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

Biochemistry of Sports
Biochemical Changes in the Body During Exercise of Varying Power and Duration
General Trends in Biochemical Process Alterations During Muscular Activity

The degree of Biochemical changes in the body during muscular activity depends on the type of exercise performed, its intensity and duration, as well as the athlete's level of physical conditioning. Primarily, these changes affect the mechanisms of aerobic and anaerobic energy production, which is most clearly illustrated by the energy supply for running various distances, representing work of differing power and duration (the maximum time limit is shown in Fig. 131).

In the 100-meter dash, the energy supply for Muscle work is provided exclusively through anaerobic processes (Fig. 133). During the initial meters of the distance or the starting acceleration, the Creatine Phosphokinase Mechanism plays a predominant role in supplying energy to the working Muscles. Over the remainder of the distance, the achieved maximum running speed is maintained through the simultaneous use of creatine phosphate and Glycogen.

The total share of anaerobic Glycolysis in the energetics of sprinting is about 50%. The rate of glycolytic glycogen breakdown during maximum-speed running over a 100-meter distance can reach a 1000-fold increase compared to the resting level. However, glycogen reserves are not depleted even during intense work to exhaustion, as indicated in Table 25, which presents the concentrations of ATP, PCr, and glycogen in human Skeletal Muscle, determined from biopsy samples taken at rest and after maximum exertion of varying durations. The ATP content changes insignificantly, whereas the PCr concentration drops from 17 mmol ⋅ kg^-1 of wet muscle mass to almost zero following maximum exertion to exhaustion.

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Fig. 133 Energy supply mechanisms for running 100 m (a) and 200 m (b)

TABLE 25 Concentration of energy substrates and pH in skeletal muscles during physical work

Note. Averaged data from numerous researchers are presented. Muscle metabolites were measured in millimoles per kilogram of wet body mass

In the 200-meter dash, creatine phosphate and anaerobic glycolysis are also used simultaneously, with a notable predominance of glycolysis in the overall Energetics of the work. However, already by the 150-meter mark, creatine phosphate reserves in the working muscles are significantly depleted, and the running pace typically drops by about 10%, at which point aerobic METABOLISM begins to play a definite role in supplying energy for the run. The total contribution of the aerobic energy production pathway at this distance can reach 10-20%.

In the 400-meter run, all three Energy Sources participate in maintaining the target running speed. Approximately 10% of the total energy cost is provided by the creatine phosphate breakdown reaction. It plays a major role during the start acceleration phase and at the finish line. Anaerobic glycolysis is of primary importance in the energetics of the 400-meter run, providing over 60% of the energy expended to cover the distance. The aerobic process accounts for about 25% of the total energy demand (Fig. 134).

The main factors limiting running speed over the 400-meter distance and contributing to the onset of fatigue are the limited capacity of the creatine phosphokinase mechanism, as well as a significant accumulation of lactic acid in the working muscles and acidification of the body's internal environment. Increasing the proportion of aerobic ATP resynthesis through carbohydrate oxidation during training can be viewed as a promising way to enhance the endurance of runners specializing in this distance.

In middle-distance running (800 and 1500 m), the creatine phosphokinase mechanism and anaerobic glycolysis make a certain contribution to ATP resynthesis during exercise, but aerobic metabolism becomes dominant. It accounts for about 45% of the total Energy Expenditure in the 800-meter run and up to 75% in the 1500-meter run (Fig. 135). In this context, glycogen reserves in the working muscles are of vital importance, which can be sufficient for approximately 1000 meters under anaerobic oxidation. To improve endurance in these types of running, it is more efficient to utilize these glycogen stores via aerobic oxidative processes. This requires an enhanced Blood supply to the working muscles, increased muscle oxygen uptake, and greater blood buffer reserves to prevent significant acidification and the early onset of fatigue.

Fig. 134 Energy supply Mechanisms for the 400 m run

Fig. 135 Energy supply mechanisms for the 800 m (a) and 1500 m (b) runs

In long-distance running (5 and 10 km), aerobic carbohydrate oxidation is the primary mechanism of energy supply, accounting for up to 87% of the total energy cost in the 5 km run and about 97% in the 10 km run (Fig. 136). At these distances, THE CONTRIBUTION OF anaerobic energy sources remains quite substantial. It can reach up to 15% of the total energy expenditure and plays a crucial role during the final sprint, which often secures victory in long-distance races. The most significant factor influencing endurance performance is the oxygen supply to the working muscles, as oxygen consumption during running sustains the maximum rate of carbohydrate oxidation. The anaerobic threshold in distance runners is observed at 75—90% of .

In marathon running, energy expenditures are covered exclusively through aerobic processes. Meeting these demands solely through carbohydrate oxidation is impossible due to insufficient glycogen reserves in the athlete's working muscles; therefore, a significant portion of energy is generated via The oxidation of fats, which can account for 10 to 50% of the total energy cost (Fig. 137). The contribution of fats to energy supply during muscular work in long and ultra-long distances is 12—20% in highly trained runners with large glycogen reserves in working muscles, and over 60% in untrained runners. In total, about 300 g of fat are oxidized during a marathon. Utilizing fats as an energy source is less efficient than carbohydrate oxidation because it occurs at a lower rate and with higher oxygen consumption. The increasing concentration of Fatty acids in the blood during fat mobilization and the high rate of their oxidation in working muscles inhibit the oxidation of CARBOHYDRATES (primarily blood glucose) within them, leading to a drop in running speed. An increase in the proportion of oxidized fats during exercise also promotes The Development of central inhibition. Fat mobilization during physical work is activated by A number of Hormones (Fig. 138).

Fig. 136 Energy supply mechanisms for the 10,000 m run

Fig. 137 Energy supply mechanisms in a marathon (indicating the point at which fat oxidation in working muscles sharply increases due to glycogen depletion)

During prolonged exercise, along with an increased utilization of fats in Energy Metabolism, the de novo synthesis of carbohydrates from non-carbohydrate precursors (Gluconeogenesis) may occur, activated by the hormone cortisol. The primary substrate for gluconeogenesis consists of Amino Acids, some of which accumulate in muscles during work As a result of tissue protein breakdown and can be utilized for glucose production.

Data on the contribution of various energy sources to ATP resynthesis during running at different distances are presented in Table 26. The contribution of individual energy sources to work performance is of great importance when choosing the optimal training strategy in a chosen sport, including those similar in intensity and duration to the running loads discussed here.

An understanding of the utilization of individual energy supply mechanisms during muscular work can be gained from changes in blood lactic acid and glucose concentrations. As shown in Fig. 139, the maximum accumulation of lactic acid in the blood is reached during the 400-m and 800-m runs, indicating the involvement of the lactate Mechanism of ATP resynthesis, whereas peak blood glucose levels occur during the 10,000-m run, when the Aerobic Mechanism of ATP resynthesis becomes predominant.

Fig. 138. Changes in the concentration of fat breakdown products (glycerol, ß-hydroxybutyrate) and lipolysis-stimulating hormones during exercise of varying duration

TABLE 26. Contribution of various energy sources to ATP resynthesis during running events of various distances

Note. For the 800, 1500, 5000, and 10 000 m events, creatine phosphate is utilized only During the first seconds of the run or in the finishing stretch

Fig. 139. Changes in blood lactate and glucose concentration in athletes after running distances from 100 to 10 000 m