To achieve record levels, highly qualified athletes work at the limit of the body's functional capabilities, which often lead to homeostasis impairment. One of the ways to preserve homeostasis is to expand the adaptive boundaries of organs and systems that provide the response to training and competitive physical loads, which ensures the achievement of an adaptive result [1].
According to literature data, as a result of low-level laser radiation (LLLR) absorption, energy is transformed into various biological reactions, which trigger the processes of self-regulation and self-healing of impaired homeostasis [2–5]. In particular, under the influence of LLLT, antioxidant defense enzymes are activated, cell metabolism is enhanced, biomembranes are stabilized. The effect of LLLR on the elasticity of erythrocyte membranes facilitates the red blood cells penetration into the capillaries of the microvascular bed, and the stimulation of the energy metabolism aerobic phase involving the incompletely deoxidized glycolysis metabolites and lipid oxidation products as well as the indirect membrane mechanism, leads to oxygen saturation of the venous blood and improves microcirculation [6]. At the same time, the influence of LLLT on microcirculatory-tissue relations remains understudied [7].
Laser therapy is an essential component of modern biomedical support of the elite sport at almost all stages of athletes’ training. Comprehensive monitoring of the athlet’s body using a complex of informative and reproducible methods for rapid assessment of body state (biochemical and hematological indicators, laser Doppler flowmetry (LDF) data, heart rate variability (HRV), neuroenergy mapping (NEM) data, etc.) allows one to correct the athlete’s homeostasis for adequate formation of fatigue not going beyond the pathology side and for acceleration of the recovery processes.

Researchers revealed the ability of LLLT to improve the physical performance of athletes of various qualifications involved in different sports. LLLR irradiation of biologically active points leaded to an increase in the aerobic performance indicator in 80% of football players with a subsequent increase in the amount of mechanical work performed [8]. A significant increase in the absolute and relative values of PWC170 after LLLT was obtained in athletes of cyclic sports. Short-term exposure to LLLR did not cause significant changes in the biochemical composition of the blood, but increased the activity of parasympathetic influences on the heart rhythm [9, 10]. The humoral-hormonal status of the athletes’ body changed due to LLLT. In particular, an increase in the concentration of beta-endorphin, glucocorticoids, triiodothyronine, thyroxine in game sports athletes and cross-country skiers was revealed [11, 12].
The systemic mechanisms that provide the effects of laser stimulation as a part of the complex training program of hockey players are described. Positive structural and functional changes in the body of hockey players and swimmers lead to a marked improvement of the physical fitness [13, 14].
The study was aimed to investigate the physiological response of highly qualified female rowers’ functional systems to the LLLR irradiation course in the special preparatory period of the annual cycle of sports training. The objectives of the study were as follows: assessment of the LLLT effect on the microcirculation system, detection of the heart rate regulation changes, investigation of the effect on the metabolic activity of the cerebral cortex neurons, evaluation of the highly qualified athletes’ physical performance.

METHODS

The study was conducted in October 2018. Twenty four highly qualified female rowers studying at the State School of the Olympic reserve (Bronnitsy, Moscow region) participated in the study which was carried out at the training center.
The participants were divided into two groups: treatment group (TG) and control group (CG). The treatment group included 12 athletes. Inclusion criteria: Master of Sports (MS) qualification level, membership in the Moscow Region combined team. Exclusion criteria: low qualification of athletes, the acute phase of the disease. The control group included 12 athletes (MS) not qualified for the combined team. Representatives of both groups used the single training program.
The study included two phases. At the first phase of the study, we evaluated the functional state of individual body systems and the physical fitness of the TG and CG athletes before LLLT. Then, the athletes of both groups in their weekly training cycle performed a special training program to prepare for the competition season.
The TG athletes were exposed to LLLT during 7 days in the morning before training. Their necks were irradiated neck symmetrically on both sides in the region of the carotid triangle using Uzor-A-2K 2-channel therapeutic laser unit (Voskhod; Russia). The laser radiation wavelength was 0.89 ± 0.02 μm; pulse mode; pulse repetition frequency 1500 Hz; 10 minute exposure time. The CG athletes were through the fake LLLT without turning on the emitting heads of the Uzor-A-2K unit. After laser therapy the studied indices were registered again.

The athletes’ heart rate variability (HRV) was evaluated using the Varicard 2.51 complex (Ramena; Russia). The recording of cardiointervalogram lasting 5 minutes was carried out using the standard method in a sitting position. The following HRV parameters were evaluated: heart rate (HR), indicators characterizing the activity of autonomous (total power of the heart rate variability spectrum (TP), high-frequency power (HF) and central (low-frequency power (LF), very-low-frequency power (VLF), amplitude mode (АМо) regulatory mechanisms, indicator of the prevalence of central regulatory mechanisms over autonomous (stress-index, SI).
The LAKK-M (Lazma; Russia) multifunctional laser diagnostic system was used as a recording instrument for studying microcirculation. After that the microcirculation parameter (PM) was analyzed in perfusion units (PU). The time-frequency analysis of blood flow oscillations was performed using the wavelet analysis LDF3.0.2.384 software (Lazma; Russia). The active mechanism contribution to the formation of vascular tone was estimated by the amplitude of sympathetic (Аs), myogenic (Аm) and endothelial (Аe) oscillations (PU). The contribution of passive mechanism was estimated by the amplitude of respiratory (Аr) and cardiac (Аc) oscillations (PU). The optical tissue oximetry method was used for evaluation of blood oxygen saturation level (SO2, %) and specific oxygen consumption rate (U, p.d.u.). The steady potentials level (SPL) parameter was used for assessment of the brain tissue metabolic activity in the frontal, parietal, occipital, right and left temporal lobes.

For topographic mapping of brain electrical activity the 5-channel Neuro-KM complex was used (STATOKIN; Russia) according to standard method [15]. The time of 2000 meter distance “passing” using the Concept 2 Model D (PM5; China) rowing simulator was the indicator of special physical fitness. The test was performed indoors, in the gym with constant temperature and illumination intensity.
Statistical analysis of the results was carried out using the IBM SPSS Statistics 19 software for Windows (StatSoft, Inc.; USA). The Mann–Whitney U-test was used to compare the studied indicators in the TG and CG athletes. To compare the indicators in TG athletes and CG athletes who experienced the imaginary effect of LLLT, as well as with the indicators of athletes after laser therapy, the Wilcoxon signed-rank test was used. The differences were considered significant at p < 0.05.

RESULTS

Percutaneous laser stimulation combined with standard training loads promotes the expansion of the body functional capabilities at various organization levels (from cellular to systemic). We studied the dynamics of the processes that occurred in the microvascular beds after the course of LLLT.
In the TG athletes, the perfusion level significantly increased by 38% compared to baseline (р < 0.05). At the same time, in the CG athletes the microcirculation parameter increased by 5% (р > 0.05) (tab. 1). The 14% decrease in SO₂ in the microvascular beds together with a tendency to SO₂ increase by 2% in the CG was the evidence of oxygen metabolism biostimulation in the TG athletes due to LLLT (р < 0.05). A significant increase by 49% in the estimated rate of oxygen utilization by tissues was an indicator of oxygen diffusion from blood into the tissue (р < 0.05). In the CG athletes the same indicator demonstrated almost no growth (1%, р > 0.05).
In our study, the myocyte tone decreased by 53% (р < 0.05) in the TG athletes, thereby increasing the lumen of the microvascular bed vessels. In addition, arterioles widened the lumen due to the decrease in the activity of the autonomic nervous system sympathetic nerves, the ends of which innervate smooth muscle cells of the blood vessel wall middle layer. According to the wavelet analysis, the indicator of sympathetic tone was reduced by 40% (р < 0.05). As a result of LLLT, the throughput of the microvascular bed exchange link increased due to vasodilation of microvessels of various diameters.
At the autonomic nervous system (ANS) level, the course of LLLT reduced the activity of the sympathetic region, while increasing the effect of the ANS parasympathetic region on the heart, which provided trophotropic recovery effect (tab. 2). As a result, stress-index reduced by 174%, АМо reduced by 48% (р < 0.05). The ТР index significantly increased by 41%. Certain spectrum parameters increased as well (LF by 121% and HF by 73%, р < 0.05). Noteworthy is the 75% increase in the VLF index, which reflects the function of cortical-humoral centers.
Without physiotherapy, during training in the CG athletes, sympathetic influences maintained high activity with a tendency to reduce the effect of the vagus nerve on the heart rhythm. High sympathoadrenal system activity promoted energy deficiency. In TG athletes, during the recovery period between training sessions, tissue anabolism increased due to LLLT, thus providing high functional readiness of the body for training activities. At the same time, increased catabolism maintained in the CG athletes caused the fatigue accumulation due to the under-recovery of the body after another training session.

LLLT indirectly stimulates functional adaptive changes in the cerebral cortex neurons. After the course of LLLT the TG athletes demonstrated enchanced metabolic activity of brain tissue in the studied areas, which was evidenced by the growth of steady potential level (SPL) value. Compared to baseline (tab. 3), there was an increase in the SCP indicator by 94% in the frontal lobe, by 109% in the parietal lobe, by 33% in the occipital lobe and 29% in the left temporal lobe (р < 0.05). The steady potential values after LLLT were distributed according to the dome-shaped curve principle (tab. 3). In the CG athletes, maximum SPL values were registered in occipital (Oz) and left temporal (Ts) lobes. Thus, the dome-shaped distribution of energy consumption by the brain was violated, i.e. some deformation occured. In the CG rowers, the SPL value tended to increase during the study, the differences were not significant. Thus, the energy metabolism of neurons in the studied regions of cerebral cortex increased after the course of LLLT. For correct assessment of the effectiveness of the LLLT use as a remedy for recovery it is advisable to evaluate the physical fitness level. In our study we evaluated the physical fitness level using testing with the Concept 2 rowing simulator. The time needed by the highly qualified female rowers to “pass” the 2000 meters distance at various phases of the study is presented in tab. 4. At the first phase (baseline) no significant differences between two groups were revealed. The CG athletes “passed” the distance in 456.55 ± 3.55 s, and the TG rowers “passed” the distance in 454.07 ± 2.43 s (p > 0.05). The course of LLLT stimulated the body of the TG athletes increasing the speed of “passing” the 2000 meter distance up to 435.63 ± 2.34 s, which was 3.32% less than baseline (p < 0.01). In the CG rowers, the time needed for “passing” the distance remained almost unchanged (453.02 ± 3.34 s) (p > 0.05). Thus, combined with the standard training process, the course of LLLT leaded to an increase in the special physical performance of female rowers during the special preparatory period of the sports training annual cycle.

DISCUSSION

The analysis of the obtained data demonstrated that the course of LLLT improved the microhemocirculation system function. We detected a significant perfusion increase, indicating an enchancement of metabolic activity at the cellular and tissue levels. An increase in the microcirculation intensity is associated with vasodilation, regulation in the microcirculation system is provided by external and internal mechanisms [16–18]. Of the internal mechanisms, the maximum contribution to the increase of the microvascular bed vessels capacity is provided by the endothelial component. Endotheliocytes take part in the formation of the vasodilation response which leads to the reduction of the microvascular tone by 62% (р < 0.05). The trigger stimulus arising in response to LLLT is the release of vasodilator, nitric oxide (NO) by Са²⁺-dependent endothelial cells, which is a precursor of the endothelium-derived relaxing factor (EDRF) [19]. Myogenic vasodilation is due to a decrease in the smooth muscle cells tone of the vascular wall. In smooth muscle cells, LLLT leads to an increase in the level of intracellular cAMP in the cytosol, leading to activation of calcium ATPase, a decrease in calcium ions level in the cytoplasm, and relaxation of vascular smooth muscle cells [20]. The pronounced effect of laser photostimulation is associated with the effect of low-intensity radiation on metabolism. Oxidation of energy materials (glucose, pyruvate, lactate) increases, leading to improvement of microcirculation and oxygen utilization in tissues [21]. According to the data obtained, the mixed blood hemoglobin saturation with oxygen of the microvascular bed decreases, the specific oxygen consumption of tissues increases, which facilitate metabolism and provide energy production in the form of ATP in the cells [22]. An increase in the cells functional activity occurs primarily due to calcium-dependent increase in the redox potential of mitochondria, an increase in their functional activity, and ATP synthesis [23–26]. In mitochondria, LLLT accelerates the electron transfer along the respiratory pathway [26].

According to PK Anokhin’s functional systems theory, structural and functional components of different level and localization are involved in the implementation of the adaptive effect by the organism. Local improvement of homeostasis at the tissue microcirculation level is a component of the vegetative balance restructuring at the system level [27]. Under the influence of high-intensity physical activity, the optimal ratio between the sympathetic and parasympathetic ANS links is violated in favor of the predominance of sympathicotonia, reflecting the imbalance of the reciprocal regulatory effects of the ANS on the athlete's cardiac system. In such conditions, a pronounced tension of the compensatory mechanisms of the athlete’s cardiovascular system is observed, which is associated with distress [28].

It was found that LLLT changes the activity of the neural pathways involved in the regulation of cardiac activity. Some authors note that LLLT activates the calcium-dependent mechanisms [29]. Calcium is an intracellular mediator of a number of hormones, primarily mediators of CNS and ANS [30], suggesting the involvement of laser-induced effects in neurohumoral regulation. At the end of the laser therapy course, the activity of the ANS sympathetic division decreases, the contribution of the parasympathetic division increases, and the total intensity of regulatory processes decreases. In general, LLLT promotes the deployment of trophotropic processes aimed at preserving energy and plastic resources.

It is well known that sports activity gives results only when the athlete’s skill is refined to automatism, that is, with minimal participation from the central regulatory systems. А system with relatively autonomous links, due to the independence of its elements, is more flexible, which facilitates its adaptation to changing environmental conditions, including adaptation to sports activity [31]. An increase in the number of the sinus node degrees of freedom helps the body to achieve a functional optimum to cope the load. As a result, SI reduces by 174% (р < 0.05). The spectral parameters dynamics indicates a transition to a higher level of adaptive capabilities ensuring the athlete’s body resistance to training loads. Thus, the growth rate of the activity of the autonomous heart rhythm control (HF) pathway increases by 73%, and of the central (LF) by 121%. At the same time, the contribution of cortical-humoral control centers (VLF) is enhanced by 75% against the background of bradycardia. Such a spectrogram reflects the high functional capabilities of the athlete’s body [32].
An increase in the functional reserve of the body after a course of LLLT has been noted earlier [33–34]. Laser radiation regulates restoration of the vegetative balance and restrains the activity of the sympathoadrenal system [35]. The evidence was obtained of the relationship between the increased relative power of a heart rhythm spectrum in the VLF frequency range with a change in the frequency and time parameters of the brain rhythmic activity [36]. Rhythmic activity was detected in the frontal, parietal and occipital lobes of the brain.
Some researchers suggest that normal energy exchange is mainly characterized by the dome-shaped curve, in which the maximum potential values are recorded in the central lead (Cz) and gradually decrease to the periphery [37]. Obviously, an SPL shift in the occipital and left temporal lobes may be associated with an increase in the functional activity of nonspecific reticular-limbic-cortical neural pathways [38].
An imbalance of regulatory influences from the higher nervous activity, depending on the brain and its cortex, leads to a violation of the speed of conditioned reflex reactions, a violation of the interaction between the first and second signaling systems, accompanied by emotional and behavioral deviations [39]. However, the adaptation mechanisms of athlete’s cortical neurons under the influence of extremely high physical activity remain understudied. The results of our study on the metabolic activity of neurons in certain areas of the cerebral cortex after the course of LLLT demonstrate an improvement in the steady potential level by 1.3–2 times.
The structural and functional adaptation changes in the body arising under the influence of LLLT promote the improvement of physical performance and physical fitness of highly qualified rowers. The interaction of low-energy laser radiation with the body allows one to create a highly effective method of using laser therapeutic units in a set of remedies for improvement of special physical performance, physical fitness and overall athletic performance of athletes [40].

CONCLUSION

Our study results demonstrated that exposure to LLLR improved the functional state of the athlete’s body and increased the effectiveness of sports training at the preparatory stage. The systemic response to LLLT was associated with perfusion increase in the exchange link of microvascular bed, facilitation of the diffusion of oxygen from blood into tissues and an increase in the efficiency of oxygen consumption in the cell. To a large extent, additional influx of blood from the main vessels ensured the microcirculation increase. After the course of LLLT, the increase of metabolic activity of neurons in certain areas of the cerebral cortex was observed. Thus, the adaptive stability of the body increase due to laser therapy, its functional capabilities expand, which helps to improve the athletes' special physical performance and accelerate the recovery process.