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

Sports Biochemistry
Biochemical Foundations of Athletes' Endurance
Biochemical Factors of Endurance

Endurance largely determines an athlete's overall level of physical performance. It is characterized by the duration of work at a given power output until the first signs of pronounced fatigue, which leads to a decrease in performance. Endurance is measured by the duration of work performed until exhaustion, i.e., the limit time (t_пр).

Endurance can be characterized by The ratio of available energy reserves to The rate of Energy Expenditure during a given type of exercise:

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In other words, endurance is determined by the duration of functioning at a given intensity until the available energy resources are completely depleted.

The specific manifestation of endurance is always of a specific nature, which depends on the utilization of various metabolic processes as Energy Sources.

In accordance with the presence of three distinct energy production mechanisms, three constituent components of endurance are distinguished: alactic, glycolytic, and aerobic. The overall manifestation of endurance, assessed by the work time to exhaustion, can in this case be represented as the sum of various combinations of power, capacity, and efficiency parameters for aerobic and anaerobic processes:

In this expression, The values of the weighting coefficients (a), corresponding to the relative contribution of each metabolic process to total energy production, reflect the respective efficiency of the metabolic sources utilized. Thus, all diverse manifestations of endurance can be quantitatively assessed using nine bioenergetic criteria: three power criteria (alactic, glycolytic, aerobic), three capacity criteria (alactic, glycolytic, aerobic), and three efficiency criteria (alactic, glycolytic, aerobic). These criteria can be established based on precise ergometric measurements of externally performed mechanical work or through direct PHYSIOLOGICAL AND BIOCHEMICAL measurements of the corresponding bioenergetic Functions. For this purpose, standardized laboratory and special ("field") tests are used, aimed at the selective assessment of each individual component of endurance. Some of the most informative indicators used as bioenergetic Criteria for the aerobic and anaerobic components of physical performance are given in Table 35.

METABOLISM/18.html">The Influence of individual components in both manifestations of endurance varies depending on the power and limit time of the exercise. In moderate exercises, where the level of total energy expenditure does not exceed the values of the maximum rate of aerobic energy production, endurance is represented predominantly by its aerobic component. With an increase in exercise power above the critical level corresponding to maximum oxygen uptake, The Role of the aerobic component of endurance gradually decreases, while The Significance of the anaerobic components increases to the same extent. In short-term exercises of maximum power, manifestations of endurance are predominantly anaerobic in nature, with approximately equal representation of the alactic and glycolytic components.

TABLE 35. Bioenergetic criteria of the aerobic and anaerobic components of endurance

Fig. 179 Dependence of exercise power on its limit execution time

This can be illustrated by data from laboratory experiments involving cycle ergometer exercises of varying limit duration, presented in Fig. 179. The dependence of power on the limit work time is described by the equation

(1)

where W_mах is the highest power that can be developed in a given type of exercise in the absence of fatigue; t is the exercise execution time; p is a constant called the endurance coefficient, which shows how rapidly power decreases As a result of fatigue. On a graph with arithmetic coordinates, this relationship is depicted as a smoothly declining curve, the relative rate of decline of which is set by the numerical value of the coefficient p. The area under the curve corresponds to the total amount of work performed and can be divided into two parts: the area corresponding to work performed through aerobic energy production, and the area corresponding to work performed through anaerobic energy sources. During subcritical power exercises, current oxygen consumption fully satisfies the body's Energy Requirements, and the work is performed under conditions of a true steady state. With increasing exercise intensity, the rate of O₂ consumption also grows until the critical power level (W_кр) is reached, at which point the oxygen demand equals the maximum O₂ uptake level.

The level of critical power is higher the greater the athlete's aerobic capabilities, i.e., the higher the individual VO_2mах. At an exercise intensity exceeding the critical power value, work is performed mainly through anaerobic energy sources with The formation of a significant oxygen debt. In these exercises, due to the low efficiency of anaerobic energy production, the highest rate of power decrease is observed. At the same time, in subcritical power exercises, when work is performed primarily through the aerobic process, the rate of fatigue development noticeably decreases with an increasing limit exercise time. These Specific features of endurance manifestation should be taken into account when developing tests and selecting the most informative criteria designed for the quantitative assessment of this physical quality.

A differentiated assessment of endurance based on power, capacity, and efficiency parameters can be performed by measuring indicators of externally performed work (ergometric criteria) or through direct physiological and biochemical measurements in exercises where maximum values for these bioenergetic parameters can be reached.

As ergometric criteria of endurance with high prognostic significance, along with indicators of limit time and limit amount of work performed, indicators of critical speed, anaerobic threshold, anaerobic reserve distance, maximum anaerobic power, etc., are used. Ergometric criteria for the quantitative assessment of athletes' endurance can be divided into partial (particular), reflecting the specific features of endurance manifestation in a single type of exercise, and generalized (zonal), characterizing endurance features in a specific group (zone) of exercises similar by some attribute. Thus, partial indicators of endurance include the limit time of work at a given intensity, the record time of covering a given distance in cyclic exercises, Cureton's endurance index, etc. Generalized indicators of endurance are usually derived through mathematical analysis of ergometric test results for various exercises. Most commonly, power–limit time and work–limit time analyses are used for these purposes. As already noted, a generalized indicator of endurance derived from the analysis of this dependence is the relative rate of decline of the power curve, which is expressed by the endurance coefficient p. This coefficient can be determined from the power–time relationship on logarithmic coordinates (Fig. 180), where the solid curve is divided into several linear segments, each corresponding to the equation

     (2)

Fig. 180 Logarithmic graph of the dependence of exercise power on its limit execution time

Upon logarithmic transformation of the power function, the endurance coefficient p becomes equal to the slope angle tangent of each straight segment. The presence of several rectilinear segments on the graph, differing in slope angle, indicates that in each time range of exercises, distinct causes operate that condition The Development of fatigue and determine the manifestation of endurance in this type of exercise. Research results show that the main causes of the observed differences in The Nature of endurance manifestation are the features of energy supply in the given type of exercise and, in particular, the ratio of aerobic and anaerobic processes in the total Energy balance of the work. If the results of direct measurements of the rate of energy production in exercises of varying limit duration are presented as a graph with logarithmic coordinates, then, as in the case of the power–limit time dependence, the energy production rate curve is divided into a series of linear sections, each characterized by a specific value of the "half-time" constant (Fig. 181).

Differences in kinetic constants indicate a shift in metabolic states with an increasing limit duration of exercise. In the range of limit time values up to 16 min (1000 s), six distinct zones differing in the Nature of the Energy supply of work are identified. When performing maximum power exercises with a limit duration of up to 6 s, the increasing rate of energy production is provided mainly through the maximum activity of the alactic anaerobic process in working Muscles. In the next time range of exercise execution from 6 to 20 s, a rapid decrease in the rate of energy production is observed with a half-time constant t¹/₂ = 21.5 s. This range is characterized by mixed alactic-glycolytic anaerobic energy supply with significant exhaustion of the capacity of the alactic anaerobic source. In the limit time range from 20 to 45 s, the rate of energy production is determined by the maximum intensification of the anaerobic glycolytic process in working muscles. The greatest magnitude of anaerobic Changes in the body, with simultaneous deployment to the maximum level of aerobic energy production in working muscles, is observed in the limit exercise time range from 45 to 180 s. The maximum increase in THE CONTRIBUTION OF aerobic energetics is achieved in the limit time range of 600 s. Subsequently, changes in the rate of energy production are mainly associated with factors limiting the capacity and efficiency of aerobic energy conversion.

To derive generalized (zonal) indicators of endurance, along with the Analysis of the power–limit time dependence, an analysis of the relationship between limit work and limit time is also widely used. An example of this type of relationship, obtained from laboratory test data in cycle ergometer work by highly qualified athletes, is shown in Fig. 182.

Fig. 181. Logarithmic plot of energy production rate versus time to exhaustion

Fig. 182. Relationship between limit work and the time to exhaustion

The dependence of the total amount of work performed until exhaustion on the time to exhaustion can be expressed by the equation

   W_пр = а + bt_пp,       (3)

where a and b are constants. According to this equation, the total amount of work performed until complete exhaustion can be divided into two components: 1 — work performed at the expense of internal reserves that are not replenished during the exercise (this work corresponds to a zero coefficient a); 2 — work performed through a metabolic source whose resources are replenished during the exercise at a rate b; this amount of work is given by the product bt_пp. The numerical value of the slope b is defined as the tangent of the line angle on the graph and represents the power corresponding to the maximum rate of energy release in a given metabolic process. As can be seen from Fig. 182, within the studied range of time to exhaustion, three linear segments can be distinguished, differing in the values of coefficients a and b. Their values can be used as ergometric equivalents of the power and capacity of aerobic and anaerobic energy sources.

Fig. 183. Distance covered versus time to exhaustion in running

In cyclic exercises (walking, running, swimming, rowing, etc.), the total amount of work performed until exhaustion is equivalent to the distance covered, S. In this case, equation (3) must be transformed as follows:

   S = а + bt_пp,       (4)

where a is the section of the distance covered using the energy of the reserve metabolic process; b is the running speed corresponding to the maximal enhancement of the primary metabolic process. The Determination of the constants of this equation from record results demonstrated by a group of the country's top runners over various distances is illustrated by the graph in Fig. 183. The ergometric constants a and b, pertaining to different distance groups, in this case — just as when Processing laboratory test results from cycle ergometer work — correspond to the power and capacity values of various energy sources.

Along with recording ergometric indices of endurance, direct measurements of bioenergetic parameters of power, capacity, and efficiency are of vital importance for the selective assessment of individual components of this quality. First and foremost, determinations are made of VO₂ max, oxygen debt values, maximum blood lactate accumulation, "excess CO₂ output," the greatest blood pH shift, etc.

Thus, endurance performance depends on both the aerobic and anaerobic energy capabilities of athletes; therefore, endurance training systems should primarily focus on improving these bioenergetic Properties of the body.