The Effects of Intermittent Hypoxic Training on Anaerobic and Aerobic Power in Boxers

nd: The aim of the study was to evaluate the e ects of intermittent hypoxic training (IHT) on anaerobic and aerobic tness in elite, national boxers. Methods: The study was conducted over a period of 6 weeks. It comprised 30 national championship boxers, divided into 2 groups: the experimental and control. Both groups performed the same boxing training twice a day (morning and afternoon training). In the afternoon, the experimental group performed training under normobaric conditions in a hypoxic chamber (IHT), while the control group undertook exercise in standard normoxic conditions. In both groups, before and after the 6-week programme, basic anthropometric indices as well as anaerobic (Wingate Test) and aerobic (graded test) tness were assessed. Results: There was a signi cant increase in anaerobic peak power (988.2 vs. 1011.8 W), mean anaerobic power (741.1 vs. 764.8 W), total work (22.84 vs. 22.39 kJ),

group undertook exercise in standard normoxic conditions. In both groups, before and after the 6-week programme, basic anthropometric indices as well as anaerobic (Wingate Test) and aerobic (graded test) tness were assessed. Results: There was a signi cant increase in anaerobic peak power (988.2 vs. 1011.8 W), mean anaerobic power (741.1 vs. 764.8 W), total work (22.84 vs. 22.39 kJ), and a decrease in fatigue index (20.33 vs. 18.6 W s 1 ) as well as time to peak power (5.01 vs. 4.72 s). Such changes were not observed in the control group. In both groups, no signi cant changes in endurance performance were noted after the training session – peak oxygen uptake did not signi cantly vary after IHT. Conclusions: Our results have practical application for coaches, as the IHT seems to be e ective in improving anaerobic performance among boxers. Keywords:aerobic capacity; anaerobic capacity; boxing; normobaric hypoxia 1. Introduction In recent years, scienti c interest in boxing has increased signi cantly. The data published are related to the structure of training in individual mesocycles, predicting sport results as well as physiological pro le of competitors [1–3]. In some publications, results of studies are presented Int. J. Environ. Res. Public Health2020,17, 9361; doi:10.3390 /ijerph17249361 /journal/ijerph

Int. J. Environ. Res. Public Health2020,17, 9361 2 of 11 concerning only the motor skills of boxers [4,5]. Only a handful of publications have been devoted to assessing indices of anaerobic capacity and muscle damage in boxers [6,7]. Currently, constant monitoring of exercise intensity and evaluation of physiological responses to boxing training can be carried out by measuring heart rate with sports testers, while the level of lactate concentration can be measured on the earlobes. Training volume is de ned as an e ective duration of exercise. Training loads are computed as the product of the training volume and its intensity using the notation of loads in energetic and informative zones [8]. In boxing, it is necessary to conduct training that provides comprehensive motor preparation, including that focused on speed, strength, power, coordination, and physical tness of athletes, which is one of the conditions for success in this sport. Unfortunately, there is a lack of speci c training guidelines, which makes it di cult to determine whether the applied training methods are optimal to maximise exercise capacity [9]. Boxing requires not only a combination of technical, tactical, mental and physical skills, but also high aerobic tness, which, along with strength and speed, is certainly one of the most important features that should be considered when planning a boxer's physical conditioning programme [9,10]. The dynamics of movement around a ring, frequent changes in e ort intensity, delivering blows and defense responses are primarily based on anaerobic metabolism, hence the need to develop them in the course of the training cycle. Therefore, physical training in boxing should be aimed at increasing both the aerobic and anaerobic tness of a boxer. A boxing competition, as in other combat sports, requires both aerobic and anaerobic metabolism in the e ort during a ght. Competitors usually attack with maximal strength, which requires anaerobic metabolism. Aerobic metabolism usually occur when a competitor relaxes after an attack or during breaks between rounds. The frequency of both types of processes also depends on the number of rounds in a bout, which is why amateur competing more frequently requires

anaerobic metabolism in the e ort during a ght. Competitors usually attack with maximal strength, which requires anaerobic metabolism. Aerobic metabolism usually occur when a competitor relaxes after an attack or during breaks between rounds. The frequency of both types of processes also depends on the number of rounds in a bout, which is why amateur competing more frequently requires aerobic energy pathway in comparison to a professional boxing competition with a greater number of rounds [11]. For this reason, new methods or combinations of training methods are sought to maximise exercise capacity. One such method is residing in the lowlands and training at an appropriate altitude (Live Low/Train High [LL/TH]). Recently, coaches, mainly of endurance disciplines, but also of strength-power disciplines, include hypoxic training in their programmes. In studies on hypoxic training, improvement in exercise capacity of athletes is suggested [12]. One of the such methods is intermittent hypoxic training (IHT). In accordance with this method, athletes undergo 1–2 h of training in a hypoxic chamber before and after being in normoxic conditions. The physiological mechanism of the body's response to hypoxia is used to increase both aerobic and anaerobic tness. In previous studies on IHT, improvement has been demonstrated in the ability to perform long-term physical e orts of sub-maximal intensity; an increase in maximal oxygen uptake (VO2max) and intensity at metabolic thresholds—particularly the second ventilatory threshold (VT2), is extremely important in competitive sports [13,14]. However, in other studies, increases in VO2max as a result of IHT were not noted, which may be explained by the insu cient exercise duration in hypoxic conditions and the overall workload [15,16]. Research results also indicate that training in hypoxic conditions increases anaerobic power, which characterises the ability to perform short-term physical exercise at maximal and supramaximal intensities [17–20]. However, some authors suggested that this type of training does not signi cantly a ect anaerobic capacity or explosive strength of the lower limbs or maximal running speed [21,22]. Due to ambiguous opinions regarding the e ectiveness of IHT [13,16], the aim of this study was to assess the impact of IHT on

exercise at maximal and supramaximal intensities [17–20]. However, some authors suggested that this type of training does not signi cantly a ect anaerobic capacity or explosive strength of the lower limbs or maximal running speed [21,22]. Due to ambiguous opinions regarding the e ectiveness of IHT [13,16], the aim of this study was to assess the impact of IHT on anaerobic and aerobic tness in elite, national boxers. We adopted the hypothesis that including IHT in standard boxing training, will signi cantly improve aerobic and anaerobic tness in boxers, further allowing to develop new training methods in this sport with the use of hypoxic training.

Int. J. Environ. Res. Public Health2020,17, 9361 3 of 11 2. Materials and Methods 2.1. Experimental Approach to the Problem The study was designed as pretest/posttest evaluation. The research was experimental, conducted over a period of 6 weeks. The study comprised 30 men: Polish elite, male boxers, who were randomly divided into 2 groups of 15: the experimental (IHT) group and control (normoxia) group, ensuring the same number of competitors from similar weight categories (light, medium and heavy) be included in both groups, which further performed the same standard physical training 5 days a week. In the morning, a 60-min technical boxing training session was performed, including exercises conducted with low to medium intensity (up to 50% of maximum load, up to HRmax), and in the afternoon, a 60-min boxing training session. Details of the training are presented in Tables. The only di erence in training was that boxers in the experimental group performed the afternoon training in normobaric hypoxic conditions in a hypoxic chamber, while competitors from the control group remained in standard normoxic conditions. The athletes trained in Krakâw (Poland), at an altitude of about 230 m (754.5 ft) above sea level. During the experiment, the subjects were asked to maintain their usual diet, which was veri ed using entries in food diaries. IHT was implemented in a normobarcic hypoxic chamber at a simulated altitude of 4000 m (FiO2=12.9%). During the tests, the chamber temperature was 21–22 C with an air humidity of 40–45%. All boxers fully completed all components of the study. Table 1. Description of exercises performed during the afternoon training session (60 min) in the experimental (hypoxia) and control (normoxia) groups. Description of exercise 1. coordination exercises 2. exercises, special warm-up exercises speci c for the sport, shadow technique. 3. tennis-ball exercises. 4. 5. with partner. 6. punching bags. 7. of jabs, hooks, uppercuts and their combination, as well as moving in di erent directions. 8. training, strength endurance training, power training. 9. Development of special abilities, intensity speci c for target competitions. 10.Pace, pace intervals, sprint. 11.Stretching, cooling down the body. 12.Exercises preventing injury

the sport, shadow technique. 3. tennis-ball exercises. 4. 5. with partner. 6. punching bags. 7. of jabs, hooks, uppercuts and their combination, as well as moving in di erent directions. 8. training, strength endurance training, power training. 9. Development of special abilities, intensity speci c for target competitions. 10.Pace, pace intervals, sprint. 11.Stretching, cooling down the body. 12.Exercises preventing injury after intense work of the body. Table 2. Course of afternoon training session (60 min) in the experimental (hypoxia) and control (normoxia) groups—description of exercises in Table. Duration Period in the Course of Testing Execution of Exercises in the Experimental Programme First 4 weeks 1st–4th week of testing Endurance-speed training. Pace intervals of 8 series, 10 s of work at maximal intensity/50 s break for each exercise interspersed with recovery intervals performed at 4 times lower intensity. Following 2 weeks 5th–6th week of testing Power training. Power interval of 5 exercises performed at submaximal speed, lasting 20 s, with a 3-min recovery period. During the session, 3 such sets with 10-min recovery (training at low intensity up to 40 percent of maximum load). Prior to participation in the tests, the competitors were informed about the research procedures, which were in accordance with the ethical principles of the Declaration of Helsinki WMADH (2000). Obtaining the competitors' written consent was the condition for their participation in the project. The research was approved by the Bioethics Committee at the Regional Medical Chamber (No. 42/KBL/OIL/2015). 2.2. Participants The studied athletes (elite, national, male boxers) had a minimum of ten years' training experience, and were winners of medals at national and international competitions. The mean age of boxers was 24.2 3 yrs (IHT) and 23.5 3 yrs (normoxia).

Int. J. Environ. Res. Public Health2020,17, 9361 4 of 11 2.3. Somatic Measurements and Stress Tests Basic anthropometric indices and the level of aerobic and anaerobic tness were assessed in both groups before and after completing the 6-week training cycle. Body mass and fat mass were determined with a body composition analyser (Tanita, MC 718, Japan) using the method of electrical bioimpedance. Body height was measured via a stadiometer. Body mass index (BMI) was calculated for each of the subjects. The measurements were carried out in the morning under euhydration (proper body hydration conditions). For 24 h before any measurements, participants did not engage in prolonged physical exercise or sauna use. Their feet were clean and degreased. All measurements were carried out at similar ambient temperatures. The exercise tests took place on 2 consecutive days: the anaerobic power test (Wingate test) was performed on the rst day, and the next day, endurance was assessed by measuring peak oxygen uptake (VO2peak) and by determining the level of the second ventilatory threshold. Exercise tests were carried out in normoxic conditions (FiO2 20.93%) at a room temperature of 21 0.5 C and relative humidity of 40 5%. During this time, the men could consume uids without any restrictions, and before going to bed and in the morning after waking up, they were advised to drink a total of 1000 mL of isotonic uids. Twenty-four hours before the stress tests, the competitors were not allowed to consume alcohol, co ee or other stimulants. All of the tests were carried out in the early morning hours, following a light breakfast. To determine the level of maximal anaerobic power, the 30-s Wingate test was applied (Bar-Or, 1987), which was performed on a Cyclus 2 cycloergometer (RBM elektronik-automation GmbH, Leipzig, Germany), with a load totalling 7.5% of the subject's body mass. The main test was preceded by a 5-min warm-up at 100 W. During the warm-up, the subjects performed two 5-s maximal accelerations in the 2nd and 4th minutes. Two minutes after the warm-up, the subjects performed the Wingate test. The athletes began the e ort in

elektronik-automation GmbH, Leipzig, Germany), with a load totalling 7.5% of the subject's body mass. The main test was preceded by a 5-min warm-up at 100 W. During the warm-up, the subjects performed two 5-s maximal accelerations in the 2nd and 4th minutes. Two minutes after the warm-up, the subjects performed the Wingate test. The athletes began the e ort in a static position, their task being to develop maximal pedaling cadence on the cycloergometer as quickly as possible and then maintain it for as long as possible. During the test, the following indices were measured: total work performed (TW), peak anaerobic power (PP), average power after 30 s of the test (MP), time to attain peak power (tPP), and power decrease index (FI). Immediately before and during the 3rd minute, as well as 20 min after the completion of exercise, 20 L of blood were collected from the ngertip for determination of lactate concentration (LA) using the Super GL2 analyser (Müller Gerätebau GmbH, Freital, Germany). In order to assess the level of aerobic tness – peak oxygen uptake (VO2peak) and the second ventilatory threshold—a running test of progressive intensity was performed on the mechanical treadmill (h/p/cosmos, Nussdorf—Traunstein, Germany). The test began with a 2-min recording of ventilatory markers at rest, the subjects in a standing position. For the rst 4 min of the test, the participants ran at a speed of 8 km h 1 , afterwards, the running speed was increased by 1 km h 1 every 2 min. The e ort was continued until volitional exhaustion, which was manifested in the inability to keep on running at the determined speed. During the test, the levels of cardiopulmonary indices were recorded based on the “breath-by-breath” method using an ergospirometer (Cosmed, Rome, Italy). The following indices were analysed: pulmonary ventilation (VE), oxygen uptake (VO2), carbon dioxide production (VCO2), respiratory-exchange-ratio (RER), expiratory carbon dioxide concentration (%FECO2), ventilatory equivalent ratio for oxygen and carbon dioxide (VE/VCO2), and heart rate (HR). Data were averaged every 30 s. The highest registered value of oxygen uptake was considered as peak oxygen uptake. The second ventilatory

(Cosmed, Rome, Italy). The following indices were analysed: pulmonary ventilation (VE), oxygen uptake (VO2), carbon dioxide production (VCO2), respiratory-exchange-ratio (RER), expiratory carbon dioxide concentration (%FECO2), ventilatory equivalent ratio for oxygen and carbon dioxide (VE/VCO2), and heart rate (HR). Data were averaged every 30 s. The highest registered value of oxygen uptake was considered as peak oxygen uptake. The second ventilatory threshold (VT2) was determined based on the dynamics of changes in respiratory indices. It was assumed that VT2was reached after the following criteria were met: (1) a decrease in %FECO2after reaching maximal level; (2) a rapid nonlinear increase in VE (second de ection); (3) the VE/VCO2ratio reached a minimum and began to increase; (4) a nonlinear increase in VCO2(second de ection) [23]. Immediately before and during the 3rd and 20th minutes after completion of the progressive test, blood was collected from the ngertip for determination of lactate concentration using a Super GL2 analyser (Dr. Müller Gerätebau GmbH, Freital, Germany).

Int. J. Environ. Res. Public Health2020,17, 9361 5 of 11 2.4. Statistical Methods The test results were statistically analysed by determining mean values and standard deviations. Normality and homogeneity of variance were con rmed using the Shapiro-Wilks and Levene's tests. Two-way analysis of variance with repeated measures was used to investigate the main e ects and interaction between the group (hypoxia vs. normoxia) and time factors (pre-training vs. post-training). In the case of signi cant e ects regarding the main factors, the signi cance of di erences between speci c averages was checked using the Bonferroni test (post-hoc analysis). Moreover, in order to quantify the size of di erences in the data, e ect size (ES: Cohen'sd), as the di erence in group means divided by the standard deviation of the pooled data, was calculated and classi ed as trivial ( 0.19), small (0.20–0.49), moderate (0.50–0.79) or large ( 0.80). For all analyses, the level ofp 0.05 was selected to indicate statistical signi cance. All calculations were performed using STATISTICA ver. 13.3 (TIBCO Software Inc., Palo Alto, CA, USA). 3. Results The physical characteristics of boxers were similar in both groups and are presented in Table. There was no signi cant improvement in somatic variables after the training in either of the groups (Table). Table 3.Participants' physical characteristics. Variables Hypoxia Training Normoxia Training Before After p d Before After p d BH (cm) 182.9 5.09 182.9 5.09 - - 179.7 4.61 179.7 4.61 - - BM (kg) 80.9 8.48 80.3 8.35 0.05 0.07 77.7 9.19 78.0 9.16 0.30 0.03 BMI 24.2 1.88 24.0 1.80 0.05 0.11 24.1 2.53 24.2 2.54 0.29 0.04 FAT (%) 15.3 3.84 14.9 3.22 0.47 0.11 14.1 4.49 14.5 4.22 0.36 0.09 FAT (kg)12.5 4.06 12.1 3.54 0.32 0.11 11.1 4.19 11.5 4.11 0.19 0.10 BH—body height, BM—body mass, BMI—body mass index. Analysis of variance showed signi cant improvement in absolute peak power (f=11.225,p=0.007), absolute (f=12.346,p=0.003) and relative mean power (f=13.829,p=0.002), fatigue index (f=7.316, p=0.002), total work (f=4.548,p=0.049), time to attain peak power (f=4.535,p=0.048) maximal pulmonary ventilation (f=6.681,p=0.02), maximal running speed (f=8.529,p=0.01), and speed at VT2(f=5.841,p=0.028)

3.54 0.32 0.11 11.1 4.19 11.5 4.11 0.19 0.10 BH—body height, BM—body mass, BMI—body mass index. Analysis of variance showed signi cant improvement in absolute peak power (f=11.225,p=0.007), absolute (f=12.346,p=0.003) and relative mean power (f=13.829,p=0.002), fatigue index (f=7.316, p=0.002), total work (f=4.548,p=0.049), time to attain peak power (f=4.535,p=0.048) maximal pulmonary ventilation (f=6.681,p=0.02), maximal running speed (f=8.529,p=0.01), and speed at VT2(f=5.841,p=0.028) in boxers after the training. Post-hoc analysis indicated that signi cant changes of these variables occurred only in the hypoxia group (Table). In the hypoxia group, we noted a signi cantly higher level of the following indices in the Wingate test performed after IHT: peak power (988.2 vs. 1011.8 W;p=0.001, d=0.192 – small improvement), mean power (741.1 vs. 764.8 W;p=0.001, d=0.275—small improvement), and total work (22.39 vs. 22.84 kJ;p=0.046, d=0.18—small improvement). Signi cant decreases in fatigue index (20.33 vs. 18.6 W s 1 ;p=0.013, d=0.594—medium-to-large improvement), and time to attain peak power (5.01 vs. 4.72 s;p=0.038, d=0.362—small-to-medium improvement) were noted. Similar changes were not found in the group of boxers training in normoxic conditions (Table). In neither of the examined groups were any signi cant changes noted in endurance abilities, i.e., peak oxygen uptake and VT2threshold level, were noted under the in uence of the training (Table). However, in the group of boxers training under hypoxic conditions, there was a signi cant increase in maximal pulmonary ventilation (161.9 vs. 156.5 L min 1 ;p=0.004,d=0.276—small-to-medium improvement), maximal running speed (15.29 vs. 14.61 km h 1 ;p=0.002,d=0.531—medium improvement), and running speed at VT2level (13.14 vs. 12.69 km h 1 ;p=0.019,d=0.454—medium improvement) (Table).

Intermittent hypoxic training improved anaerobic performance in elite boxers.