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

Biochemistry of Sports
Bioenergetics of Muscular Activity
Activation of Energy Systems under Various Physical Loads and Their Adaptation during Training

Under conditions of relative rest and moderate-intensity exercise, ATP in skeletal Muscles is restored primarily via the aerobic pathway. It reaches peak power by the 2nd–4th minute of exercise in untrained individuals and by the 1st minute in athletes, maintaining this level for several hours.

During anaerobic physical exertion, the power of the phosphocreatine (alactic) and glycolytic (lactic) energy production mechanisms increases. Under very intense physical loads (maximal and submaximal power), anaerobic mechanisms become predominant in ATP resynthesis: the alactic mechanism during exercise lasting 10–30 s, and the lactic mechanism during exercise lasting 30 s to 6 min. The relative contributions of anaerobic and aerobic energy production mechanisms to the energetics of various exercises are illustrated in Fig. 130.

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Fig. 130. Sequence and contribution of anaerobic and aerobic energy production mechanisms to the energetics of various exercises

Exercises of varying power and duration are supported by different energy production mechanisms. This is clearly evident from the overall energetic contribution of each mechanism to the Energy supply of running over various distances (Fig. 131). As the duration of the run increases, the share of anaerobic energy production mechanisms decreases, while the share of aerobic energy production increases. However, under competitive conditions, a maximal mobilization of all systems ensuring specific performance is observed, with the predominance of any particular system depending on the duration of the exercise. Therefore, training programs must be designed to develop the capacity of each energy system.

In sports practice, physical exercises in which THE CONTRIBUTION OF anaerobic alactic and glycolytic processes exceeds 60% of the energy demand are generally classified as anaerobic exercises. Prolonged physical exercises, where the relative contribution of the aerobic process to Energy Expenditure exceeds 70%, are classified as aerobic exercises. Exercises in which the aerobic and anaerobic energy supply processes are of approximately equal importance are referred to as mixed anaerobic-aerobic loads. Examples of such exercises include 1000-m and 3000-m runs (see Fig. 131).

Each energy production mechanism possesses specific reserves that are unlocked or enhanced through adaptation to specific physical training. The aerobic capacity of athletes specializing in endurance sports depends on adaptive Changes in the power and capacity of the aerobic mechanism supplying energy for muscular activity. The capacity of the aerobic mechanism—largely determined by Glycogen stores in the skeletal muscles and Liver, as well as The rate of O₂ utilization by muscles—increases significantly within just 1.5–2 months of endurance training (Fig. 132). The power of the aerobic mechanism, which depends on _VO2max and The activity of oxidative Enzymes, also increases due to adaptation to muscular activity after 2–3 months of training. There is a marked increase in the activity of oxidative enzymes (Table 24). The expansion of the capillary network and the delivery of oxygen to muscles occur at a slower pace. The concentrations of Hemoglobin in the Blood and Myoglobin in the muscles increase, as do the number, size, and density of Cell/35.html">Mitochondria, thereby enhancing the capacity of muscles to utilize oxygen and carry out aerobic ATP resynthesis. Under these conditions, the ability of trained muscles to oxidize pyruvic acid improves, preventing the accumulation of lactic acid while also enhancing fat oxidation. This ensures more efficient execution of prolonged work.

Fig. 131. Relative energetic contribution of anaerobic (An) and aerobic (Ae) mechanisms to running performance across various distances

Fig. 132. Adaptive changes in the power (VO₂) and capacity of individual energy supply mechanisms during muscular work in the course of specific training

TABLE 24. Activity of Oxidative Muscle Enzymes

* Significant difference compared to the untrained subject

In the course of speed training, anaerobic energy supply mechanisms undergo significant alterations, marked by an increase in their power and capacity (see Fig. 132). This is associated with elevated activity of anaerobic enzymes (see Table 22) and larger stores of energy substrates. For instance, during adaptation, the concentration of phosphocreatine in skeletal muscles can increase by 1.5–2 times, and glycogen content by nearly 3 times. Post-exercise blood lactate levels in elite sprinters can reach 25–30 mmol·L^-1, whereas in untrained individuals performing the same physical work, it reaches 6–12 mmol·L^-1. This is linked to an increase in blood buffer capacity, which rises by 20–50% following anaerobic training. In highly qualified sprinters, the alactic mechanism can sustain high-speed work for 15–45 s, and the lactic mechanism for up to 3–4 min. This factor must be taken into account when selecting training loads.

CONTROL QUESTIONS

1. What is meant by ATP resynthesis mechanisms or pathways? What pathways of ATP resynthesis do you know in skeletal muscles?

2. What criteria are used to evaluate the energetic capabilities of ATP resynthesis pathways?

3. Provide a general characterization of the ATP resynthesis mechanisms.

4. Which of the ATP resynthesis mechanisms exhibits the highest maximal power, metabolic capacity, and efficiency?

5. What is meant by the rate of activation (response speed) of an ATP resynthesis mechanism? What does it depend on, and what is its significance in the energy supply of muscular work?

6. What type of physical work is supported by the phosphocreatine pathway of ATP resynthesis? What are its Advantages and disadvantages?

7. How does the phosphocreatine Mechanism of ATP resynthesis change during exercise?

8. In which types of muscle fibers do anaerobic ATP resynthesis mechanisms function most effectively?

9. What are the Key Enzymes of The Glycolytic Pathway of ATP resynthesis, and what are its energetic capabilities?

10. What kind of physical work is supported by the glycolytic pathway of ATP resynthesis? What adaptive changes in muscular activity are characteristic of it?

11. What is The Essence of the myokinase pathway of ATP resynthesis and what is its significance during muscular activity?

12. Name the main energy substrates, reactions, and energy yield of the Aerobic Mechanism of ATP resynthesis.

13. Why does the aerobic pathway of ATP resynthesis have a slow rate of activation?

14. How is the rate of mitochondrial oxidation regulated during exercise?

15. How does training affect the aerobic mechanism of ATP resynthesis?

16. What is The sequence of activation of various energy supply mechanisms during muscular work?

17. What is meant by the anaerobic threshold (AnT), and how is it determined?

18. How does the anaerobic threshold (AnT) change in athletes with training, and what are the BIOCHEMICAL FOUNDATIONS OF these adaptive changes?