Energy and  Exercise  Metabolism from the Perspective of Exercise Physiology

 

Energy

I'm posting this because there are scientific and easily explained materials on the subject of exercise and energy metabolism from the perspective of exercise physiology.

A continuous supply of energy is required for human activity. The body's direct energy source is a chemical called ATP. Just as a car needs gasoline to move, the human body needs an energy source called ATP to exercise. The energy source that the human body needs are ATP, whether for a short time or for a long time.

Exercise is achieved by contracting the muscle by the energy generated when the ATP stored in the muscle is decomposed. There are ATP-PC system, lactic acid system, and oxygen system as the methods of supplying ATP. The aerobic system is divided into three stages: aerobic glycolysis, the Krebs cycle, and the electron transport chain. It is important to understand which energy mobilization system to mainly strengthen when training an athlete to show explosive power in a short time or to show sustained power over a long period depending on the characteristics of the event.

Definition of Energy 

In the field of life sciences, energy is described as the ability of the body to perform the movement. Energy can be divided into six types: chemical energy, mechanical energy, thermal energy, light energy, electrical energy, and nuclear energy. The forms of energy used for the movement of the human body are chemical energy and mechanical energy.

Chemical energy is also expressed as a potential energy source. For example, food is decomposed through a chemical reaction in the body to release energy, and this chemical energy is used to synthesize or decompose other chemicals. In other words, some of the chemical energy or potential energy obtained from the food we eat is converted into mechanical energy or kinetic energy during the process of contraction and relaxation of skeletal muscles.

Energy used for physical activity and muscle contraction

What energy source is used by the human body to perform mechanical work such as muscle contraction? The decomposition of the food we consume does not directly contribute to mechanical work such as muscle performance, and only the energy released through the decomposition of a chemical compound called ATP stored in muscle cells is used by human cells to perform specific tasks. can be In other words, the energy source directly used by human cells is ATP. 

ATP is a very complex compound, consisting of 1 molecule of adenosine and 3 molecules of a phosphate group, and has two linkages connecting phosphoric acid and phosphoric acid. When this link breaks out of the stable state and one of the bonds is broken, ATP is converted into adenosine diphosphate and organic phosphate (Pi), and about 7-12 kilocalories (kcal) of energy is released. do. As described above, the energy generated when ATP is decomposed into diphosphate acts as an immediate energy source that can be used for body cells to function.

The only energy that can be used to contract or relax a muscle is ATP, and the total amount of ATP stored in the body is stored in such a small amount that it is almost depleted after sprinting 100 meters. Therefore, since human beings have to continuously contract and relax their muscles from birth to the moment they die, ATP replenishment must be replenished at the same time as the amount used. Stored energy sources such as carbohydrates and fats are not converted directly to ATP. 

Free phosphate (Pi) is released when the chemical bond of carbohydrate or fat is broken down, and this released energy (Pi) is then resynthesized into ATP by combining diphosphate with free phosphate. The method to supply free phosphate to the decomposed diphosphate during muscle contraction or relaxation is the decomposition of creatine phosphate (CP) stored in the body like ATP, and the decomposition of carbohydrates and fats by aerobic and anaerobic oxidation. is possible by

Supply of free phosphate by decomposition of creatine phosphate, which is its storage fuel.

During muscle contraction, ATP is decomposed into diphosphate and free phosphate, and at about the same time as creatine phosphate is decomposed into creatine and free phosphate, free phosphate is converted into diphosphate. ATP is resynthesized by transfer to Free phosphoric acid, supplied by the breakdown of creatine phosphate, which is important for exercise lasting less than 30 seconds, such as 100 m running. When exercise intensity is low and sustained, creatine phosphate is not depleted as rapidly as brisk, explosive strenuous activity.

After ingestion of carbohydrates, glucose molecules transported into the liver or muscle cells in the form of glucose through digestion and absorption are mainly oxidized to lactic acid, carbon dioxide, and water in muscle cells by two methods. Glucose molecules enter the cell from the blood of the capillaries through the muscle fiber membrane and are stored or used. Glucose entering the cell is broken down by a decomposition process called glycolysis.

The storage form of intracellular glucose is called glycogen, and glycogen molecules are linked to glucose molecules. Glucose and glycogen are not only decomposed under conditions of sufficient oxygen supply such as at rest or jogging but also decomposition under conditions of insufficient oxygen supply such as 200 meters or 400 meters running. An insufficient supply of oxygen is evidence that we breathe heavily after running while resting. 

When glucose and glycogen are decomposed in the presence of insufficient oxygen, lactic acid is produced as a by-product. Lactic acid is known as a substance that causes fatigue by slowing the cellular activity of active muscles. The reason why this energy source should be used even when fatigue is accumulating is that, above all, only this energy can be supplied for sprinting in a short time such as running 200 meters or 400 meters.

As mentioned above, when exercise intensity is low, such as at rest or jogging, glucose and glycogen are completely oxidized, so lactic acid is not produced. In general, when the exercise intensity is low, the oxidation of fat is relatively increased, and the amount of free phosphoric acid supplied by the decomposition of fatty acids is greater than the amount supplied from glucose or glycogen, but the amount of oxygen supplied is also required. In addition, the decomposition of fatty acids is not decomposed unless oxygen is insufficient or supplied, unlike glucose or glycogen. The decomposition of fatty acids increases proportionally as the amount of oxygen supply increases. However, it is not decomposed indefinitely according to the amount of oxygen supplied through the decomposition of fatty acids, but the amount of decomposition of fatty acids is determined according to the oxygen utilization capacity in the cell. Exercising for a long time, such as jogging, is a good training method to improve intracellular oxygen utilization.

Energy is used at rest and during exercise two-thirds of food fuel used for energy generation at rest is fat, and the remaining one-third is carbs. Protein utilization at rest is negligible. At rest and during daily activities, fat and carbohydrates are oxidized by oxygen supplied to supply free phosphoric acid. That is because the oxygen delivery system (heart and lung) can supply sufficient oxygen to each cell, the necessary energy can be satisfied with the supply of oxygen.

The category of short-duration exercise includes sprints such as 100m, 200m, and 400m runs, as well as exercises that can be sustained for 2-3 minutes at peak load. The main food fuel used here is mostly carbohydrates, with some fat being used. These movements are mainly performed under insufficient oxygen supply. During short-term exercise, ATP is mainly supplied through the decomposition of creatine phosphate and anaerobic glycolysis, which does not mean that the oxygen system is not involved at all. Short, high-intensity exercise causes a sharp drop in creatine phosphate (PC) levels, which remain low until the end of the exercise. This creatine phosphate is quickly replenished during the post-workout recovery phase.

In the initial stage of any exercise, energy production by oxygen supply is limited, and there are two reasons. First, there is a limit to the maximum rate that everyone can take in oxygen, that is, the maximum aerobic power. Second, it takes at least 2-3 minutes to adapt to the increased oxygen demand immediately after starting exercise. The reason why the rate of increase in oxygen intake is slow right after the start of exercise is that a very complex biochemical and physiological control process is required in the meantime. This is also true when switching from low-intensity exercise to high-intensity exercise. 

The part minus the actual oxygen intake from the oxygen intake required to supply all the ATP required for a certain exercise, that is, the part lacking in oxygen is called oxygen deficiency. Accumulation of lactic acid occurs rapidly during exercise, and lactic acid accumulation appears to play a significant role during exercise lasting 2-10 minutes. During exercise lasting more than 3 minutes, the depletion of creatine phosphate and the rate of ATP resynthesis are very important. When intense exercise stress is unbearable, exercise should be stopped or switched to a lower intensity. After a short period of exercise at extremely high intensity, the blood lactate concentration may reach up to 200 mg/dl. This is an accumulation of about 20 times when the normal value at rest is 10 mg/dl.

Long-term exercise

Carbohydrates and fats are the main fuels for energy sources during exercise that can last for a relatively long time, that is, about 10 minutes or more. In an exercise that can last up to 20 minutes, the main energy source that can resynthesize ATP is carbohydrates, and when this type of exercise is performed, a lot of lactic acids is accumulated in the blood. However, during exercise lasting more than 1 hour, glycogen storage begins to decrease significantly, and at the same time, fat is used as an important source of ATP resynthesis.

The combined use of glycogen and fat may vary depending on various conditions, such as the state of training, the ratio of muscle fibers, and the amount of glycogen storage before exercise. ATP required for long-term exercise can be synthesized by free phosphoric acid generated by oxygen supply. Anaerobic glycolysis also contributes in part but is limited only immediately after the start of exercise, until oxygen uptake is acclimatized to a new demand (steady-state). When oxygen intake reaches a new steady-state approximately 2-3 minutes after the start of exercise, all ATP energy (Pi) required for exercise is achieved by oxygen supply. Because of this, lactic acid in the blood does not accumulate much during exercise lasting more than an hour.

When glycolysis in the state where oxygen supply is insufficient at the beginning of exercise, lactic acid accumulates when it reaches a steady-state with sufficient oxygen supply, but it gradually decreases and the accumulation remains relatively low at the end of this exercise. A marathon is a good example. Marathon runners run 42.195 kilometers for more than 2 hours, but when the marathon is almost over, the blood lactate concentration increases only 2-3 times compared to at rest.

Relative roles

of anaerobic and anaerobic energy systems Let's take an example of running, swimming, and other sports to see how anaerobic and anaerobic energy systems interact during exercise. For example, the amount of adenosine triphosphate required during a 100-meter run is sufficient only by the breakdown of adenosine ATP and creatine phosphate, which are energy stored in the body.

In terms of the rate of supply of ATP required for a 200-meter run, it is similar to that for a 100-meter run. In other words, even with this type of exercise, the energy stored in the body is the main way to supply ATP. However, it can be seen that not all the necessary ATP is supplied only by the energy stored in the body, and the decomposition of glucose is required auxiliary in the state of insufficient oxygen supply. During this 200m run, these two anaerobic systems are important for ATP energy supply. 

As the speed of movement decreases and the duration and distance of movement increase, the primary energy system mobilized shifts to a system that relies solely on anaerobic glycolysis and oxygenation. That is, during 400-meter running and 100-meter swimming, there is mainly an interaction between stored energy and anaerobic glycolysis, whereas in 800 and 1500 m running and most swimming events, all three energy systems participate in ATP energy supply. It can be seen that in marathons and 1,500-meter swims, ATP energy supply is mainly dependent on the aerobic energy system.

Because there are so many sports, it is impossible to discuss them one by one. Therefore, let's find out what kind of energy is used based on the exercise time rather than the individual exercise itself. The purpose of using exercise time as a reference is that exercise time is the time required until the end of the game and at the same time, it can be defined as the time required to perform individual functions. For example, a basketball game usually consists of a total of 40 minutes, with 20 minutes each in the first and second half. In games that take a long time like this, energy is mainly supplied by the aerobic energy system. 

However, the game of basketball requires several skills, such as jumping, shooting, and defending. That is, short, high-intensity movements are performed intermittently throughout the game. As such, although the game itself lasts for a long time, the movements of the players during the game are anaerobic activities. So, there are anaerobic elements as well as anaerobic elements in basketball. Sports included in this category include baseball, fencing, soccer, golf, ice hockey, tennis, volleyball, and wrestling.

In sports such as track events, swimming, cycling, skiing, rowing, and speed skating in athletics, the duration of an exercise, independent of motor skills, mainly refers to the duration of time spent in competition. For example, it takes about 4-5 minutes for a good sprinter to run the 1500m, and for a good freestyler to run for 400m in about 4-5 minutes. Because these two movements have similar exercise times, they become the same form of supply of free phosphoric acid.

The reason marathon runners are extremely exhausted at the finish point even though lactic acid, a fatigue-inducing substance, does not accumulate excessively is that the blood glucose concentration decreases according to the depletion of stored liver glycogen, and local muscle fatigue occurs due to the depletion of stored muscle glycogen. The loss of water and electrolytes leads to an increase in body temperature and psychological boredom. 

Just as anaerobic capacity is important when performing the short-term exercise, maximal aerobic power is an important factor when performing long-term activities. This is based on the fact that most of the energy required for prolonged exercise is supplied by the aerobic energy system. In general, it is evaluated that the greater the maximum aerobic power, the better the endurance exercise performance.

Test of aerobic capacity refers to energy supply capacity by aerobic metabolism. The rate of energy supply by aerobic metabolism depends on the biochemical ability of the muscle tissue to produce energy using oxygen and fuel and the ability of the respiratory and circulatory systems to transport oxygen to the muscle tissue. 

In general, these two abilities can be assessed by either maximal oxygen uptake or anaerobic threshold. Aerobic capacity or energy consumption is measured by the method of oxygen intake and the amount of work performed in parallel. Therefore, it is necessary to understand how to calculate the energy expenditure based on the oxygen intake and respiratory exchange rate measured during exercise. 

The maximum oxygen intake can be used as a useful index for diagnosing endurance capacity, and it can be measured by a respiratory gas analyzer during maximum exercise under increasing exercise load. In principle, the incremental exercise load method uses equipment with high similarity depending on the exercise type, and equipment such as bicycle ergometers and treadmills are used for weight lifting. When it is difficult to directly measure the maximum oxygen uptake, there are several indirect methods, but estimation errors must be taken into account. The anaerobic threshold is the maximum exercise intensity that can be continued for a long time without fatigue.