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

Biochemistry of Sports
Bioenergetics of Muscular Activity
General Characteristics of Energy Production Mechanisms

The resynthesis of ATP can occur via reactions that proceed without the participation of oxygen (anaerobic mechanisms) or with the participation of inhaled oxygen (aerobic mechanism).

Under normal conditions, the resynthesis of ATP in Tissues occurs predominantly aerobically. However, during intense muscular activity, when oxygen delivery to the Muscles is impaired, anaerobic ATP resynthesis mechanisms are also enhanced within the tissues. Three anaerobic and one aerobic pathway of ATP resynthesis have been identified in human skeletal muscles (Fig. 122).

Class="center">

Fig. 122 Mechanisms of ATP resynthesis in muscles. The boxes show energy substrates and highlight the names of the mechanisms

Anaerobic mechanisms include:    

✵ the creatine phosphokinase (phosphagen or alactacid) mechanism, which provides ATP resynthesis through transphosphorylation between creatine phosphate and ADP;

✵ the glycolytic (lactate) mechanism, which provides ATP resynthesis through the enzymatic anaerobic breakdown of Muscle Glycogen or Blood glucose, culminating in The formation of lactic acid (hence the term lactate mechanism);

✵ the myokinase mechanism, which carries out ATP resynthesis via a transphosphorylation reaction between two ADP molecules with the participation of the enzyme myokinase (adenylate kinase).

The Aerobic Mechanism of ATP resynthesis primarily involves Oxidative Phosphorylation reactions occurring in the Cell/35.html">Mitochondria. The energy substrates for aerobic oxidation are glucose, Fatty acids, partially Amino Acids, as well as intermediate metabolites of Glycolysis (lactic acid) and Fatty acid oxidation (Ketone Bodies).

Each mechanism possesses distinct energetic capabilities, which are characterized by the following evaluation criteria for energy production mechanisms: maximum power, activation rate, metabolic capacity, and efficiency. Maximum power is the highest rate of ATP formation in a given metabolic process; it limits the ultimate intensity of work performed via that mechanism. The activation rate is assessed by the time it takes to reach the maximum power of a given ATP resynthesis pathway from the onset of activity. Metabolic capacity reflects the total amount of energy substrates available to yield ATP through a specific resynthesis mechanism; this capacity limits the volume of work that can be performed. Metabolic efficiency represents the fraction of energy captured in the high-energy bonds of ATP; it determines the economy of the work performed and is evaluated by the total coefficient of performance (COP), representing The ratio of all usefully expended energy to the total energy released in the given metabolic process.

The overall efficiency of converting the energy of metabolic processes into mechanical work (E_m) depends on two indicators: a — the efficiency of converting The energy released during metabolic transformations into the energy of resynthesized high-energy phosphorus compounds (ATP), i.e., phosphorylation efficiency (E_p); b — the efficiency of converting ATP into mechanical work, i.e., chemomechanical coupling efficiency (E_c):

E_м = (Е_ф/Е_е)100

The efficiency of chemomechanical coupling in aerobic and Anaerobic METABOLISM is approximately equal at about 50%, whereas phosphorylation efficiency is highest in the alactacid anaerobic process at roughly 80%, lowest in anaerobic glycolysis averaging 44%, and approximately 60% in the aerobic process.

A comparative characterization of the creatine phosphokinase, glycolytic, and aerobic mechanisms of energy supply for muscular activity according to evaluation criteria is presented in Table 22.

TABLE 22. Evaluation Criteria for the mechanisms of energy supply in muscular activity

* The maintenance time of maximum energy production power does not evaluate the overall capacity of a given bioenergetic process, but rather corresponds to the fraction of that capacity that can be utilized when working at maximum power. For example, the depletion of PCr during maximal-effort exercise accounts for roughly 1/3 of its total reserves in working muscles.

As seen from the table, the creatine phosphokinase and glycolytic mechanisms feature high maximum power and ATP formation efficiency, but a short maintenance time for maximum power and a small capacity due to limited energy substrate reserves. The aerobic mechanism exhibits a maximum power nearly three times lower than that of the creatine phosphokinase pathway, yet it can sustain this power for a prolonged period, alongside a virtually inexhaustible capacity owing to large energy substrate reserves in the form of carbohydrates, fats, and partially Proteins. Thus, relying on fat reserves, the body can work continuously for 7–10 days, whereas the energy substrate reserves of anaerobic energy production mechanisms are significantly smaller.

Anaerobic mechanisms serve as the primary energy supply for short-term, high-intensity exercises, whereas aerobic mechanisms predominate during prolonged, moderate-intensity work.

The bioenergetic criteria presented in Table 22 were obtained through direct experimental measurements of energy production in highly qualified athletes; in untrained individuals, these values are substantially lower.