Effects of four weeks of inspiratory muscle training with different resistance modalities on pulmonary function and specialized athletic ability in synchronized swimmers: a randomised trial

Abstract

This study applies resistance inspiratory muscle training to synchronized swimmers and through two resistance setting methods to explore that the differences in the effects of four weeks of dynamic constant load resistance training versus constant load inspiratory muscle resistance training on improving pulmonary function and specialized athletic ability in synchronized swimmers. Fourteen synchronized swimmers were randomly assigned to the dynamic constant load resistance inspiratory muscle training group (n = 7) and the constant load resistance inspiratory muscle training group (n = 7) for 4 weeks of inspiratory muscle resistance training. Group 1 was trained with the resistance setting of dynamic constant load resistance, and 50% of the maximum inspiratory muscle strength index (S-index) measured every Friday was used as the training load for the following week, while group 2 was trained with the constant load, and 50% of the initial value of S-index was used as the training load. The training schedule of both groups was 5 days per week, 2 sets per day, 30 repetitions per set. The subjects’ related index of inspiratory muscle function, pulmonary function and specialized athletic ability were tested before and after the experiment. Through statistical analysis, we concluded that after four weeks of inspiratory muscle training, the related index of inspiratory function, pulmonary function and specialized athletic ability of both groups were significantly different from those of the pre-training period (p < 0.05), while the peak inspiratory flow (PIF) and the maximal inspiratory pressure (MIP) of inspiratory muscle function indices were significantly different in the group-to-group comparison (p < 0.05). Pulmonary function and specialized athletic ability also had significant differences between two groups (p < 0.05). Compared with constant load resistance inspiratory muscle training, dynamic constant load resistance inspiratory muscle training is more effective in improving inspiratory muscle function, pulmonary function and specialized athletic ability of synchronized swimmers;

Trial registration China Clinical Trial Registry, ChiCTR2300072833,26/06/2023. https//www.chictr.org.cn/.

Introduction

Synchronized swimming is a program that requires both the swimming foundation of marching and the artistic foundation of dancing, as well as a strong physical foundation. As the competitive level of synchronized swimmers increases, the difficulty of the choreography also increases, which puts higher demands on the physical ability as well as the cardiorespiratory function of synchronized swimmers. Since the choreography of competitive synchronized swimming requires long time underwater breath-holding to complete various high-intensity and high-difficulty movements, which demands strong aerobic capacity—or a high oxygen reserve—to fuel the body’s demands. Consequently, the training of inspiratory muscles is particularly important for synchronized swimmers.

Respiratory muscle training (RMT) refers to the training of respiratory-related muscles, which is commonly used in clinical medicine to treat cardiopulmonary-related diseases, while in sports science, this training method is used to improve lung function and ultimately athletic performance. In rowing, cycling, running, swimming and other sports that require high endurance, the daily training generally includes specialized technical training and specialized physical training, and these programs, in addition to the limb muscles used to produce movements and the core muscles that stabilize the center of gravity and transfer the strength of the upper and lower limbs, also require the synergistic movement of respiratory muscles, but specialized respiratory muscle training is often neglected, resulting in respiratory muscle fatigue¹ that often occurs during the exercise or early termination of exercise due to respiratory discomfort, or worse, respiratory injury², which affects athletic performance. Therefore, respiratory muscle training, which enhances the working capacity of respiratory muscles by increasing respiratory resistance and hyperventilating for a long period of time, can help to delay the emergence of respiratory muscle fatigue, reduce the fatigue level of the athletes, prolong the exercise time, and enhance the athletic ability, which in turn improves the athletic performance.

In the process of completing a set of movements, synchronized swimmers consume 50%−65% of the time in the facial immersion state^3,4, and as the competitive level of synchronized swimmers continues to improve, the synchronized swimming movement choreography is also developing in the direction of higher difficulty, more intense and faster, which requires the athletes to hold their breath frequently and for long periods of time in the water while it also needs to complete a large number of difficult technical movements, which puts a high demand on the athlete’s cardiopulmonary function. In addition, due to the special environment of this sport, athletes are also subject to the resistance of water and heat conduction energy consumption in the process of exercise. The above characteristics make the athletes have a great demand for the improvement of respiratory efficiency in the process of exercise.

In summary, we performed inspiratory muscle resistance training for synchronized swimmers. Inspiratory muscle training is currently used in some sports, but most of them use constant-load inspiratory muscle resistance training, whether there are other training methods or dosages that can maximize the training effect is still worth exploring. In this regard, we attempted to compare dynamic constant load inspiratory muscle resistance training with constant load inspiratory muscle resistance training to observe the effects of these two load-giving methods on the inspiratory muscle function, quiet lung function, and athletic ability of synchronized swimmers, and we expect to find more effective training methods to be added into the daily training of synchronized swimmers to help athletes to improve their athletic performance.

Methods

Participants

Fourteen synchronized swimmers (mean age 13.57 ± 1.76 years, all with more than 4 years of training) were recruited from Beijing Muxiyuan Sports School. Athletes with metabolic disorders, endocrine disorders, abnormal lung function, heart disease, respiratory disease, gastrointestinal disease, diabetes, hypertension, etc. will not be recruited. Informed consent was obtained from all subjects, and the experiment was reviewed by the ethics committee (ID: TKSLL202201). We confirm that all experimental protocols were approved by the licensing committee (Medical Ethics Committee of Beijing Institute of Sports Science) and all methods were carried out in accordance with relevant guidelines and regulations. The sample size is calculated by G∗Power (version 3.1.9.7; Dusseldorf, Germany) programme. The results of calculation showed that the study could be completed with 3 subjects in each group (effect size: 3.56; actual power: 0.95 for MIP as the outcome measure)⁵. The subjects were randomly divided into dynamic constant load resistance inspiratory muscle training group (DIMT, n = 7) and the constant load resistance inspiratory muscle training group (IMT, n = 7).

Study design

This study is a single-blind randomized controlled trail and only researchers know the grouping situation. Before the formal start of the trial, the subjects’ inspiratory muscle function, pulmonary function and specialized athletic ability were tested which were used as their baseline indices, while the results of the maximum inspiratory muscle strength index(S-Index) were used to adjust the training load in the formal trial. In this study, 50% of the initial S-Index was used as a constant training load in IMT group. The DIMT was carried out in the form of dynamic constant resistance. 50% of the baseline S-Index was taken as the training load of first week and the S-Index was tested every Friday afternoon at the Beijing Institute of Sports Science to adjusted the training load to be 50% of the tested value in the next week of DIMT group. The tester are relevant researchers from the Beijing Institute of Sports Science. The training cycle selected for this study was 4 weeks and the training frequency was 5 days per week with two sets of 30 times per day. Inspiratory muscle training is carried out every morning before and after special training, and the training location is the Swimming Pool of Beijing Muxiyuan Sports School. After 4 weeks of training, the pretest content was repeated. The test process is carried out in strict accordance with the relevant requirements of the guidelines.

Training equipment

The training equipment during the trial is PowerBreathe Plus (PowerBreathe International Ltd. Southam, Warwickshire; England UK). The PowerBreathe Plus trainers are equipped with loads based on a specified percentage of the S-index from the PowerBreathe K5 test. The PowerBreathe Plus series is a pressure threshold load trainer for the inspiratory muscles, applying scalar springs to provide adjustable resistance with 11 levels, with the higher the level, the higher the load.

The basis for selecting the PowerBreathe Plus resistance load is detailed in the table below. The resistance load of the PowerBreathe Plus was set strictly in accordance with its official specification. The corresponding relationship between the load level and the S-Index is shown in Table 1.

Table 1.

Resistance load setting principle of the PowerBreathe Plus.

Level	0	1	2	3	4	5	6	7	8	9	10
S-Index	29	53	78	102	127	151	176	200	225	249	274

Inspiratory muscle function test

Inspiratory muscle function related parameters were tested by PowerBreathe K5 and PowerBreathe KH2 for the S-Index, peak inspiratory flow (PIF) and maximum inspiratory pressure (MIP). In advance, the related supporting software was separately downloaded on the computer and the information about 14 subjects was registered in the software. The PowerBreathe K5 was connected to the computer and then the software page was switched to the test interface. The system set the test mode which needs to inhale for 30 times explosively without a time limit automatically, and when the inhalation curve was in the shaded part, the maximum value of the previous test would appear, which could be a good evaluation of stage training effect of the subject by quantifying the basic data. During this test, the subjects would be instructed to inhale explosively and avoid compensatory actions. The test methods and requirements for PowerBreathe KH2 are the same as those for PowerBreathe K5. The inspiratory muscles are trained in the same way as they are tested. It is important to note that during training and testing, you need to be able to inhale quickly and exhale slowly, and inhale and exhale to the extreme.

Resting pulmonary function test

The test was carried out by the resting pulmonary function tester (CHEST H-101) made in Japan, which can measure the flow and volume of exhaled or inhaled gas, and finally analyze the relative lung ventilation indices such as vital capacity (VC), minute voluntary ventilation (MVV) and forced vital capacity (FVC) according to the time-volume curve and flow-volume curve. The device is powered on when we start to test and the VC, FVC and MVV tests are performed orderly. The VC and FVC tests are repeated twice to record the best results, and the MVV test is done once.

Specialized athletic ability testing

Due to the special sports environment of synchronized swimming, therefore, in order to verify the translation of the effect of these two resistance modes of inspiratory muscle training on the improvement of synchronized swimmers’ aquatic athletic ability, the following two special tests were selected to test their special athletic ability.

1. 50 m torpedo: special swimming capacity;

2. 50 m diving: breath-holding capacity.

Statistical analysis

SPSS 25.0 statistical software was used for data processing, and the data were assessed for normality by the Kolmogorov-Smirnov test. The independent samples T-test was used for comparative analysis of training effects between groups and the paired sample T-test was used for comparison of the effects before and after training within groups. The experimental data were presented as Means ± SD, and statistical significance was set at P < 0.05. All data generated or analysed during this study are included in this published article.

Results

The specific basic information of each group is shown in the Table 2. There were no significant differences between two groups for the basic information (p > 0.05).

Table 2.

Specific basic information of each group.

	IMT(n = 7)	DIMT(n = 7)	p
Height (cm)	159.29 ± 5.82	159.57 ± 6.05	0.930
Weight (kg)	43.59 ± 6.47	43.01 ± 5.95	0.866
Age (year)	13.00 ± 1.73	14.14 ± 1.86	0.258

Inspiratory muscle function

The outcomes of inspiratory muscle function are shown in the Table 3. After 4 weeks of inspiratory muscle training, significant differences are found in S-index, PIF and MIP compered to pre-training in each group (p < 0.05). No significant differences before training in the indices above between groups (p > 0.05). But there are significant differences in PIF and MIP after training in the group-to-group comparison (p < 0.05).

Table 3.

Outcomes of inspiratory muscle function pre- and post- training.

	IMT(n = 7)		DIMT(n = 7)
	pre-	post-	pre-	post-
S-index	108.69 ± 19.10	117.81 ± 18.60*	115.80 ± 43.87	126.56 ± 44.86**
PIF	6.10 ± 0.76	6.60 ± 0.86*	5.80 ± 1.34	7.64 ± 0.91**#
MIP	97.43 ± 27.50	110.39 ± 18.46*	91.81 ± 28.07	135.52 ± 22.91**#

*Significant difference change from baseline(p < 0.05).

**Extremely significant difference change from baseline(p < 0.01).

#Significant difference change between these two groups after training(p < 0.05).

Resting pulmonary function

The resting pulmonary function related indicators are shown in the Table 4. Both groups are shown significant differences after 4 weeks of inspiratory muscle training in resting pulmonary function indices (p < 0.05) which is same as post-training comparison result between groups (p < 0.05) while there’s no statistically significant comparison before training (p > 0.05).

Table 4.

Outcomes of resting pulmonary function pre- and post- training.

	IMT(n = 7)		DIMT(n = 7)
	pre-	post-	pre-	post-
VC	3.74 ± 0.43	4.05 ± 0.37*	3.84 ± 0.69	4.74 ± 0.65**#
FVC	3.41 ± 0.45	3.68 ± 0.23*	3.46 ± 0.68	4.32 ± 0.64**#
MVV	105.23 ± 16.37	113.54 ± 10.02*	105.20 ± 26.90	131.56 ± 17.77**#

*Significant difference change from baseline(p < 0.05).

**Extremely significant difference change from baseline(p < 0.01).

#Significant difference change between these two groups after training(p < 0.05).

Specialized athletic ability

The outcomes of specialized athletic ability are shown in the Table 5. The 50 m torpedo and diving results of post-training in both groups are significantly different from which of pre-training (p < 0.05). There’s no significant difference between groups before training (p > 0.05), but after inspiratory muscle training, the specialized athletic ability in DIMT improved significantly (p < 0.05).

Table 5.

Outcomes of specialized athletic ability pre- and post- training.

	IMT(n = 7)		DIMT(n = 7)
	pre-	post-	pre-	post-
50 m torpedo	58.91 ± 2.86	57.27 ± 3.73*	60.29 ± 2.87	52.21 ± 4.65**#
50 m diving	39.39 ± 1.18	37.37 ± 1.03*	39.91 ± 1.67	35.66 ± 1.76**#

*Significant difference change from baseline(p < 0.05).

**Extremely significant difference change from baseline(p < 0.01).

#Significant difference change between these two groups after training(p < 0.05).

Discussion

Synchronized swimming is a competitive sport which combines swimming, music, dancing, and special techniques, and it requires athletes’ explosive power, endurance, strength, softness, flexibility, and artistic expression. In addition, in a complete set of movements, synchronized swimmers spend 50%−65% of the time in the face immersion state^3,4, which requires athletes to hold their breath frequently for a long time in the water while completing a large number of difficult technical movements, which puts high demands on the athletes’ lung function. With the continuous development of the synchronized swimming and the increasing competitive level of athletes, it requires the synchronized swimming to develop in the direction of more difficult, more intense, and faster movements. At the same time, the breathing mechanism of synchronized swimming is similar to that of swimming, which is divided into a rapid forceful inhalation phase and a prolonged breath-holding phase underwater⁶, which prompts synchronized swimmers to establish a “controlled breathing pattern” that must be coordinated with the technical movements.

Breath-hold and underwater effects in synchronized swimming indicate a decrease in gas exchange and an increase in physiological stress during exercise^7–9. Underwater exposure creates a physiological environment that restricts gas exchange because the vast majority of energy required during breath-holding must be produced in the presence of carbon dioxide accumulation and oxygen supply reduction, meanwhile, the muscles of synchronized swimmers contract vigorously and rapidly during exercise, which contributes to an increase in physiological stress^7,9. In normal physiological conditions, respiration is controlled by chemoreceptors, which are sensitive to increases of CO₂¹⁰. Thus, the impulse of respiration is determined by an increase of CO₂ rather than a decrease of oxygen¹¹. It has been suggested that the decrease of pH is due to the accumulation of CO₂, as it cannot be eliminated during breath-holding and accumulates in the blood and muscle tissues^12–14. Moreover, during breath-holding, the body’s metabolism rapidly exhausts effective oxygen, and continuous oxygen consumption may reach a level insufficient for muscle function during prolonged periods of no breathing, which can lead to alterations in PaCO₂ and PaO₂. Altered body sensations resulting from prolonged repetitive breath-holding and the accumulation of CO₂ could alter cognitive functioning and may make decisions to impair motor performance^15,16, which in turn may lead to a delayed conscious response¹⁵ or even unconsciousness¹⁷.

The specific training challenges faced by synchronized swimming can be summarized as high metabolic demands and recurrent apnea in the aquatic environment, which causes a bradycardic response and a hypoxic respiratory response¹¹. This condition may stress the respiratory system through lung hyperinflation and hyperventilation¹⁸ in order to increase alveolar ventilation. In a study by García I¹⁹, breath-hold swimming training increased lung diffusing capacity (+ 9.2%) compared to pre-training. Thus, swimmers⁶, divers²⁰ and synchronized swimmers²¹ have greater lung capacity and diffusion than land-based athletes, which may be explained in the fact that, because those programs require repeated apneas in the aquatic environment, their daily aquatic training routine trains them in breath-holding, and this type of training requires a low respiratory frequency and high tidal volume, and such a breathing pattern involves large inspiratory muscle forces²². Therefore, the breath-holding ability and their inspiratory explosiveness of synchronized swimmers during gas exchange also need to be further improved. It was mentioned in the previous section that inspiratory muscle training can improve ventilation efficiency by increasing inspiratory explosive force and tidal volume, which coincides with the respiratory demands of synchronized swimming events.

In summary, we suggest that adding inspiratory muscle strength training to the daily training of synchronized swimmers may be effective in improving the inspiratory muscle function and lung function of synchronized swimmers in order to improve their respiratory control as well as their respiratory efficiency, and thus improve their athletic performance.

The present study investigated the effects of four weeks of two different resistance methods of inspiratory muscle training on inspiratory muscle function, quiet lung function, and specialized ability of synchronized swimmers, and to find a more effective resistance method of inspiratory muscle training to improve lung function and athletic ability of synchronized swimmers through this research. The participants were randomly assigned to the DIMT and IMT groups. The subjects’ S-index and PIF were quantitatively assessed by Powerbreathe K5, and MIP was assessed by Powerbreathe KH2 for evaluating the inspiratory muscle function of synchronized swimmers; VC, FVC and MVV were quantitatively assessed by CHEST H-101 for evaluating the rest lung function of synchronized swimmers; the specialized abilities of synchronized swimmers were evaluated by timing tests of 50 m diving and 50 m torpedo. Powerbreathe PLUS was used for training in this study. The Powerbreathe range of products, has proven its effectiveness in a large number of studies^23,24. No adverse events occurred during the entire training and evaluation process in this study.

Effects of inspiratory muscle training on pulmonary function in synchronized swimmers

Effects of inspiratory muscle training on inspiratory muscle function in synchronized swimmers

The S-index reflects the maximal contraction force of the inspiratory muscles; the larger the S-index, the greater the amount of gas that can be inhaled in a single breath, and the relatively higher the contraction rate of the inspiratory muscles. After four weeks of inspiratory muscle training, there was a significant increase in S-index in both the IMT and DIMT groups compared to the pre-training period (p < 0.05), but there was no significant difference between the two groups after training (p > 0.05). The above results indicated that the working capacity of the inspiratory muscles of synchronized swimmers was improved after constant load inspiratory muscle training and dynamic constant resistance load inspiratory muscle training, but the two resistance modes of training had comparable effects on the improvement of inspiratory muscle strength.

Peak inspiratory flow rate reflects the level of explosive force of the inspiratory muscles. The higher the peak inspiratory flow rate, the higher the rapid contraction capacity of the inspiratory muscles.There was a significant increase in PIF in both the DIMT and IMT groups compared to the pre-training period (p < 0.05), and significant differences were found between the two groups after training (p < 0.05). This result suggests that the level of explosive force of the inspiratory muscles increased after IMT and DIMT, and that dynamic constant-load inspiratory muscle training has a better lifting effect compared to constant load training.

Maximum inspiratory pressure is an important index for assessing the function of inspiratory muscles, which reflects the strength of inspiratory muscles. In this study, it can be found that both training modalities improved it (p < 0.05), and DIMT improved it more significantly compared to IMT (p < 0.05). The study verified the improvement of IMT on inspiratory muscle strength and proved that DIMT was more effective.

It has been suggested²⁵ that the S-index represents the maximum dynamic inspiratory pressure, while the MIP is the static maximum inspiratory pressure. And Fabiola et al.²⁶ compared the reliability and consistency of S-index measured by Powerbreathe K5, MIP measured by Powerbreathe KH2, and MIP measured by standard manometers, which are most commonly used to assess MIP, and found that the reliability of S-index was higher, and that all three measurements were similar. Strong correlations were found between both S-index and MIP measured by the Powerbreathe range of products and MIP measured by standard manometers. The reliability of the S-index has also been found in other studies^27,28, and the S-index provides a dynamic real-time display of inspiratory pressure from the residual volume to the total vital capacity. It has been demonstrated^25,26 that at least 8 inspiratory movements are required for S-index testing to obtain maximum and reliable values. At the same time, it was concluded²⁵ that compared to the MIP, which is a static isometric assessment of inspiratory muscle strength (inspiratory muscle strength in only a small range of lung volumes), the S-index, as a dynamic assessment, which allows for inspiratory muscle output across the entire lung volume, is also more representative of the maximal capacity of the inspiratory muscles.

In this study, both the S-index and the MIP were used to assess the maximal contraction force of the inspiratory muscles, but we found that the inspiratory muscle function improved after the training, however, there were different results in the between-group comparison of the S-index and the MIP. Fabiola et al.²⁶ also found in their study that the S-index test results were slightly lower than the MIP, but this was not found to be universally lower than the MIP during our study. We analyzed that the reason for the different results may be the different criteria for the maximal contraction force of the inspiratory muscles in the two devices. The principle of measurement of Powerbreathe KH2 is to make the subject to perform the maximum ability to inhale and required to maintain a period of time (1.5s), during this process the device will automatically give a certain amount of resistance according to the real-time inhalation pressure, and with the inhalation force continues to stimulate and increase the resistance given to maximize the activation of the inspiratory muscles, when there is three consecutive times of the test results of the decline in the results of the performance, then the results will be obtained. The best score is the MIP, which is usually tested more than 3 times, and the results can be obtained in about 5 times. Whereas, the Powerbreathe K5 test process has only a small resistance, which may not be able to fully stimulate the inspiratory muscles to maximize the recruitment of muscle fibers, and at the same time, the use of the device to test the maximum contraction force of the inspiratory muscles needs to be performed for 30 times of maximal inhalation, and during this test process, the subject may have already experienced fatigue of the respiratory muscles, which led to the results of the S-index test not being able to represent the subject’s maximal capacity. Therefore the two indexes yielded different results after four weeks of inspiratory muscle training. Overall, both groups showed significant improvements in inspiratory muscle strength and explosiveness through four weeks of inspiratory muscle training, and the dynamic constant load resistance format improved inspiratory muscle explosiveness better compared to constant load training. The two inspiratory muscle strength indexes only MIP yielded between-group differences, while S-index did not find differences between the two groups, in this regard, we concluded that the dynamic constant load group is more conducive to the improvement of the inspiratory muscle strength of the synchronized swimmers, and more obvious differences may require longer-term training as well as observation. Meanwhile, we also expect to deeply explore the changing rules of S-index and MIP in the subsequent studies.

In summary, it is reasonable to assume that respiratory muscle training is compatible with the principle of stimulus adaptation in quadriceps muscle training. After training with a constant load the inspiratory muscles become adapted to that stimulus load, and in later stages the training effect is focused on maintenance and recovery. Dynamic constant-load inspiratory muscle training changes the training resistance accordingly to the variation of the S-index, so that the inspiratory muscles are not only receiving 50% of the appropriate load during a small portion of the training session, but throughout the entire training session, during which new adaptations are constantly made to obtain further growth in inspiratory muscle function. The effect of inspiratory muscle training on the improvement of inspiratory muscle function in athletes, in addition to the results of the present study, we found similar results in runners^29,30, soccer players^31–34, swimmers^5,35–39, rowers^40–42, and cyclists^43–48.

Effects of inspiratory muscle training on quiet pulmonary function in synchronized swimmers

Resting pulmonary function mainly indicates the pulmonary ventilation function of respiratory physiology, and vital capacity is one of the relevant indexes of resting pulmonary function; the higher the VC, the higher the respiratory efficiency, and the stronger the aerobic metabolism capacity. The results of the study showed that the VC of both IMT and DIMT groups improved significantly after four weeks of inspiratory muscle training (p < 0.05); and there was a significant difference between the VC situation of the DIMT group and that of the IMT group after the training (p < 0.05), which suggests that inspiratory muscle resistance training helps to improve the aerobic capacity of the athletes, which is mainly reflected in the increase of lung storage capacity, and dynamic constant load inspiratory muscle training has a better effect.

After dynamic constant load inspiratory muscle training, there was a highly significant increase in FVC compared to the pre-training period (p < 0.01), as well as a significant improvement in FVC in the constant-load group (p < 0.05), and the difference between the two groups was significant after 4 weeks of training. This result showed that inspiratory muscle training in both resistance modalities improved expiratory explosive force, and that dynamic constant load inspiratory muscle training was more effective in comparison of the two modalities. Regarding the fact that inspiratory muscle training improves expiratory capacity, we suggest that it is because after forceful inhalation of gas, the volume taken in becomes larger, and the elastic deformation of the lungs becomes greater, which induces an increase in the lung’s tensor reflexes, so that the volume of exhalation becomes larger, and the expiratory speed becomes faster.

Maximum Ventilation Volume (MVV) indicates the maximum volume of respiration per unit of time, which requires both depth and speed of breathing, and mainly monitors the combined level of respiratory muscle strength, explosive power and endurance. The results of the study showed that both training groups showed a significant increase in MVV after four weeks of training compared to the pre-training period (p < 0.05), and there was a significant difference between the groups (p < 0.05). This suggests that both training modalities are effective in improving the speed endurance of the athletes’ inspiratory muscles, and dynamic constant load inspiratory muscle training is more effective.

In conclusion, both inspiratory muscle training modalities had an enhancement effect on resting pulmonary function, and dynamic constant load training was able to improve pulmonary function more effectively compared to constant load training. It has been shown⁴⁹ that inspiratory muscle training improves MVV in soccer players and mixed martial arts athletes, but not in kickboxing athletes⁵⁰.Najafi A concluded that inspiratory muscle training does not improve VC in soccer players³³.Parodi-Feye A S et al.⁵¹ concluded that the use of the Powerbreathe trainer for inspiratory muscle training is feasible in terms of time and effort, but experimentally demonstrated that the training did not improve the level of pulmonary function in handball players. We believe that the reason for these differences in the results of the study may be due to the short training period, which is not enough to observe significant changes, the second possible reason is that the differences in the physiological characteristics of athletes in different sports make the results of the training show inconsistent improvement, In addition, the mastery of the training method may also have an impact on the effectiveness of the training.

The effect of inspiratory muscle training on the specialized athletic ability of synchronized swimmers

Synchronized swimming requires athletes to achieve a high level of aerobic, anaerobic, strength, endurance, flexibility, performance skills and artistic expression. Due to the specificity of the sport environment of synchronized swimming program, general fitness tests cannot fully represent the athletes’ athletic ability, therefore, two aquatic tests closely related to synchronized swimming were selected in this study to determine the transforming effect of the inspiratory muscle training on the synchronized swimmers’ specific athletic ability. Because the athletes are more familiar with aquatic movement patterns, which are more representative of the abilities required by the program, and in order to avoid the test results not accurately representing the athletes’ corresponding athletic abilities due to the unfamiliarity with the methods, rules and precautions of the test items, the specialized tests were selected from the items that the coaches of the team tested the athletes in their daily practices, which were the 50 m diving and the 50 m torpedo tests. The reason for the selection is that the program characteristic of synchronized swimming to perform strenuous exercise under breath-holding state for a long time makes the athletes produce a greater demand for oxygen metabolism or oxygen storage. In order to verify whether the inspiratory muscle training can supply the oxygen demand of synchronized swimmers in the exercise by improving the strength of the inspiratory muscles and thus increasing the amount of oxygen inhaled in a single maximal effort, we selected 50 m diving to test the athlete’s ability of the breath-holding. In addition, we selected the torpedo from the specialized movements of synchronized swimming to verify the effect of inspiratory muscle training on their specialized swimming ability.

After four weeks of inspiratory muscle resistance training, the 50 m diving and 50 m torpedo performances of both IMT and DIMT groups were significantly improved compared with the pre-training period (p < 0.05), and there was a significant difference in the inter-group comparison of 50 m torpedo performances between the DIMT and IMT groups after four weeks of training (p < 0.05). Therefore, we believe that both resistance modes of inspiratory muscle training can effectively improve the breath-holding ability and specialized swimming ability, which are highly related to the demands of synchronized swimming events, and improve the athletes’ performance while helping them to improve their respiratory efficiency. Numerous studies have supported our analysis⁵² that inspiratory muscle resistance training improves sport-specific abilities^{32,33,40,46,53–59}. Ohya T et al.³⁸ study by comparing the effects of 50% as well as higher intensity inspiratory muscle resistance training on swimmers showed that inspiratory pressure threshold loading of 50% MIP was sufficient to improve MIP and swimming performance under controlled frequency breathing conditions and that higher intensity IMT did not show better improvement in athletic performance. And from the results of the present study, inspiratory muscle training in the form of dynamic constant load resistance was more effective in improving the mentioned abilities above.

Synchronized swimmers in the process of completing a set of movements has more than 50% of the time in the breath-holding state⁶⁰, at the same time, the muscles have been kept in motion, muscle metabolism is enhanced, at this time, the body continues to consume oxygen, and carbon dioxide cannot be discharged in a timely manner, which results in the body’s metabolic wastes, CO2, is constantly increasing, so that the partial pressure of carbon dioxide in the blood will be increased, which will inhibit the activity of the central nervous system, and ultimately may lead to the athlete’s consciousness disorder during exercise, or even lead to coma. Meanwhile, respiratory drive is the intensity of nerve impulses sent from the respiratory center⁶¹, and respiratory effort is the mechanical output of respiratory muscles, which includes the amplitude and frequency of respiratory muscle contraction, and respiratory drive determines the mechanical output of respiratory muscles^62,63, which means that it determines the depth and speed of respiration. Prolonged hypoxia also causes the body to produce hypoxic respiratory responses such as slowed respiratory rate, shallow respiratory depth and dyspnea, and the partial pressure of carbon dioxide (PaCO2) in the body goes up, which affects the respiratory drive to gradually decrease⁶⁴ and may suddenly disappear, which leads to apnea⁶⁵, and ultimately affects the respiratory efficiency. Therefore, we need to train the respiratory muscles of Synchronized swimmers to try to provide more oxygen to the body, buffer the elevated partial pressure of carbon dioxide, and maintain the supply of oxygen demand to all parts of the body for a longer period of time by increasing the ventilation. In the current research, inspiratory muscle training is more focused on cycling, swimming, rowing and other sports, but there are fewer studies related to synchronized swimming sports. The aim of this study is to investigate the effects of inspiratory muscle training on the pulmonary function and exercise capacity of Synchronized swimmers, and to compare the training effects of the two load application methods, with the expectation of eventually finding more effective training methods and approaches for Synchronized swimmers’ sports programs.

Limitation

The training equipment used in this study was mainly Powerbreathe Plus, which has a single load setting level and a large interval between two adjacent levels, resulting in a possible error in the accuracy of the actual training load resistance setting, so the accuracy of the load setting in training should be further improved in later studies. As the Synchronized swimmer program has high requirements in terms of athlete selection, which makes the number of synchronized swimmer teams small, we hope to further increase the sample size as well as lengthen the training period in the later studies, so that the results will be more reliable.

Conclusion

Dynamic constant load resistance inspiratory muscle training and constant load inspiratory muscle training both help to improve the inspiratory muscle function, pulmonary function, and specialized exercise capacity of synchronized swimmers, and compared with constant load inspiratory muscle training, dynamic constant load resistance inspiratory muscle training has a better effect on the improvement of inspiratory muscle function, pulmonary function, and specialized exercise capacity. Based on the results of this study, we consider that inspiratory muscle resistance training can be introduced into the daily training of synchronized swimmers, and we recommend dynamic constant load resistance inspiratory muscle training to better improve the performance of Synchronized swimmers.

Author contributions

X.H. wrote the main manuscript text and prepared figures in the paper. F.Y. sorted out all the data. All authors were involved in the trial implementation and reviewed the manuscript.

Funding

Key Laboratory of Performance Evaluation and Integrated Enhancement, General Administration of Sport.

Data availability

All data generated or analysed during this study are included in this published article [and its supplementary information files]. Due to the confidentiality of athlete-related data and the refusal of athletes and their coaches to disclose the data, so it is inconvenient to provide the original data, but the relevant research data are reflected in the paper.

Declarations

Competing interests

The authors declare no competing interests.

Ethics Committee

Medical Ethics Committee of Beijing Institute of Sports Science.