainly active, is at rest. The largest part of your daily glucose consumption is to keep your organs running. Your brain is a major consumer of energy in this regard.
With the moderate intensity of endurance sports, often after 90 minutes of exercising, the largest store of glycogen in the body will be depleted. Through both training and high carbohydrate intake, it is possible to store more glycogen in the body by making better use of the maximum storage capacity. Many endurance athletes make use of this. “Carbohydrate loading” therefore has a beneficial effect on endurance. It doesn’t make one run faster, but it can help maintain a certain pace for a longer duration.
Summary
The body can provide energy in different ways. The phosphagen system/ATP-CP system, the anaerobic system/lactic acid system, and the aerobic system/oxygen system. It’s important to know that all three systems are always active together. The duration and the effort determine which system is most active. The Aerobic system is the most complex system and can provide the largest amount of energy. Mainly because in this system fats are burned, which people generally have enough of.
References
NASM Essentials of Personal Fitness Training
The Three Metabolic Energy Systems by Jason Karp PhD
All About your Metabolic Energy Systems by Andrew Heffernan
