In recent years, several studies have demonstrated that inspiratory muscle training (IMT) is a purposeful strategy with which to extend exercise tolerance across a range of activities (7,13,35). However, the efficacy of this treatment remains debated (23,29), as physiological indices such as maximal aerobic power and oxygen uptake kinetics are unchanged after IMT (7). This is most likely because of the relatively minor cardiorespiratory challenge posed by IMT (11). However, IMT is known to positively influence the efficiency of inspiratory muscles (26), whereas also attenuating both respiratory and peripheral effort perceptions (32). This suggests further investigation of IMT may, therefore, be meaningful to sports where reduced effort perceptions could be influential and the requirement to tolerate high-intensity cardiorespiratory exercise is important.
The sport of soccer is widely acknowledged as a high-intensity prolonged duration intermittent activity that requires performers to undertake regular repeated sprints across a 90-minute game and where the sustained level of effort approximates that of the anaerobic threshold (∼75% of maximal effort) (30). As a consequence, the ability to tolerate a high cardiorespiratory load during soccer, coupled with the capacity to retain sufficient energy to sprint when required, presents an intriguing physical challenge. It has been suggested that soccer players pace themselves during match play to avoid the debilitating consequences of fatigue (10), and it is possible that greater functionality of inspiratory muscles may directly facilitate either the maintenance of higher workloads or greater efficiency in match play performance.
In soccer, there are relatively few opportunities to positively influence the physical performance characteristics of performers because of the regular occurrence of matches over a season that can last 7–9 months (6,30). Therefore, the preseason period, immediately before the start of the competition phase, represents the main opportunity for coaches and athletes to improve fitness and match readiness. However, the soccer preseason period is relatively short (∼6–7 weeks), and in the absence of detailed, published data, much conjecture remains as to the optimal practice, composition, and training load of this period (1,4). For amateur and recreational players, the frequency of training is commonly 2–3 sessions per week (e.g., 2- × 2-hour sessions per week) (19), and although it is unlikely this training load substantially enhances physical fitness across a 6-week preseason period, the addition of an intervention such as IMT may maximize outcomes from training of that frequency.
Studies of IMT have thus far identified that cycling time-trial (18), rowing (35), and endurance running (22) may be improved using this intervention; however, few studies have yet explored whether the addition of IMT to existing training practices may augment the outcomes from training compared with a placebo or control condition.
A recent precedent for using IMT as an addition to existing training practice was set in a study by Edwards et al. (12), which reported that the concurrent use of IMT and a repetition-based endurance running training program significantly improved 5,000-m track-running performance (4.6% gain) compared with a placebo condition, over 4 weeks. However, to our knowledge, the study by Edwards et al. (12) is currently the only study in which IMT has been used as a concurrent training strategy. This lack of research may be attributable to the ongoing debate over the efficacy of IMT and speculation as to the mechanisms by which performance outcomes are affected (11,14,16).
Several putative mechanisms have been proposed to explain the effect of IMT on exercise performance (24). These include a delay in respiratory muscle fatigue (17,21), a redistribution of blood flow from respiratory to locomotor muscles (24), and a decrease in the perceptions of respiratory and limb discomfort during exercise to fatigue (32,33,35). Research also suggests that IMT may evoke lower postexercise blood lactate (BLa) levels as a result of increased breathing efficiency after IMT, which in turn could increase oxidative or lactate transport capacity or both (3,26). Conversely, it has also been suggested that IMT could compromise limb blood flow (increasing BLa) during maximal exercise as a result of an increased cost of breathing (15). As yet, no consensus exists; however, it has been postulated that improvements in performance in exercise tolerance as a result of IMT are most likely because of positive sensations of respiratory efficiency, which lead to greater cardiorespiratory comfort, confidence, and voluntary willingness to extend duration (11). To our knowledge, there are currently no studies examining the concept of IMT as a mechanism to extend exercise tolerance in soccer players or whether this minimally intrusive technique offers tangible sport-specific benefits such as improved soccer performance efficiency for amateur players with limited training opportunities during the preseason period. The purpose of this study, therefore, is to examine whether the addition of IMT to an existing preseason training program for recreational soccer players facilitates increased tolerance to high-intensity exercise and maximizes performance outcomes from a twice-weekly training frequency.
Experimental Approach to the Problem
The experiment required the completion of baseline physiological testing, 6 weeks of concurrent IMT, and preseason soccer training, followed by posttraining testing of a cohort of participants undergoing the same exercise treatment (preseason training), but which was separated into experimental and placebo groups. A further (control) group of aerobic fitness-matched participants underwent the same baseline testing and posttraining testing without an intervention to investigate whether any posttraining changes in test performances could be attributed to learning effects or other nontraining systematic artifacts.
Twenty-four recreational soccer players from the same soccer club were randomized in matched pairs (based on aerobic fitness) into either experimental (EXP; n = 12) or placebo (PLA; n = 12) groups for the duration of their scheduled preseason training program, which consisted of twice-weekly training sessions. A further cohort of aerobic-fitness matched, recreational soccer players (CON; n = 12) group not undertaking preseason training or IMT.
All participants undertook the same battery of tests at baseline on 2 separate occasions. On the first occasion, this included anthropometry, spirometry measures, maximal inspiratory mouth pressure, familiarization for a soccer-specific fitness test (SSFT), and completion of a maximal multistage fitness test (MSFT) as the index of exercise tolerance. After a 48-hour recovery period under instruction not to exercise, participants revisited the laboratory for a second baseline occasion to complete the SSFT for which they had been familiarized 48 hours earlier (7). On completion of all baseline assessments, training loads for IMT were established for EXP (55% of maximum effort) and PLA (15% of maximum effort) from maximal inspiratory muscle pressure tests. These 2 groups thereafter completed the same 6-week program of preseason training unaware of their grouping as either EXP or PLA or that the placebo resistance setting for PLA has previously been shown to only exert a negligible effect (9,12,18,35). The controls were requested to maintain their habitual activity patterns over the 6-week intervention period and were included in the study to determine whether systematic drift occurred between test occasions for the selected outcome measures.
At the conclusion of the 6-week intervention period, all 3 groups repeated the battery of tests performed at baseline in the same order and under the same physical conditions (27.3 ± 1.2° C and 91 ± 8% relative humidity). An overview of the testing timeline is shown in Figure 1.
Thirty-six men agreed to participate in this 6-week training study. For reasons unrelated to this investigation, 3 participants from the PLA and 2 participants from the CON condition did not complete all testing requirements and, consequently, were not included in statistical analyses (Table 1). This study was approved by the Research and Ethics Committee of James Cook University, and all subjects provided written informed consent before participating.
All subjects were requested to refrain from consuming alcohol and caffeinated products for 24 hours before each experimental trial. Dietary guidelines were provided for the participants over 3-day periods before baseline and posttraining evaluations. Exercise testing was held at the same time of the day on each occasion to avoid diurnal variations.
Inspiratory Muscle Training
The EXP and PLA groups were both instructed to use the handheld pressure threshold-breathing device (POWERbreathe International Ltd, Warwickshire, United Kingdom) twice daily (morning and evening) for 30 self-paced inspiratory breaths, each to maximal voluntary inspiratory capacity across the 6-week intervention period. The resistance of the inspiratory pressure threshold device for EXP was preset and was not changed for the duration of the study in accordance with earlier studies of this duration (7,12). All individuals (across both EXP and PLA groups) were informed that they were undertaking an individualized IMT program. The device setting was disguised from all participants and the adjustment function disabled.
Respiratory and Lung Function
Spirometry and inspiratory muscle measures of forced vital capacity (FVC), forced expiratory volume in 1 second (FEV1) (Microlab-Spirometry SN M20364; USA), and maximal inspiratory mouth pressure (MIP) (POWERBreathe KH1 INSPIRATORY METER; Gaiam) were collected with the use of portable handheld devices. Three trials were performed for each with the best result being recorded.
Following a standardized warm up of 5-minutes steady jogging at rating of perceived exertion (RPE) 13 (somewhat hard: steady pace) (2), participants completed the MSFT, running continuously back and forth on a 20-m track in time with an audible signal, running speed progressively increased as the test continued, and the participants were required to run until they reached volitional exhaustion. Their final score was determined by the level reached immediately before stopping. Predicted V[Combining Dot Above]O2max and total distance covered (exercise tolerance) was calculated from the level reached by the participants. Rating of perceived exertion and rating of perceived breathlessness (RPB) (20) scores were recorded immediately on cessation of the test.
Soccer-Specific Fitness Test Protocol
The SSFT uses walking, sprinting, high-intensity running, and jogging activities in a cyclic pattern for evaluation of ability to sustain soccer-type intensity and movement patterns for 45 minutes (8). Although only replicating the demands of one-half of a game, the SSFT was selected in preference to other shorter duration protocols, such as the Yo-Yo Intermittent Recovery Test (YYIRT), as the SSFT is designed to mimic the demands of soccer match play rather than to predict fitness characteristics that are suitable for soccer (i.e., as is the case with the YYIRT). The SSFT is a well-known protocol that uses movement patterns originally specified in the Loughborough Intermittent Shuttle Test, has previously been shown to be highly reproducible, based on soccer match play observations, and is of ecological validity to soccer performance (8,27,28).
The SSFT was conducted in an indoor gymnasium on wooden floors. Participants were required to run between 2 lines 20-m apart, at various speeds related to the cohort’s mean maximal speed attained from the MSFT on the first baseline test occasion. Similar to other experiments using this test procedure (e.g., Ref. 8), this corresponded to standardized speeds for shuttles of high-intensity runs (95%), jogging (55%), and walking (35%). A 4-second recovery period followed each sprint to facilitate individual variation in maximal sprinting speed. The test required participants to complete 3 cycles of 10- × 20-m blocks comprising: 3× walking; 1× sprint, 3× high-intensity runs, and 3× joggings bouts. At the conclusion of each cycle, participants were given a 3-minute passive recovery. The running and walking speeds were dictated by a prerecorded verbal countdown to an audio signal to assist pace judgment.
Soccer-Specific Performance Outcome Measurements
During the SSFT, sprint times were recorded as the mean of 2 hand timings using a hand held stopwatch (JS-7064; Junsd, Inc., Japan). Fatigue rates (%) were determined by the percentage difference from the fastest to the slowest sprint times of each block and averaged across the 3 cycles. The 3-minute recovery period between each of the three 15-minute exercise blocks was used to record RPE and RPB scores and to consume water ad libitum. Heart rate (HR) was collected throughout the SSFT at 1-second intervals via wireless HR monitors (Polar Team 2; Polar Electro, Kempele, Finland). For posttest evaluation, a recovery HR was determined by calculating the mean HR from the last 30 seconds of the recovery period for each of the 3 blocks while participants waited passively for the test to recommence (8). A capillary blood sample was drawn from the fingertip from which BLa was determined before the beginning and immediately following the cessation of the SSFT (LT-1710; Lactate Pro-Akray, Kyoto, Japan).
Preseason Training Program
The frequency and structure of activities comprising the preseason physical training program undertaken by EXP and PLA were determined externally to the research team by the existing coaching staff of the amateur soccer club. The training design was similar to that of comparable recreational soccer groups (19). The preseason training period comprised 2 sessions of 2-hour duration per week (12 sessions in total), and each session was carried out under supervision of a qualified soccer coach and observed by a representative of the research team. Typical sessions were 2 hours in duration, which began with a 15-minute warm-up period that used dynamic stretching. After the warm-up, players underwent a conditioning portion of training, which included the following: sprinting; small-sided games (3 vs. 3, 6 vs. 6); and 20-m interval running training with and without the ball. This portion of the session was 45–60 minutes in duration. Technical drills were then performed for the remainder of the session. Intensity during sessions was monitored by use of the Borg CR-10 scale, and verbal encouragement was given to all players to ensure appropriate intensity during drills was maintained. Study and intervention adherence were checked 3 times weekly by the research team.
Two-way (group × time) analyses of variance with repeated measures (pre- and posttraining) were used for performance outcomes. Where differences were indicated, post hoc evaluations were analyzed by Tukey’s HSD test. Following dropouts for PLA and CON, the Tukey’s HSD formula for uneven group sizes was used appropriately for all comparisons among EXP (n = 12), PLA (n = 9), and CON (n = 10). Sample size analysis was calculated on the dependent variable of distance covered (meters) in response to the exercise tolerance test based on a statistical power (1-beta) of 90% and alpha of 0.05. A sample size of 8 was required to generate a moderate effect (>0.5) according to Cohen’s d effect size for the target population. Statistical analyses were performed on SPSS for Windows (version 19; SPSS, Inc., Chicago, IL, USA).
Respiratory and Lung Function Tests
The EXP group significantly improved MIP (p = 0.002) across the 6-week period, whereas the PLA group demonstrated a nonsignificant trend for improvement in MIP (p = 0.08). There was no change in MIP for the CON condition (p = 0.9). The EXP group also demonstrated a significant between-group effect (p = 0.002) for MIP compared with CON at the conclusion of the 6-week intervention. There was no change in FVC and FEV1 across time or between groups for any condition. The MIP and other respiratory values are reported in Table 2.
Analyses of variance indicated, and post hoc analysis confirmed, a significant time effect from baseline to postintervention (p < 0.05) by EXP in the MSFT (Table 3). Markers of perceived exertion or breathlessness (RPE and RPB) in response to the MSFT did not show change over time or between groups (Table 3).
Soccer-Specific Fitness Test
Blood lactate concentrations were significantly lower for EXP (p = 0.04) after the posttraining SSFT compared with baseline; however, PLA (p = 0.3) and CON (p = 0.3) conditions did not show a significant difference in BLa over this time (Figure 2). There was no significant difference in BLa between conditions. There was no difference in any performance measures that were taken during the SSFT, which comprised mean sprint times per cycle (Figure 3A); mean HR throughout the exercise cycles (Figure 3B); total fatigue rate, taken as a mean of the 3 fatigue rates from each cycle (before: EXP 77.1 ± 14.5%, PLA 76.4 ± 16.6%, CON 84.2 ± 5.8%; after: EXP 77.8 ± 10.8%, PLA 82.1 ± 7.4%, CON 83.3 ± 6.6%); mean recovery HR for each of the 3 recovery periods (Figure 3B); and neither RPE or RPB (Table 4).
The main finding of this study was that 6 weeks of concurrent IMT and twice-weekly pre-season soccer training for recreational players resulted in a significant improvement to exercise tolerance as detected by accumulative distance in a running test (p = 0.02) but did not result in substantial change to sport-specific fitness performance variables. Posttraining BLa concentration following the sport-specific SSFT was significantly reduced in EXP (p = 0.04), but no other results or performance indices suggested a sport-specific benefit from IMT use during preseason training, which indicates that its impact may be limited.
Although the observations of this study do not strongly support the inclusion of IMT to soccer preseason training, it is apparent that twice-weekly training is an inadequate stimulus from which to expect observable change to the fitness indices in this study when not coupled with IMT. It is a common training load of recreational athletes (19) and, in this study, was preset by the coaching staff of the soccer club. It was hoped that the inclusion of IMT may lead to greater than usual outcomes from this limited training stimulus and to some extent that does appear to be the case with significant gains in MSFT distance covered and attenuated posttraining BLa after the SSFT, which were all isolated to the EXP group. Collectively, these results suggest positive change to exercise tolerance (MSFT distance covered) and performance efficiency (posttraining SSFT BLa was reduced with no significant decline in test performance). Therefore, the addition of IMT to the preseason training may represent a measured success of the concurrent strategy.
It had been anticipated by the soccer club coaching staff that the twice-weekly training stimulus would be sufficient to demonstrate a significant gain in MSFT distance covered by both of the groups (EXP and PLA) undertaking 6 weeks of preseason training. This was not the case as PLA did not show any performance gains after 6 weeks, which strongly suggests that recreational or amateur clubs should carefully consider issues of training load if gains in fitness are sought over the preseason period. Our results suggest further research is required to examine adequate frequency, load, and type of training in preseason training for recreational and amateur soccer players. Currently, there is a considerable lack of research in this area (19). As observed in this study, the addition of IMT may facilitate greater adaptations to training stimuli by increasing tolerance to high-intensity exercise but equally adding greater frequency of training is more likely to facilitate more consistent fitness gains.
The significant change to MSFT distance covered by EXP is consistent with other recent studies, which have identified that IMT can influence performance to volitional exhaustion in the absence of concomitant change in physiological variables such as maximal oxygen uptake or cardiopulmonary factors (7,13,22). The findings of this study support those observations insofar as the distance covered in the test of exercise tolerance (MSFT) was significantly extended by EXP group across the 6-week intervention. This MSFT performance increase is similar to that observed after a previous IMT intervention that used a shuttle run test to exhaustion (5). Although PLA and CON groups did not improve significantly, the magnitude of change for EXP group was not sufficient to result in an appreciable between-group effect. As discussed elsewhere (7,11), IMT is unlikely to directly stimulate improvements to cardiovascular fitness per se.
Improvements to lung and chest wall compliance as a consequence of IMT (9) could be expected to reduce respiratory discomfort and perceptions of effort at high ventilatory loads such as those experienced during the MSFT and SSFT, thus increasing fatigue resistance. However, this does not necessarily indicate that fitness has been improved but rather that improvement to respiratory muscle strength probably increases the willingness to persist with exercise for extended periods via afferent feedback to the control center. As EXP group significantly increased the total distance covered in the MSFT by 12% (compared with: PLA 9.6%; CON 8.9%), IMT may have been influential in this regard. No change in RPE or RPB was observed, but this is not surprising as the MSFT is maximal in nature, and therefore when participants reach volitional exhaustion, they experience sensations of maximal effort, and so similar sensations of breathlessness and fatigue are reported regardless of the total distance covered during the test. It is suggested that future studies of this nature should record submaximal markers of perceived effort periodically at benchmarked intervals during tests that require participants to exercise to volitional exhaustion. It is plausible that the EXP group may have experienced attenuated perceptions of breathlessness and effort during the lower stages of the test when compared with their baseline outcomes as a direct result of decreased afferent feedback from the cardiorespiratory system.
The significant improvement to MIP (p = 0.002) in EXP group is in accordance with previous research among homogenous populations (12,25,31,34). In this study, the mean MIP for participants improved but only significantly so for EXP group (EXP: 15% gain; PLA: 9% gain; CON: 2% gain). Although the EXP group was the only group to significantly improve MIP, the 9% gain by PLA (p = 0.08) indicates that 2 × 30 daily inspiratory breaths against a minimal (∼15% MIP) resistance could be potentially beneficial for increasing inspiratory muscle strength. Additional work examining inspiratory training techniques and loads without a pressure threshold-training device may indicate alternative strategies to improve IM performance. In addition, in a review and analysis of IMT, Hajghanbari et al. (14) recommend that the muscle contraction parameters, such as range of motion and speed of contraction, of IMT protocols should match those required by the specific sport and that IMT is best suited to sports with a high ventilatory threshold, soccer is one such example. It was also suggested that an aggressive progression of intensity should be adopted to ensure an adequate training load is present so that the optimum benefit of IMT is received. Although this progressive increase is recommended, this method was not used in the current study, and there was no change in device settings throughout the intervention period. In this study, the initial setting of 55% MIP (unchanged) for 6 weeks was of adequate load to promote physiological adaptations, although a more aggressive stimulus may have promoted greater changes to occur.
Concurrent preseason soccer training and IMT resulted in posttest attenuation of BLa concentration (p = 0.04) following the SSFT for the EXP condition only. However, there was no significant change in any performance variables (i.e., sprint times, HR, or fatigue rate) collected during the SSFT after the 6-week intervention for any group. The selection of the SSFT as the sport-specific test was based on its similarity to soccer-match demands and work patterns. The work patterns of the SSFT reflect typical intensities of soccer match play (∼85–90% of maximum HR; Figure 3B) (8,27). However, as repeated sprint performance throughout the 45-minute protocol did not change across the test (Figure 3A), nor were there any significant modifications to HR (Figure 3B), perceived breathlessness, or RPE (Table 4), it seems that sport-specific physiological adaptation was largely unaffected. Nevertheless, the lower BLa concentrations (attenuated by 20% from baseline) for EXP group do suggest that greater respiratory muscle efficiency may have facilitated SSFT performance with greater physiological reserve for subsequent action. It has been postulated that IMT causes a change in respiratory muscle function via affecting lactate clearance, therefore contributing to lower BLa (26), or that there may have been an increase in the oxidative and/or monocarboxylate transport protein content of the inspiratory muscles (3). This could be meaningful for soccer performances in that match play is not a maximal activity and players perform a match in the knowledge that they could be expected to sprint to the ball at any moment during a game (10). Therefore, reduced posttest BLa indicates some level of usefulness for IMT in the context of soccer performance. Additional studies examining post-SSFT performance of skilled activities or volitional fatigue or both are required to substantiate whether the reduced BLa concentrations are meaningful to soccer.
A number of practical applications for IMT may be identifiable with caution as a result of this study. Briefly, these include the following: (a) the use of IMT as an effective concurrent training aid to augment training outcomes, (b) the use of IMT as a means to extend exercise tolerance in tests such as the MSFT, (c) potential performance-efficiency benefits of soccer match play following IMT, and (d) changes should be made to the current frequency and volume of amateur preseason soccer training if significant fitness gains are aspirational. However, further studies using a greater soccer or IMT training load or both and further sport-specific performance tests to exhaustion are warranted to examine whether IMT is useful in these circumstances. As undertaking the MSFT requires motivation and respiratory muscle efficiency at high ventilatory loads, it seems likely that IMT has greater efficacy for tests that assess exercise tolerance rather than for sport-specific testing per se. Inspiratory muscle training may prove beneficial as an added training stimulus to amateur and subelite teams who are limited by available training time. It does seem that greater exercise tolerance and perhaps a sport-specific reduction to peak postexercise BLa may be meaningful, but these observations require further substantiation.
The authors would like to acknowledge the participants and data collectors for their tireless efforts and generous time commitment throughout the study, without which this project would not have been possible. They would also like to thank Stratford United Football Club for access to their players and facilities.
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