The Effect of Respiratory Muscle Training on the Pulmonary Function, Lung Ventilation, and Endurance Performance of Young Soccer Players

his study investigated whether the addition of eight weeks of inspiratory muscle training (IMT) to a regular preseason soccer training program, including incremental endurance training (IET), would change pulmonary function, lung ventilation, and aerobic performance in young soccer players. Sixteen club-level competitive junior soccer players (mean age 17.63 0.48 years, height182 0.05 cm , body mass 68.88 4.48 kg) participated in the study. Participants were randomly assigned into two groups: experimental (n=8) and control (n=8). Both groups performed regular preseason soccer training, including endurance workouts as IET. In addition to this training, the experimental group performed additional IMT for eigght weeks with a commercially available respiratory muscle trainer (Threshold IMT), with a total of 80 inhalations (twice per day, ve days per week). Pre- and post-intervention tests of pulmonary function, maximal inspiratory pressure, and the Cooper test were implemented. Eight weeks of IMT had a positive impact on expiratory muscle strength (p=0.001); however, there was no signi cant e ect on respiratory function parameters. The results also indicate increased e ciency of the inspiratory muscles, contributing to an improvement in aerobic endurance, measured by VO2max estimated from running distance in the cardiorespiratory Cooper test (p<0.005). Keywords:soccer; inspiratory training; lung ventilation; muscle strength; endurance; running 1. Introduction The e ectiveness of soccer players depends on morphological, functional, and psychological factors, which

on respiratory function parameters. The results also indicate increased e ciency of the inspiratory muscles, contributing to an improvement in aerobic endurance, measured by VO2max estimated from running distance in the cardiorespiratory Cooper test (p<0.005). Keywords:soccer; inspiratory training; lung ventilation; muscle strength; endurance; running 1. Introduction The e ectiveness of soccer players depends on morphological, functional, and psychological factors, which determine appropriate playing tactics and success in soccer matches [1–3]. Consequently, soccer is widely recognised to be a prolonged, high-intensity, intermittent activity that requires players to perform regular, repeated sprints throughout the 90-minute game, and where the average exercise intensity is close to the anaerobic threshold of 80–90% of maximum heart rate (HR) [4–8]. The ability to perform intense exercise declines towards the end of a match, as well as immediately after the most intense periods of the game [9,10]. In this context, the implementation of a well-developed aerobic tness training program helps soccer players to maintain repetitive high-intensity actions during a soccer match, accelerate their recovery process, and maintain their physical condition at an optimum level throughout the entire game and competition season [11]. One of the basic and most important parameters of motor preparation in Int. J. Environ. Res. Public Health2020,17, 234; doi:10.3390 /ijerph17010234 /journal/ijerph

Int. J. Environ. Res. Public Health2020,17, 234 2 of 14 soccer is cardiovascular tness, which is often de ned as aerobic endurance [12,13]. In most sports, including soccer, a type of workout is endurance running performance [14,15], often referred to as incremental endurance training (IET). It's mainly based on aerobic e orts [14], but at the climax of its development, may fall into mixed aerobic-anaerobic energy release training, where the level of training intensity is 80–85% of maximum HR [16,17]. This type of training takes on special signi cance during the preseason and is the subject of the abovementioned research experiments. Aerobic endurance performance is dependent on three important components: maximal oxygen uptake (VO2max), anaerobic threshold, and work economy [18]. VO2max is an important component, de ned as the highest oxygen uptake that can be achieved during dynamic exercise with large muscle groups [19]. Therefore, VO2max is an extensively used index for measuring the aerobic tness of athletes and can be determined in both laboratory and eld tests [16,20,21]. Some studies have indicated a signi cant relationship between VO2max and distance covered during a match [21]. During exercise, respiratory muscles are subject to fatigue which limits their ability to work optimally, translating into insu cient oxygen supply to the working muscles [22]. Therefore, it seems appropriate to draw attention to the work of respiratory muscles in sports training, including the training of soccer players [23]. Inspiratory muscle training (IMT) applies an additional load to the diaphragm, being an accessory to the inspiratory muscles, to enhance their strength and endurance. Some research in the literature has assessed the e ects of IMT in soccer players [24–27], demonstrating that the inclusion of such training in the primary soccer training process can improve inspiratory muscle strength (PImax), expiratory muscle strength (PEmax), and exercise tolerance, and also reduce blood lactate (BLa) levels after a cycle of exercises [26]. Research conducted by Archiza et al. [28] on a group of 18 female soccer players indicated that IMT could relieve the metabolism of inspiratory muscles and, as a consequence, improve muscle oxygen supply during high-intensity exercise.

inspiratory muscle strength (PImax), expiratory muscle strength (PEmax), and exercise tolerance, and also reduce blood lactate (BLa) levels after a cycle of exercises [26]. Research conducted by Archiza et al. [28] on a group of 18 female soccer players indicated that IMT could relieve the metabolism of inspiratory muscles and, as a consequence, improve muscle oxygen supply during high-intensity exercise. This process can be translated into an improvement in fatigue tolerance and running e ciency of soccer players [28]. Consistent with this, Ozmen et al. [27] evaluated the e ect of ve weeks of IMT on the respiratory system function and aerobic endurance of soccer players, nding a signi cant improvement in respiratory muscle strength for the tested respiratory parameters. However, respiratory training did not signi cantly improve the soccer players' tolerance to high-intensity exercise. The essence of this type of training is to cause an appropriate amount of resistance during inspiration, with simultaneous quiet exhalation. An important part of this training is the correct breathing technique [29]. Inspiratory muscle training can be an important procedure in sports training. It increases an athlete's physical performance, reduces the concentration of blood lactate, and improves the ventilation of the lungs, diaphragm, and inspiratory muscles [30–33]. With the exception of PImaxand PEmax, the peak expiratory ow (PEF), also called peak expiratory ow rate (PEFR), plays an important role in the assessment of pulmonary function. A measurement PEF is especially useful for assessing bulbar-innervated, inspiratory, and expiratory muscle function [34,35]. The PEF ranges from 500 to 700 litres/min for men and from 380 to 500 litres/min for women, and from 150 to 840 litres/min for children and adolescents, with variations due to age, race, and sex [36]. Peak ow readings are higher when a person is healthy, and lower when the airways are constricted. Lung functionality can be determined from changes in recorded values [34]. A problem to be considered when designing a well-balanced cardiovascular tness training program for soccer players is that the maximal oxygen uptake is most e ectively trained at an intensity of 90–95% of HRmax, which is generally achieved by

person is healthy, and lower when the airways are constricted. Lung functionality can be determined from changes in recorded values [34]. A problem to be considered when designing a well-balanced cardiovascular tness training program for soccer players is that the maximal oxygen uptake is most e ectively trained at an intensity of 90–95% of HRmax, which is generally achieved by running intervals. It is also believed that for VO2max to be improved, playing only soccer games is not enough as it does not provide su cient exercise intensity over time [14,18,20]. It seems reasonable that this type of workout may represent excessive training stimuli for junior soccer players. Therefore, a combination of breathing training with a more intense running training, not exceeding 85% of the anaerobic threshold, should compensate for the intermittent activity discussed above (at 90–95%). Thus, this study aimed to determine whether the addition of eight weeks of IMT to a regular program of preseason soccer training, including IET, would change pulmonary function, lung ventilation, and aerobic performance in young soccer players.

Int. J. Environ. Res. Public Health2020,17, 234 3 of 14 2. Materials and Methods 2.1. Participants Sixteen club-level (Football Academy) competitive junior soccer players (mean age 17.63 0.48 years, height 182 0.05 cm, body mass 68.88 4.48 kg) participated in this study. Participants were randomly assigned into two groups: inspiratory muscle training (IMT, n=8) and control group (n=8). Medical examinations showed no history of pulmonary disease in the participants and acknowledged their good health condition to participate in the study. All participants were non-smokers (self-report), with no evidence of respiratory restrictions or obstruction upon examination of the maximum ow-volume loops. The experimental protocol was approved by the University Institutional Ethics Committee, and all participants and their parents provided written informed consent for voluntary participation in the study. Before the study commenced, the participants were informed of the procedures and potential risks of the training. 2.2. Experimental Design and Task Participants were randomly assigned to the experimental or control group. During the 8-week experimental period, both groups continued their regular preseason soccer training program including endurance workouts as IET. Regular soccer training sessions were performed four times per week under the supervision of a coach. Supervised training sessions consisted of about 2 hours of soccer activities, including strength conditioning, drills for skill improvement, team scrimmaging, and sparring. The soccer training program was developed by the Football Academy. The experimental group performed additional IMT with a commercially available respiratory muscle trainer (Threshold IMT; Philips Respironics, Inc., Murrysville, PA, USA). The players were instructed not to participate in any physical activities during the study period other than those stated. The experiment required an assessment of respiratory muscle strength, pulmonary function, and indirect aerobic endurance (VO2max) via an estimated performance of the Cooper test. The tests were carried out at similar times of the day (morning) for both groups, although the IMT test for the experimental group was performed rst. The Cooper test was performed after that. Both tests were performed before the respiratory and running endurance intervention training was started. The initial performance assessment took place at the beginning of January when the

The tests were carried out at similar times of the day (morning) for both groups, although the IMT test for the experimental group was performed rst. The Cooper test was performed after that. Both tests were performed before the respiratory and running endurance intervention training was started. The initial performance assessment took place at the beginning of January when the participants were starting the preparation phase for their spring/summer competition season. Participation in the experiment required a familiarisation session with the test procedure. The participants were instructed to adhere to their usual diet and to not engage in strenuous activity the day before testing. On the test day, the participants were asked to not eat for at least 2 h before testing. For each participant, testing was scheduled at a similar time of day ( 0.5 h) to minimise the e ects of diurnal uctuation. 2.3. Measures 2.3.1. Dynamic Lung Function Pulmonary function (forced ow-volume loops) was assessed using a spirometer (Flowscreen; Jaeger, Wuerzburg, Germany) with a special adapter (780, 578, version 1.3). Measurements were made according to the recommendation of the European Respiratory Society [37]. During testing, the participants assumed their normal sitting position. The following variables were determined for all participants before and after IMT: vital capacity (VC), forced vital capacity (FVC), forced expiratory volume in rst second (FEV1) and peak expiratory ow (PEF). 2.3.2. Maximal Inspiratory Pressure A spirometer (Jaeger, Wuerzburg, Germany) with a shutter module was used for analysis of muscular respiratory pressure. The obtained values were expressed as a percentage of the normal values. The force of respiratory muscles can be evaluated using static measurements (PImaxand

Int. J. Environ. Res. Public Health2020,17, 234 4 of 14 PEmax) or dynamic manoeuvres (maximal voluntary ventilation, MVV). PImaxrepresents the highest sub-atmospheric pressure that can be generated during an inspiration against a blocked airway (Muller manoeuvre). PEmaxis the highest pressure that can be achieved during a high expiratory e ort against a blocked airway (Valsalva manoeuvre). These methods are usually performed by starting the manoeuvre from the residual volume (for MIP determination) or from the maximal capacity (for PEmaxdetermination). There are a few contraindications for these exploratory manoeuvres: aneurysm, uncontrolled hypertension, urinary infection, and recent abdominal, or thoracic surgery. The participants underwent three to ve maximal acceptable and reproducible manoeuvres (with di erences of 3–9% between values). For the statistical evaluation, the maximal value obtained from these successive trials was taken into consideration. A 1-min interval was permitted between consecutive e orts. The technique used was performed in accordance with the adopted norms [37]. A minimal leak of air (shutter module) was used to prevent blocking of the epiglottis. This minimal leak does not in uence the measurements recorded. The inspiratory or expiratory e ort was sustained for a minimum of 1 s [38]. 2.3.3. Aerobic Endurance Measurement The assessment or measurement of aerobic endurance is achieved by determining the maximum oxygen consumption (VO2max). Laboratory tests using step-like or ramped protocols with incremental intensities can be used for accurate measurement of VO2max [39,40]. Because the current experiment could not measure VO2max directly, the researchers decided to use a cardiorespiratory test, speci cally the Cooper test [41,42], and then estimate the achieved results into VO2max. The formula was as follows: (distance covered in meters 504.9)/44.73 [41]. The Cooper test was conducted in the 400-m track stadium with all players running together, and the total distance covered by each player was recorded. An electronic timer display was placed on the side of the track so the players were familiar with their time. At the end of each lap (every 400 m), the coach told each player his number of laps to run. In addition, markers were set at 100-m intervals around the

and the total distance covered by each player was recorded. An electronic timer display was placed on the side of the track so the players were familiar with their time. At the end of each lap (every 400 m), the coach told each player his number of laps to run. In addition, markers were set at 100-m intervals around the track to help trainers accurately measure the distance covered at the end of 12 min. To reach VO2max at 55–57 mL/kg/min (calculated from the run), the player had to get a result over 3 km in the Cooper test. Calculated in terms of time, this gives a pace of 4 min/km, i.e., 80–85% of the intensity for maximum running e ort. We are aware that this is an indirect measurement, however, the results achieved by young soccer players are comparable to Reilly's [43] suggestion that a VO2max greater than 60 mL/kg/min is required at elite levels of soccer. Before the test, participants conducted their regular 20 min warm-up consisting of a 10-min light run, stretching, running drills, and 100-m rhythm strides. Heart rate was monitored with a Polar RS300X GPS (Polar Electro Oy, Kempele, Finland). The validity of the Cooper test as a correlation between VO2max and the distance covered during a 12-min run was 0.94. 2.3.4. Inspiratory Muscle Training Immediately prior to IMT, instructional classes were held to ensure proper use of the hand-held Threshold IMT device (Philips). Each player received an individual training load according to the results of their preliminary PImax assessment. Due to the nature of the training device, inspiratory muscle strength was converted from kPa to cm H2O (1 kPa=10.2 cm H2O). The IMT group performed dynamic inspiratory e orts that increased in frequency during the study period ( ve repetitions in week 1, 15 repetitions in week 8), twice daily from Monday to Friday for eight weeks (total of 80 sessions) against a pressure threshold load equivalent to 40% MIP in the rst week. After the initial setting of the training load at 40% MIP, participants in the IMT group were instructed to periodically

the study period ( ve repetitions in week 1, 15 repetitions in week 8), twice daily from Monday to Friday for eight weeks (total of 80 sessions) against a pressure threshold load equivalent to 40% MIP in the rst week. After the initial setting of the training load at 40% MIP, participants in the IMT group were instructed to periodically increase the load from 45% MIP in week 2 to 75% MIP in week 8 (Table). One repetition was the equivalent of a 1-min work period, where the participants exercised for 45 s followed by a 15-s break. The rst training session was held at home. The players trained alone each morning between 7:00 a.m. and 9:30 a.m. The second session was executed in the evening (approximately 6:30 p.m.) immediately after the regular football practice. Three times a week during the intervention period, the evening session

Int. J. Environ. Res. Public Health2020,17, 234 5 of 14 training was conducted under the supervision of a physiotherapist to ensure a correct technique and appropriate load. The training took place in a sitting position. The players set up an appropriately selected training load, then put on a nose clip and gripped the mouthpiece with their lips. After this, the participants performed fast, energetic inhalations and slow, quiet exhalations. Participants were requested to complete a daily training diary (load) for IMT training throughout the study. Table 1.Speci cation of the load application during eight weeks of inspiratory muscle training (IMT) and incremental running endurance (IRE) workouts. Training Period Week 1 Week 2 Week 3 Week 4 Week 5 Week 6 Week 7 Week 8 Inspiratory Muscle Training Training load (cmH 2O) 40% PImax 45% PImax 50% PImax 55% PImax 60% PImax 70% PImax 75% PImax 80% PImax Time/session (min.) 5 7 9 10 12 13 14 15 Periodized endurance running training 6 km 5.30 min/km 5.20 min/km 5.10 min/km 5.05 min/km - - - - 3 (5 200 m) - - - - 42 s 42 s 40 s 38 s 2.3.5. Incremental Endurance Training Incremental endurance training is a periodised form of endurance running training that aims to increase the player's endurance. The IET training protocol included four weeks of endurance running training, consisting of a 6-km steady run and 10–15 100 m stride rhythm (65–70% of maximum intensity) executed at the end of training. This training was performed once per week. The initial work rate was based on participants' known work capacity. The soccer players were asked to start running with a 5.30 min/km pace, ending in week 4 with a 5.0 min/km pace. The work rate increments for each subsequent week increased by 5–10 s/km. Heart rates were monitored using a Polar RS300X GPS HR monitor (Polar Electro Oy, Kempele, Finland). During the endurance run, the players' HR should not exceed 135–140 bpm, which is a typical manifestation of aerobic performance. All participants ran together, as both groups of the players of both groups (experimental and control)

for each subsequent week increased by 5–10 s/km. Heart rates were monitored using a Polar RS300X GPS HR monitor (Polar Electro Oy, Kempele, Finland). During the endurance run, the players' HR should not exceed 135–140 bpm, which is a typical manifestation of aerobic performance. All participants ran together, as both groups of the players of both groups (experimental and control) belong to the same team and carry out joint special soccer training for the whole team. After four weeks, participants started the second type of training. This also featured an incremental training load consisting of a 2-km steady run followed by submaximal interval training consisting of three series of ve 200 m run repetitions at 75–80% maximum intensity. The work rate increments increased from 45 s per 200 m in week 5 to 38 s per 200 m in week 8 (Table). Heart rate was also monitored for each 200 m repetition. They started with a target of 150 bpm and ended with 165 bpm. Interval training for both groups took place at the same time, but control and experimental groups trained separately. Each 200-m repetition was performed in pairs of two players in the same group. Pairs were matched according to the coach's speci cations, which was based on the individual running capabilities of players. 2.4. Statistical Analysis Descriptive statistics (mean SD) were calculated for all dependent variables. The Shapiro–Wilk test indicated a normal distribution for all variables. Comparisons between pulmonary function, maximal inspiratory pressure, and Cooper test performance pre- and post-IMT intervention were examined by two-way analysis of variance (ANOVA). Duncan post-hoc tests were performed to determine pairwise di erences when signi cant F ratios were obtained. A comparison of somatic features between the groups was carried out by Student'st-test. The relationship between variables was determined using the Pearson product-moment correlation. The level of signi cance for all statistical comparisons was set atp<0.05.

The study assesses the impact of inspiratory muscle training on young soccer players' performance.