Dr.Med., Professor Yu.E. Vagin¹
PhD I.E. Zelenkova^2
1I.M. Sechenov First Moscow State Medical University under the Ministry of Health, Moscow
²P.K. Anokhin Research Institute of Normal Physiology under the Academy of Sciences, Moscow

Keywords: freediver, physical endurance, breath-hold, hypoxia, heart rate variability.

Background. Physical endurance and tolerance to progressing shortage of oxygen in arterial blood are ranked among the key factors of importance for competitive success in many sports. It is a matter of common knowledge that competitive diving sport (freediving) develops high hypoxia tolerance in the athletes. The physical loads in the underwater swimming process with long breath-holds trigger growth of the parasympathetic nervous system tonus associated with bradycardia [5, 7, 9]. Aerobic metabolic processes are replaced by the anaerobic ones in the bodily peripheral tissues with the lactate contents being on the rise [4, 5]. Blood vessels in limbs are constricted to increase the inflow of blood to the heart and brain [3]. Blood buffer capacity grows and the metabolic rate in the vital organs slows down [2, 6]. These changes in the body functions are geared to support performance of the vital organs. The sport theory and practice, however, offer no comparative analyses of the functionality-changing processes in freedivers, other athletes and untrained people exposed to static/ dynamic hypoxic effects.

Objective of the study was to explore the physiological mechanisms of physical endurance and hypoxia tolerance in freedivers, basketball players and untrained people based on the sympathetic/ parasympathetic nervous system tone profiling by the heart rate variability indices.

Methods and structure of the study. Subject to the study performed at laboratories of P.K. Anokhin Research Institute of Normal Physiology (Moscow) were 12 freedivers (including 4 Class I Athletes, 6 Candidates for Master of Sport and 2 Masters of Sport) having 2-7 year-long formal sport records, 175.1±3.2 cm tall, weighing 71.4±3 kg and aged 30±1,7 years; 15 basketball players (including 3 Class II Athletes, 6 Class I Athletes, 5 Candidates for Master of Sport and 1 Masters of Sport) having 3-7 year-long formal sport records, 185,4±2,9 cm tall, weighing 84±2.4 kg and aged 21±0.6 years; and 14 healthy untrained adults engaged in regular physical education practices – 169.8±3.3 cm tall, weighing 69±5.1 kg and aged 20±0.5 years.

At the onset of the study, the primary hypoxia tolerance rates and physical endurance rates of the subjects were obtained by the tests in which the subjects were required to hold breath for maximum possible time at rest. In 10-15 minutes they were exposed to physical loads on a cycle ergometer with the primary load rated at 2 W per kilo of body weight and then the load rate was raised by 30 W every 30 seconds. The rotation rate was to be maintained at 70-75 revolutions per min by a speedometer which additionally recorded the distance run by the tested subject. Breath rhythms of the subjects under the physical loads were to be kept deliberate as required by the actual metabolic needs of the body with no influence from the test assistant. The rotation rate was also controlled by the subjects on their own based on the speedometer readings.

In 20-30 minutes of the test time, combined effects of hypoxia and physical load on the body were tested. The subjects performed uninterrupted physical work on the cycle ergometer with repeated breath-holds growing with time. This combination of the heavy loads with the breath-holding cycles was designed to simulate one a long-distance swimming with growing breath-holding times that is one of the key freedive training exercises. In these tests, the cycle ergometer load was permanent rated at 1 W per kilo of the body weight. The breath holding times were set to grow, with the first time of 20 s followed by the breath-holds of 30, 35, 40, 45, 50, 55 and 60 s. In between the breath-holds, the tested subjects made 3-5 breaths for 3-5 s. The test was deemed completed when the tested subject was unable to continue due to fatigue.

The hypoxia was rated by pulse-oxygen-meter with the arterial blood being tested for arterial oxygen saturation (SpO₂). The vegetative nervous system tone was rated by the heart rate variability indices [1], with Neurosoft software being used to process the RR-intervals on the ECG. In the statistical processing of the test data, average values and mean arithmetic deviations were computed, with t-tests applied to find the statistical difference significance rates for two arrays of the study data. A difference was rated as significant at p<0.05, high at p<0.005 and very high at p<0.001.

Study results and discussion. The resting breath holds of the tested freedivers averaged 2 min 34 s±21 s that were statistically significantly higher than those of the basketball players (1 min 14 s±5 s) and the untrained subjects (1 min 25 s±4 s). The free-of-breath-hold distances run by the freedivers in the cycle ergometer tests averaged 303±20 m compared to 270±31 m run by the basketball players and 165±22 m run by the untrained subjects.

At the prestart stage, duration of heart cycles (RRNN) averaged 644±31 ms in the freedivers compared to 579±16 ms in the basketball players and 574±27 ms in the untrained subjects. The standard deviation of the heart cycle durations from the average value (SDNN) was found to average 52.8±5.6 ms in the freedivers compared to 30.3±2.4 ms in the basketball players and 29.1±3.3 ms in the untrained subjects. The standard deviation of the actual duration difference of the adjacent heart cycles from the average value of the difference (RMSSD) was found to average 28.8±4.6 ms in the freedivers compared to 13.3±1.6 ms in the basketball players and 17.0±2.4 ms in the untrained subjects (see Figure 1).

Figure 1. Standard deviation of basic cycle length (SDNN) and the standard deviation of the actual duration difference of the adjacent heart cycles from the average value of the difference (RMSSD) in ms, as found by the prestart tests of freedivers (I), basketball players (II) and untrained subjects (III).

*Statistically significant (p<0.05); and ** (p<0.005) difference of the SDNN and RMSSD values in freedivers versus basketball players and freedivers versus untrained subjects

In the prestart conditions, the total wave amplitudes (TP) in the heart rate variability spectra of the freedivers were found to make up 2809±471 ms² compared to 970±138 ms² of the basketball players and 849±224 ms² of the untrained subjects. Amplitudes of the very-low-frequency (VLF), low-frequency (LF) and high-frequency (HF) waves in the prestart-state heart rate variability spectrum of the freedivers were notably higher than those in the basketball players and the untrained subjects (see Figure 2).

Figure 2. Amplitudes of very-low-frequency (VLF), low-frequency (LF) and high-frequency (HF) waves (ms²) in the prestart-state heart rate variability spectrum of freedivers (I), basketball players (II) and untrained subjects (III).

*Statistically significant (p<0.05); and **(p<0.005) difference of the VLF, LF and HF values in freedivers versus basketball players and freedivers versus untrained subjects

The cycle ergometer load tests with repeated breath holds totalled 2 min 51 s±26 s for the freedivers compared to 1 min 28 s±9 s for the basketball players and 54 s±6 s for the untrained subjects, with the relevant speedometer-fixed cycling distances found to average 406±41 m, 264±17 m and 184±21 m, respectively.

The SpO₂ values showed no statistically significant variations in all the tested subjects during the first three breath-holds, followed by the statistically significant fall of the SpO₂ values in the freedivers from the prestart 97.8±0.3% to the fifth-breath-hold 89.7±2.7%. The freedivers were still active in the cycle ergometer test in the sixth (and some even in the seventh and eighth) breath holds. The basketball players were tested with the SpO₂ sagging from the prestart 96.5±0.3% to the fourth-breath-hold 86.7±3.5%. Thereafter the tested basketball players were no more able to hold breath for the fifth time and had to stop the cycling test. The fourth-breath-hold SpO₂ rates of the basketball players were statistically notably lower than those of the freedivers. The untrained subjects were tested with the SpO₂ rates staying statistically unchanged varying from the prestart 97.5±0.4% to the third-breath-hold 96.0±1.1. Thereafter the untrained subjects could no more hold breath for the fourth time and had to stop the cycling test.

The RRNN rates showed the statistically significant fall from the prestart 644±31 ms to the sixth-breath-hold 476±21 ms in the freedivers; from the prestart 574±27 ms to the fourth-breath-hold 457±19 ms in the basketball players; and from the prestart 574±27 ms to the third-breath-hold 453±24 ms in the untrained subjects.

The SDNN rates showed the statistically significant growth from the prestart 52.8±5.6 ms to the third-breath-hold 101.4±17.6 ms in the freedivers; from the prestart 30.3±2.4 ms to the second-breath-hold 64.9±9.3 ms in the basketball players; and from the prestart 29.1±3.3 ms to the first-breath-hold 61.7±14.4 ms in the untrained subjects. Thereafter the SDNN rates were found to go back to the starting levels in every tested subject.

Therefore, the untrained people were naturally tested with low physical endurance and hypoxia tolerance rates. The basketball players showed fairly high physical endurance rates but their hypoxia tolerance rates were found insufficient for high accomplishments in the sport. The freedivers showed the highest physical work capacity rates due to their excellent hypoxia tolerance rates developed by the special training methods they apply.

The prestart sympathetic nervous system (SNS) tone was increased in all the subjects making them prepared for the physical loads; and the freedivers were additionally tested with an increased prestart parasympathetic nervous system (PSNS) tone. The SNS tone continued to grow under the physical loads. In the performance tests with the repeated breath-holds, all the subjects were tested with an increased PSNS tonus, the growth being most expressed in the freedivers. The growing parasympathetic nervous system tone is believed to protect brain and heart from the nearing or rising hypoxia caused by the breath-holds [2, 5-7, 9]. The PSNS tone being on the rise may be considered an inherent physiological mechanism activated to protect vital organs from hypoxia, the effect being particularly expressed in the freedivers due to the special training methods they apply.

Conclusion. The study identified varying hypoxia tolerance mechanisms in all the subjects, albeit their effects were found less expressed in the untrained subjects and basketball players. The freedivers were tested with the highest hypoxia tolerance rates contributing to their overall high work capacity rates. Therefore, there are good reasons to believe that high accomplishments in any sport are attainable based not on physical endurance only, but on the hypoxia tolerance qualities too. Both of the physiological qualities are equally important for athletes being imperative for their high competitive performance standards and, hence, stable success in sports.

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Corresponding author: yuvaguine@yandex.ru

Abstract
Objective of the study was to explore the physical endurance and hypoxia tolerance mechanisms in freedivers, basketball players and untrained people based on the sympathetic/ parasympathetic nervous system (SNS/ PSNS) tone profiling by the heart rate variability indices. The untrained subjects showed the lowest physical endurance and hypoxia tolerance rates in the tests. The basketball players showed fairly high physical endurance rates but their hypoxia tolerance rates were found insufficient for high accomplishments in the sport. The freedivers showed the highest physical work capacity rates largely due to their excellent hypoxia tolerance achieved by the special training methods applied. The prestart sympathetic nervous system tone was found to increase in all the subjects and was on the rise under the physical loads. The prestart parasympathetic nervous system tone was found to grow in the freedivers and in all the subjects under the physical loads with repeated breath-holds. The growing parasympathetic nervous system tonus may be considered an inherent response mechanism that is activated to protect vital organs from hypoxia, and it is only natural that these responses are mostly expressed in the freedivers.