Biochemical Foundations of Human Vital Activity - Volkov N.I., Nesen E.N. 2000

Biochemistry of Sports
Bioenergetics of Muscular Activity
Aerobic Mechanism of ATP Resynthesis

Under normal conditions, the aerobic Mechanism of ATP resynthesis accounts for about 90% of the total ATP resynthesized in the body. The enzyme systems responsible for aerobic METABOLISM are located primarily in the Muscle Cell/35.html">Mitochondria. The Mechanism of aerobic nutrient oxidation is known as "Oxidative Phosphorylation," the essentials of which are described in Chapter 3. The General scheme of the aerobic ATP resynthesis mechanism in Muscles is shown in Fig. 129.

The substrates used for aerobic oxidation include glucose, Higher Fatty acids, specific Amino Acids, Ketone Bodies, lactic acid, and other partially oxidized metabolic products. All of these substances are progressively converted into a common intermediate—acetyl-CoA—which is subsequently oxidized in The Citric Acid Cycle into the end products CO₂ and H₂O. This process relies on numerous oxidative Enzymes and inhaled oxygen delivered to the Tissues by erythrocyte Hemoglobin in the Blood, as well as the oxygen stored by the Myoglobin protein in skeletal muscles (see Fig. 129). The energy derived from oxidation is captured in the reduced forms of hydrogen carriers, NADH₂ and FADH₂, which transfer high-energy electrons down the Respiratory Chain to inhaled oxygen. Meanwhile, hydrogen protons establish a proton gradient (H⁺) across the mitochondrial membrane, serving as the driving force for ATP generation during oxidative phosphorylation. Once the proton gradient reaches a specific threshold (200 mV), the protons flow back across the mitochondrial membrane and react with oxygen to form H₂O. The energy released by the movement of H⁺ is harnessed to synthesize ATP from ADP and H₃PO₄ via the highly specific enzyme ATP synthase.

The rate of ATP production during oxidative phosphorylation depends on the following factors:

✵ the ATP/ADP ratio; in the absence of cellular ADP, ATP synthesis halts;

✵ The amount of cellular O₂ and the efficiency of its utilization;

✵ The activity of numerous oxidative enzymes;

✵ the Abundance of respiratory enzyme systems within the mitochondria;

✵ the integrity of the mitochondrial membranes;

✵ the number of mitochondria within The Cell;

✵ the concentration of Hormones, Ca²⁺ ions, and other regulators of aerobic metabolism.

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Fig. 129 Schematic diagram of the aerobic mechanism of ATP resynthesis, including anaerobic transformation (I) and aerobic oxidation—tissue Respiration (II)

A drop in ATP concentration, which occurs immediately upon the onset of intense physical exertion, stimulates the respiratory and cardiovascular systems to deliver oxygen to the Cells.

The volume of oxygen consumed by the Lungs is directly proportional to the amount of O₂ utilized in oxidative phosphorylation. This relationship makes it possible to determine the magnitude of aerobic energy production by measuring oxygen uptake. Normalization of respiratory rate and Heart rate occurs only after the increased cellular demand for ATP has been met.

When consuming an identical amount of oxygen, the volume of work performed will be greater if CARBOHYDRATES rather than fats serve as the energy substrate. Carbohydrates are a more efficient "fuel" than fats because their oxidation requires 12% less oxygen per molecule of synthesized ATP. Consequently, under conditions of restricted oxygen availability during exercise, energy production relies primarily on carbohydrate oxidation. Because the body's carbohydrate stores are limited, so is their usability in sports requiring general endurance. Once carbohydrate reserves are depleted, fat metabolism is recruited to sustain energy production, with fat reserves enabling extremely prolonged activity. For instance, during a marathon, muscle Glycogen sustains Muscle contraction for approximately 80 minutes, while additional ATP can be mobilized from Liver glycogen. As a result, carbohydrates can provide the energy for 75% of a marathon distance, with the remaining energy generated through the Oxidation of Fatty acids. Given that Fatty acids are densely packed with energy, cultivating an athlete's physiological capacity to mobilize them earlier for energy supply is critically important. To achieve this, periodic aerobic training—such as ultra-long-distance running (30–40 km or more)—is recommended.

Proteins can also serve as an oxidation substrate by breaking down into amino acids that can be converted into glucose or other metabolites of the aerobic pathway. However, proteins contribute only 5–10% to total energy production during muscular activity.

The maximum power of the aerobic mechanism is the lowest among energy systems, amounting to 1.2 kJ ⋅ kg^-1 ⋅ min^-1, and depends equally on the rate of O₂ delivery and its utilization within cells. The capacity of aerobic energy production is evaluated by the maximum oxygen uptake (VO₂max) achieved during muscular work. In athletes, this value averages 5.5–6 L ⋅ min^-1, whereas in non-athletes, it ranges from 2.5 to 3.5 L ⋅ min^-1. Because this metric reflects the rate of O₂ consumption in working muscles—and Skeletal Muscle constitutes the majority of active body mass—VO₂max is typically expressed relative to 1 kg of body weight to compare the aerobic capacities of different individuals. In untrained young adults, VO₂max is 40–45 mL ⋅ kg^-1 ⋅ min^-1 (800–1000 kJ ⋅ kg^-1 ⋅ min^-1), whereas in endurance athletes, it reaches 80–90 mL ⋅ kg^-1 ⋅ min^-1 (1600–1800 kJ ⋅ kg^-1 ⋅ min^-1).

The peak power of the aerobic process is attained by the 2nd to 3rd minute of low-intensity exercise in athletes and by the 4th to 5th minute in non-athletes, and it can be sustained for up to 15–30 minutes. In longer-duration exercises, it gradually declines. During a marathon, the average level of aerobic energy production accounts for 80–85% of maximum aerobic power.

Aerobic energy production processes occur most intensively in slow-twitch muscle fibers. Consequently, the higher the percentage of such fibers in the muscles bearing the primary load during exercise, the greater the athlete's maximum aerobic power and the higher their physical performance during prolonged exertion.

The metabolic capacity of the aerobic mechanism is practically limitless due to massive energy reserves capable of yielding large amounts of resynthesized ATP. For example, the aerobic oxidation of 1 glucose molecule yields 38 ATP molecules, whereas anaerobic Glycolysis produces only 2 ATP:

The oxidation of higher fatty acids, such as palmitic acid, yields even greater amounts of energy:

The energetic efficiency of this mechanism is also high, reaching approximately 50%. It is evaluated using the anaerobic threshold (AnT): in untrained individuals, the AnT is reached at an oxygen uptake of roughly 50% of VO₂max, whereas in elite endurance-trained athletes, it occurs at 80–90% of VO₂max. The elevation of the AnT metric through specialized training is associated with the enhancement (adaptation) of the oxygen-transport system, as well as enzymatic, regulatory, and other physiological systems.

The aerobic energy system is the primary pathway during prolonged, high- to moderate-intensity exercise, such as 5,000 m and 10,000 m runs, 25,000 m marathons, cycling, 800 m and 1,500 m swimming, and 5,000 m and 10,000 m speed skating. It serves as the biochemical foundation of general endurance.