Dr.Med., Associate Professor A.L. Pokhachevsky¹
V.M. Mikhailov²
PhD A.B. Petrov³
PhD D.A. Donskov¹
D.A. Faleev¹
D.A. Pokhachevsky⁴
V.V. Donskova⁵
Scientific Research Laboratory of Diagnostic and Recreational Technologies, Academy of the Federal Penitentiary Service of the RF, Ryazan
Medical Center Mega, Ivanovo
Lesgaft National State University of Physical Education, Sport and Health, St. Petersburg
Moscow Institute of Physics and Technology, Moscow
Ryazan State University named after S.A. Yesenin, Ryazan

Keywords: cycle ergometry, cardiorhythmogram, exercise tolerance, chronotropic index, insufficiency.

Introduction. Studies of physical exercise tolerance are relevant not only due to continued importance of sport physiology, but due to the appearance of positive cross effects of adaptation that determine the survival rate. The Framingham and modern multicenter studies, that had involved dozens of thousands of subjects, proved conclusively that exercise tolerance is the quantitative standard of health. At the same time, it is not the ECG criteria that serve as the survivability indices (coronary events and total mortality) but chronotropic insufficiency (CI) - inability to adequately change the HR parameters and power of the performed load registered during maximal load tests [2]. The predictors, that exceed the diagnostic range of survivability, can probably indicate the adaptive capability of the body, determine the quantitative standard of health, and determine the criteria of physical endurance [1, 4].

Objective of the study was to study the chronotropic index (CI) application benefits for the exercise tolerance test in the groups with different levels of motor activity.

Methods and structure of the study. We examined a mixed (S) population (68 persons) of apparently healthy senior school children and students under 23 years old split up into two groups. The first group (32 persons) was made of 18 active track and field athletes (Class 1 and Candidate Masters, middle distance) and 14 racing skiers. The second group consisted of 36 persons who were not engaged in systematic training sessions and exercised 2–3 times a week according to the curriculum.

The maximal cycle ergometer test was carried out of an individual record [2]. The capacity W₁ (Watt) of the first 3-minute stage was calculated based on the value of a due basal metabolic (DBM) rate in kilocalories by the formula W₁(W)=DBM×0.1 (DBM was determined by the Harris-Benedict tables). Further, we used a ramp-protocol (increase by 30 W, duration - 1 minute) by an individual maximal value - a decrease in the pedaling speed below 30 rpm indicating the load end and the beginning of a recovery period (duration - 7 minutes).

The load tests were performed before noon using a Corival (Lode) bicycle ergometer. During the testing, the ECG system "PolySpectrum-12" (Neurosoft) recorded a digitized electrocardiogram (ECG), from which an array of RR-intervals was distinguished - a cardiac rhythmgram (CRG). CI was calculated by the formula: CI=(100*(HRpeak-HRrest)/((220-A)-HRrest)), where HRpeak – maximal HR at the peak of loading, HRrest – average resting HR on a bicycle ergometer before exercise, A – age of the subject.

CI, HR: resting, peak and average values during exercise were calculated based on the time series resulted from a CRG using Microsoft Excel. When studying physical load (W), the difference between the maximum (Wmax) and the first stage (W₁) values was taken into account. The findings were processed using the statistical package Statistica 6.0. Since the value distribution differed from the standard, the data were represented in the form of percentile (Pc) series. The nonparametric correlation (Spearman) and comparison (Mann-Whitney) methods were used to process them.

Results and discussion. The study of exercise tolerance involves assessing the dynamics of HR during exercise - chronotropic myocardial response to exercise, therefore, CI is one of the objective markers of this process. A decrease in its value is diagnosed if it is impossible to reach 85% of the maximum HR with due regard to age. In this case, reduced CI is associated with greater prevalence of IHD and considered as an independent predictor of coronary events and total mortality [5]. According to our data, the CI values of the mixed population correspond to 85% only at the level of 75Pc, which from the point of view of diagnostic significance may indicate chronotropic insufficiency (Tables 1, 2). Moreover, while the group of non-sporting youth was characterized by this indicator at the level of 60Pc, which, at first sight, could reasonably be accepted as the norm, the group of athletes reached the survivability level only at the population maximum level exceeding 90Pc. CI statistically dominated in the group of non-sporting young individuals. The same pattern characterizes the index components – peak and resting HR.

Table 1. Exercise tolerance measures

At the same time, the distinctiveness of differences, still being statistically significant, decreases in the series: resting HR, average HR during exercise, peak HR, ending with CI itself. In other words, the comparative minimum of differences in CI is due to reducing dominance of peak HR versus the maximum dominance of resting HR in the group of non-sporting youth. By contrast, significant dominance of the load achieved is most expressed in the group of athletes (6.72).

Table 2. Distinctiveness of differences between the first and second groups (Z) and correlation between values and power of load performed

Several questions arise in this context: firstly, what does CI indicate in the population of apparently healthy young individuals; secondly, can it be used as a criterion for exercise tolerance; thirdly, what are the standards for this indicator and what do they depend on?

It should be noted that CI in the mixed population is not dependent, and its components - peak and resting HR are characterized by the negative correlation with the performed load; what is more, its intensity shifts significantly towards the resting HR level. Higher exercise tolerance at lower resting HR values can be easily explained from the physiological point of view, as this increases the homeokinetic range of HR that supplements physical exercise. A similar pattern for peak HR is determined not by the physiological appropriateness, but by the heterogeneity of the group, in which, in contrast to non-sporting young individuals, higher physical load corresponds to lower peak HR. By the way, the same pattern is applicable to resting HR, and in determining the correlation relationships it turns out to be the leading one. In this case it coincided with the physiological appropriateness, and in terms of peak HR – not. In addition, the lack of correlation between CI and performed load in the mixed population is also due to the heterogeneity of the sample. Herewith, the correspondence of higher load to increasing CI in each group forming the mixed population was compensated by a lower CI with better exercise tolerance in athletes compared to non-sporting young individuals. At the same time, a moderate positive correlation between CI and physical loading in the group of non-sporting youth was coupled with the same, but more pronounced, correlation between peak HR and absence of any pattern for resting HR. In the group of athletes the same, but more pronounced patterns were observed manifesting themselves in stronger correlations with CI and emergence of moderate positive effects of resting HR. In other words, when dividing the mixed population into the component parts, CI becomes dependent on the load performed; moreover, enhancing tolerance is due to increasing CI. At the same time, this is to a greater extent typical of the group of athletes. The revealed strength of correlation is reinforced by the involvement of exclusively peak HR in the group of non-sporting young individuals combined with the involvement of resting HR in the group of athletes. The display of positive correlation with resting HR in the group of athletes is associated with the availability of a well-functioning functional system (FS) that provides high working capacity [3]. The current functional system in terms of the afferent synthesis (presence of a memory trace, dominant motivation, situational, triggering afferentation) is pending a decision to start (command to start work), which probably leads to a certain increase in the resting HR. At the same time, it should be noted that the increase in HR from 57 to 63 bpm can hardly be considered significant, energy-consuming or physiologically inadequate - significantly reducing the chronotropic reserves before exercise. The direct correspondence of the peak HR to the performed load is a physiologically justified reaction, since the load increase requires an increase in chronotropic supply. However, this adaptation reaction (under 170 bpm) in this group is far from saturation.

The lack of adequate FS in the body of non-sporting individuals leads to ineffective provision of FR, which is manifested in the inability to regulate a high level of resting HR (76-93 bpm), meeting the chronotropic demands of physical load solely by means of an increase in the peak HR values (175-190 bpm) with no mechanism of inhibition (preservation) of the chronotropic reserve and physiological limit of the peak HR values themselves.

In turn, chronotropic variability, despite the significant dominance of the peak HR in the group of non-sporting youth, nevertheless, significantly (p<0.01) dominates in the group of athletes (99 versus 106 bpm on the median), which is achieved due to significantly lower values of the resting HR.

It is known that a high level of exercise tolerance is characterized by a delayed increase in HR and attainment of maximum values ​​not exceeding 170 bpm. The group of athletes is no exception, since the peak HR value reaches this level only at 75Pc. The latter, by the way, makes it impractical to conduct a PWC170 testing among athletes engaged in endurance training. In turn, high HR values under submaximal load and rapid achievement of the peak HR values may be due to hypodynamia, as well as pathological conditions related to a decrease in the circulating blood volume, cardiac output and peripheral resistance. However, it is only hypodynamia that is related to our analysis of the apparently healthy population and, as a consequence - minimal exercise tolerance.

At the same time, inability of athletes to reach the threshold rates, which is associated with a delayed chronotropic response to load and low peak HR values, should be considered as an indicator of high exercise tolerance. Herewith, a higher level of CI in the group of non-sporting youth, on the contrary, should be considered as an indicator bordering the diagnostic threshold of probability of coronry events and indicating low exercise tolerance. In this case, ideally, the CI range is to be determined depending on the level of exercise tolerance, when each level of the maximum achieved in Watts or MET will correspond to its percentile range (25-75Pc) of the norm. In addition, it is necessary to take into account the specific nature of training loads, both in terms of specialization and intensity.

Conclusions. In the population of apparently healthy young individuals, CI can be used to analyse exercise tolerance. However, its interpretation differs principally from clinical one. Due to the expansion of adaptation reserves, the formation of mixed endurance, which manifests itself in the improvement of exercise tolerance - CI decreases equivalently to increasing level of FR, which visually brings it closer to the survivability criteria. The revealed specificity requires the CI standards to be developed in view of the level of FR, but within the framework of formation of the functional system, accounting for specialization as well as the specifics of training loads.

References

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2. Mikhailov V.M. Nagruzochnoe testirovanie pod kontrolem EKG: veloergometriya, tredmill-test, step-test, khodba [Stress testing under control of ECG: cycle ergometry, treadmill test, step test, walking]. Ivanovo: Talka publ., 2008, 545 p.
3. Pavlov S.E., Pavlova T.N. Tekhnologiya podgotovki sportsmenov [Athletic training technology]. Shchelkovo: Markhotin P.Yu. publ., 2011, 344 p.
4. Pokhachevskiy A.L. Opredelenie adaptatsionnogo potentsiala po raspredeleniyu kardiointervalov pri veloergometrii [Distribution of load RR intervals to determine adaptive capability]. Voenno-meditsinskiy zhurnal, 2010, no. 6 (331), pp. 46-47.
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Abstract

Rates of predictors in access of the tested viability limits may be indicative of the body adaptive capability and physical endurance as verified by the relevant criteria. Objective of the study was to analyse the chronotropic index application benefits for the exercise tolerance tests. Subject to the study was a mixed sample (n = 68) of practically healthy senior schoolchildren and university students under 23 years of age split up into the following groups: Group 1 (n = 32) including sporting young people, and Group 1 (n = 36) composed of non-sporting young people. The sample was tested using maximal cycle ergometer tests to obtain the following test rates: chronotropic indices (CI), HR, mathematical model markers for cardiac rhythmgrams; and Spearman probe rates versus exercise tolerance. The tests showed the CI values being lower than the viability rates (85%) in both of the groups, with the values falling in the following sequence: S, 2, 1. It should be noted that the CI under load and HR after exercise statistically dominated in Group 2; and the post-exercise rates and dHR (difference of the maximal and resting HR) – in Group 1. Correlations of the test rates found by the study are indicative of the CI formation logics for different physical working capacity rates. Based on the study data and analyses, we came to conclusion that the CI values vary inversely to the physical working capacity rates; and this finding must be taken into account by analyses of the exercise tolerance, with due consideration given to the training process priorities and specifics.