Complex network model indicates a positive effect of inspiratory muscles pre-activation on performance parameters in a judo match

This study investigated the effects of inspiratory muscle pre-activation (IMPA) on the interactions among the technical-tactical, physical, physiological, and psychophysiological parameters in a simulated judo match, based on the centrality metrics by complex network model. Ten male athletes performed 4 experimental sessions. Firstly, anthropometric measurements, maximal inspiratory pressure (MIP) and global strenght of the inspiratory muscles were determined. In the following days, all athletes performed four-minute video-recorded judo matches, under three conditions: without IMPA (CON), after IMPA at 15% (IMPA15), and at 40% (IMPA40) of MIP using an exerciser device. Blood lactate, heart rate and rating of perceived exertion were monitored, and the technical-tactical parameters during the match were related to offensive actions and the time-motion. Based on the complex network, graphs were constructed for each scenario (CON, IMPA15, and IMPA40) to investigate the Degree and Pagerank centrality metrics. IMPA40 increased the connectivity of the physical and technical-tactical parameters in complex network and highlighted the combat frequency and average combat time in top-five ranked nodes. IMPA15 also favoured the interactions among the psychophysiological, physical, and physiological parameters. Our results suggest the positive effects of the IMPA, indicating this strategy to prepare the organism (IMPA15) and to improve performance (IMPA40) in judo match.

www.nature.com/scientificreports/ techniques and influencing the result of the score during the combat 30 . Merola et al. 31 applied the high-intensity IM PA before a specific test for judo with projection techniques (Special Judo Fitness Test). Even though that study has not found positive results on test performance, the evidence suggests that inspiratory efforts applied using lower intensities before the exercise may demonstrate the benefits of this strategy.
In addition, the cause-effect analyses of traditional statistics seem insufficient to understand multifactorial responses to performance 32 , including in combat sports. The complex network can investigate the relationships between the performance parameters of athletes in different modalities [33][34][35] . Sports science lacks studies on the interaction among technical-tactical parameters and physical and physiological aspects under the intervention of strategies to improve performance in combat sports. Therefore, this study aimed to investigate the effects of inspiratory muscle pre-activation on the interactions among technical-tactical, physical, physiological, and psychophysiological parameters in a simulated judo matches, based on the analysis of centrality measures for complex network models. Considering previous studies involving acute inspiratory load strategies and the judo characteristics, we hypothesize that IM PA at 40% of MIP (IM PA 40) will be promote positive effects on the technical-tactical, physiological, and psychophysiological parameters of a simulated judo match. Additionally, the centrality measures obtained by complex network models will be able to identify the impact of the IM PA on judo match, improving the interaction and connectivity among the technical-tactical, physical, physiological, and psychophysiological parameters, especially after the IM PA 40.

Methods
Subjects. Ten male judo athletes, medallists in official competitions (2018-2019) at state and national levels participated in the study. The athletes' profile is described in Table 1 (see "Results" section). The athletes mentioned no metabolic, cardiovascular, respiratory, or orthopaedic disease and no use of medications or drugs. All athletes were evaluated in the pre-competitive period and aware of sleep, food, and physical training conditions before each test.
Experimental design. Four assessment sessions were performed, separated by 24-48 h. In the first section, all subjects (n = 10) received information about the experimental design and signed the consent form. In addition, they performed the assessment of anthropometric and inspiratory measures, and familiarization with the equipment and protocols. The other sessions were performed, in random order, to evaluate the effects of inspiratory muscles pre-activation (IM PA ) previously to a judo match (Fig. 1). All subjects performed these sessions in three different conditions: (1) control protocol, in which the judo match was carried out without the  40). The participants performed a specific judo warm-up composed of mobility exercises for all joints (20 repetitions) and 2 sets of 20 techniques without projection (Uchikomi) on each side with 30 s of interval before each intervention, followed by 5 min of passive pause and application of the IM PA protocol. After 2 min of passive pause, the athletes performed a judo match, followed by 10 min of recovery. In control protocol, the athletes completed only the specific judo warm-up and started the match immediately after the passive pause of 5 min. Information on the inspiratory loads and the possible effects of the protocols was omitted from the participants.
Inspiratory measures. The maximal inspiratory pressure (MIP) was obtained through the inspiratory effort initiated from the residual volume after maximum expiration 37 . Participants remained seated and performed at least 5 maximum inspiration efforts with a 1-min interval 38 , three acceptable and two reproducible (that is, a variation of values ≤ 10%). The efforts were sustained by at least 1 s to register the highest inspiratory pressure, considering the measure of greatest value. The analog manovacuometer (± 300 cmH 2 O) (Ger-ar®, Brazil) was used for these measurements. After 30 min, the global strength of the inspiratory muscles (S-index) of the participant in the standing position was evaluated. The participants performed a sequence of 30 inspirations in an inspiratory muscle exerciser (POWERbreathe® K5 model, IMT Technologies Ltd., Birmingham, UK), with verbal encouragement to inspire greater air capacity 39 . The average and maximum values of the global force (S-index), peak inspiratory flow (PIF) and volume measures were obtained using the Breathe-Link Version 1.1 software. All participants used a nose clip in both tests.
Inspiratory muscle pre-activation (IM PA ). According to previous studies that applied warm-up to inspiratory muscles 7,9,13,14 , our protocols were conducted under three conditions. The control protocol was characterized by the judo match without IM PA . On the other hand, in IM PA 15 and IM PA 40 sessions, the athletes were submitted to these acute strategies using loads equivalent to 15% and 40% of the MIP, respectively. The IM PA interventions were performed with an inspiratory muscle exerciser POWERbreathe® K5 model (IMT Technologies Ltd., Birmingham, UK). During the procedure, the subjects remained in a standing position and performed 2 sets of 15 maximum inspirations (1-min interval between sets), maintaining the diaphragmatic inspiratory muscle pattern. All repetitions were monitored by the equipment's software.
Technical-tactical parameters. The simulated judo matches followed the official rules (2018-2020) 30 . To guarantee the same conditions in all assessment sessions, the matches lasted 4 min, independent of the score achieved. The participants performed all matches with the same opponent of equivalent technical level (difference in body mass < 10%) 40 . The technical-tactical parameters were obtained by notational analysis from videorecorded. Offensive actions: number of attacks, number of scores (Ippon and Wazari) and penalties (Shido), for effectiveness ((number of scores/number of attacks) × 100) 41  Physiological and psychophysiological parameters. The physiological parameters of blood lactate [Lac] and heart rate (HR) were monitored at rest, pre-and post-combat, and at each 2 min unitl 10 min of recovery. Heart rate (bpm) was continuously recorded every second by a monitor (Polar® model V800, Finland). Blood samples (25 µL) were extracted from the ear lobe with a heparinized capillary and deposited in microtubes (Eppendorf, 1.5 ml) containing 50µL of 1% sodium fluoride and frozen at -20 °C before reading lactate concentrations. The blood lactate concentrations were determined on a lactate analyser (YSI-2300-STAT-Plus™, Yellow Springs, OH, USA). The Rating of Perceived Exertion (RPE) 44 was applied as a psychophysiological parameter.  www.nature.com/scientificreports/ parameters of the judo match and the common parameters to the three scenarios that consist of the characteristics anthropometric and body composition, sport profile and the inspiratory measures of athletes. The edges (E) represent the interactions mediated by the values of "r" statistically significant Spearman correlations (p ≤ 0.05); and w is the weight function 45 . The topology of the networks can be seen in Fig. 4. The centrality metrics of Degree and Pagerank were applied to the scenarios 34,35,46 . The degree metric representing the number of edges of the node that connects to the other nodes. The influence of one node on the others in the network is highlighted by the Pagerank metric 35 . Data processing for the elaboration of the complex network was performed using a specific algorithm in MATLAB environment. The analyses were obtained by software Gephi (0.9.2 version) implemented in the JAVA programming language applying the Fruchterman-Reingold layout 47 to construct the graphs.

Complex networks analysis.
Statistical analysis. The results are described as mean and standard error of the mean (SEM). Nonparametric statistics were adopted, since the normality of the data was not attested by the Shapiro-Wilk test. Homogeneity was verified by the Levene test. The Friedman test followed by the Bonferroni post-hoc test, were applied to compare the technical-tactical, physiological, and psychophysiological parameters in each intervention performed by all athletes (CON × IM PA Table 1 shows the athletes' profile, anthropometric characteristics, body composition, and inspiratory measurements (mean values ± SEM).

Results
Traditional analysis of technical-tactical parameters. The number of attacks, Wazari Scores, and penalties for Shido did not differ among the protocols (Table 2). However, IM PA 40 significantly increased Ippon Scores compared to IM PA 15. Figure 2A,B shows that effectiveness (15.5 ± 4.7%) and efficiency (20.9 ± 6.4) were also higher by IM PA 40 when compared to IM PA 15 (8.0 ± 3.4%; p = 0.020 and 12.2 ± 5.8; p = 0.020, respectively). In time-motion, the parameters related to combat periods (average values) were not changed by the interventions ( Table 2). IM PA  www.nature.com/scientificreports/ Differently others, in this scenario important technical-tactical parameters were highlighted. For both metrics used, the physical parameters presented the greatest number of interactions and importance among all the performance parameters of a judo match, with emphasis on BM that was among the first two positions in the rankings in all scenarios.

Discussion
To the best of our knowledge, this is the first study that analysed the effects of IM PA on the interactions among the technical-tactical, physical, physiological, and psychophysiological parameters in a simulated judo match. Partially confirming our hypothesis, the IM PA 40 improved the performance of Ippon Scores, effectiveness and efficiency compared with IM PA 15, besides promoting changes in time-motion. However, this strategy did not modify the physiological parameters before and after the judo match, at least using traditional analysis. In an innovative way to judo modality, our study reinforces the complex network model to improve the data interpretations into athlete´s performance. Using this model, IM PA 40 scenario showed interactions mainly among the physical and technical-tactical parameters important to judo match. Differently of the IM PA 40, the acute load using 15% of the MIP did not highlight the technical-tactical parameters as top-five nodes in centrality metrics obtained by complex network. Despite that, IM PA 15 showed the highest connectivity among physical and physiological parameters, suggesting that lower inspiratory load already prepare the organism of this athletes. www.nature.com/scientificreports/ The quality of movements of techniques in standing and groundwork combat can influence the result of the score in a judo match 30 . Using the non-parametric statistical analysis, we observed the effect of IM PA 40 on Ippon Scores (Table 2) and effectiveness and efficiency ( Fig. 2A,B) when compared to IM PA 15. In relation to CON, IM PA 40 provided changes in the total values of standing combat, groundwork combat and pause. Although the official rules of judo advantage standing combat, especially when there is no objectivity of offensive actions on the groundwork 30 , changes in total standing and groundwork combat times (Fig. 2) are associated with previous results, since the positive effects on the offensive actions may have induced the choice of strategy to conduct the combat standing, in which the frequency of scores 48 and efficiency 42 are generally larger than in groundwork combat. The other time-motion parameters ( Table 2) that did not differ among the protocols, indicate that there was no effect of IM PA 15 or IM PA 40 on the dynamics of the match. These results reflect the behaviour presented by the physiological and psychophysiological parameters (Fig. 3).   (Fig. 3A-C) decreased during post-combat recovery but were not affected by IM PA strategies. In line with our results, previous studies that applied the traditional IM PA protocol in high-intensity efforts or in exhaustive tests did not observe differences in ventilatory and metabolic responses 8,9,12,14,17-19. High-intensity efforts cause fatigue in the inspiratory muscles, activating the metaboreflex 25 . The accumulation of metabolites caused by intense exercise stimulates the types III and IV afferent fibers of the inspiratory muscles, especially the diaphragm 49 . This process increases the sympathetic activity of the muscle that promotes adrenergic vasoconstriction, redistributing the blood flow from the active musculature to the respiratory muscles 50 . Here, at least through investigations using traditional analysis, the physiological and psychophysiological parameters were not altered regardless of the acute loads adopted (15 or 40% of MIP) suggesting that IM PA was not able to significantly inhibit the metaborreflex, probably due to the ventilatory and postural demands of the inspiratory muscles during the judo match. On the other hand, these results reinforce that the positive effect of IM PA 40 on the technical-tactical parameters was not explained exclusively by blood lactate, HR or RPE responses.
The literature indicates the inspiratory acute load at 15% of MIP as a placebo condition 8,9,14,18 . If considered this point, we observed the impact of the IM PA 40 in judo match by traditional statistical analysis, without www.nature.com/scientificreports/ modifying physiological responses. In this way, probably the technical-tactical alterations could be associated with neural control of inspiratory muscles that assist in ventilatory mechanics and postural function 51 . The neural control of the inspiratory muscles can be activated automatically by the bulbospinal pathways and voluntarily, by corticospinal pathways 51 . Although without direct evidence, we could suggest that the resistance caused by IM PA 40 during voluntary inspiration contribute to postural adjustment and the application of techniques during combat, since the diaphragm is also activated in tasks with upper limb movements 28 and intercostal muscles in trunk rotations 29 . In line with this reasoning, studies have attributed the increase in MIP after IM PA to improved intra and intermuscular coordination of inspiratory muscles 7,[9][10][11][12][13] considering that resisted inspiration can cause changes in the heavy chain of myosin that modulates the recruitment of motor units 52 . On the other hand, as follow discussed, the integrative analysis applied in our study did not confirm the IM PA 15 as a placebo conditioning to judo match. It is important emphasize that many studies involving acute muscle inspiratory protocols used 2 sets of 30 maximum inspirations with a load equivalent to 40% of MIP [7][8][9][10][11]14,18 . Here, we adopted IM PA protocols as 2 sets of 15 maximum inspirations using the exercise device. Our choice was based on Merola et al. 31 who investigated the Special Judo Fitness Test 53 parameters under the intervention of an IM PA (2 sets of 15 maximum inspirations, at 60% of MIP) and due our athletes having no previous experience with respiratory training. Accordingto our results, this method using fewer number of inspiratory repetitions, already provide benefits to judo athletes, which was confirmed by an integrative analysis as discussed below.
Complex networks (Fig. 4) demonstrated the effects of IM PA in a simulated judo match, mainly on physical and technical-tactical parameters. IM PA 15 and IM PA 40 promoted greater connectivity among performance nodes  www.nature.com/scientificreports/ (an increase of connections around 59.7% and 15.2%, respectively). The centrality metrics emphasized on the physical parameter's BM and THC (see top-five nodes- Fig. 4). In a similar way, Gobatto et al. 35 investigated two scenarios of laboratory and field tests in basketball players, highlighted the body mass and the vertical jump power (measure related to the lower limbs) by complex network analysis. The BM and THC are so important for judo, as athletes compete in matches divided by body mass and high levels of lower limb strength are required during the application of projection techniques 54 . Thus, THC can be an indirect method for measuring the muscle 55 , where the cross-sectional area of the muscle is related to the ability to generate strength 56 .
The centrality metrics showed that IM PA 15 presented more connections, mainly on psychophysiological, physiological, and physical parameters, demonstrating that this inspiratory load cannot be considered placebo as pointed by literature 8,9,14,18 , at least to high-performance judo athletes studied here. Thus, this scenario can be applied as a prior task in the athlete's organism preparation for combat.
Interesting results were observed in the IM PA 40 scenario. In this case, both centrality metrics (Degree and Pagerank) indicated technical-tactical parameters in the top-five nodes ranking. For example, the combat frequency (CF) and average combat time (ACT) so important to judo performance, occupied the 2nd position in the classification of Pagerank together with the time to reach the peak of blood lactate (TRLac). Considering that this metric represents the influence of one node on the others in the network, the highlighted technical-tactical nodes confirm the positive effect of the IM PA 40 on judo match. These findings obtained through an integrative analysis detail the effects of IM PA , reinforcing the multifactorial aspects that can determine competitive success in judo 54 and suggest acute strategies to assist coaches and athletes in training and competition.
Despite our study providing promising and positive effects of the IM PA to judo matches, some limitations must be considered. First, we investigated only ten judo high-performance athletes. In next opportunities, we suggest increasing the sample size, as well as extending this protocol to different levels of competitive athletes. In addition, after this detailed investigation into the application of IM PA in a single judo match, we point out the suggest to expand this investigation to assess the effects of IM PA on successive matches as it occurs in a competitive way.
In summary, our study suggests the use of IM PA 40 as a safe, legal, and non-invasive resource that plays a positive role in the judo match. Based on the integrative analysis by complex network model, IM PA 40 increased connectivity and the influence of physical and technical-tactical parameters, and highlighted the important combat nodes to support performance in judo. According to the centrality metrics, IM PA 15 also stimulates interactions among psychophysiological, physical, and physiological parameters. These results confirm the positive effect of the IM PA in the judo modality, pointing out this strategy to prepare the organism (IM PA 15) and to improve performance (IM PA 40) in judo match.