The occurrence of significant inspiratory muscle fatigue (IMF) after participation in a range of different sports is well documented (9,10,17,19). For example, IMF has been quantified after various modes of land-based exercise, including marathon running (9), indoor rowing (19), simulated cycling time trials (TTs) (17), and triathlon (6) with %IMF for such sports typically ranging between 16 and 26%.
For water-based exercise, however, the occurrence (prevalence and magnitude) of IMF is less well understood. Although both Lomax and McConnell (10) and Jakovljevic and McConnell (7) have quantified IMF after 200-m freestyle swimming trials, these were not maximal efforts (i.e., only 90-95% of race pace). Regardless, the 11-29% drop in maximal inspiratory pressure (MIP) does suggest that IMF for water-based exercise might be substantially greater than the IMF typically observed for on-land sports. However, without measures of IMF after maximal ‘all-out’ swim trials, this cannot be confirmed.
The opportunity to further assess IMF after intense swimming exercise is of particular interest because of the unique challenge that exercising in water poses on breathing, compared with on-land exercise. For example, swimmers are required to precisely coordinate their frequency of breathing and tidal volume with stroke mechanics, which results in a different breathing pattern compared with on-land exercise (16). Coupled with the increased hydrostatic pressure on the chest, and the potential effect of body position on breathing, these factors individually and collectively likely result in an increased load on the respiratory system.
To our knowledge, no study has quantified and reported the degree of IMF across a range of events in the same subjects. There is some limited evidence to support that the distance or intensity intensity of an event during on-land exercise may influence IMF. For example, Johnson et al. (8) showed that the magnitude of diaphragm fatigue, as determined by bilateral transcutaneous supramaximal phrenic nerve stimulation, and the likelihood of its occurrence, increased as the relative intensity of the exercise exceeded 85% O2peak during constant-load cycle exercise. In support, Romer et al. (17) also showed that greater IMF was observed after a 20-km TT compared with a 40-km TT in well-trained athletes. The effect of distance on IMF for water-based exercise is unknown. In response to the lack of research to date in this area, the aim of the present study was to quantify and compare the exercise-induced global IMF after 3 standard competitive swimming distances. We hypothesized that short-duration events would result in greater IMF.
Experimental Approach to the Problem
A randomized crossover design was used to determine the effect of 3 standard swimming race distances on global IMF, physiological responses, and perceived exertion. On separate days, after familiarization trials for all measures, trained swimmers performed 100-, 200-, and 400-m swimming TTs as fast as possible. Before and after each trial, maximal inspiratory mouth pressure (MIP) was determined and the global IMF calculated and compared across distances. The study was conducted near the competition phase during which race-specific swim training predominated their program.
Ten well-trained, nonasthmatic, nonsmoking, freestyle male swimmers from a local swim team participated in the study. The swimmers characteristics are presented in Table 1. All procedures were approved by the institutional ethics committee, and participants consented, in writing, before the study.
All swimmers completed 4 testing sessions. During session 1, swimmers completed an assessment of dynamic lung function and familiarization tests for measures of MIP. Sessions 2-4 involved 3 performance TTs, each separated by 48 hours and performed at the same time of the day. Before and after each TT, measures of MIP were determined. Before all tests and TTs, swimmers were requested to abstain from heavy physical exercise and consuming caffeine in the 24 hours preceding assessments and prepare in their usual way for competition.
Pulmonary function (forced flow-volume loops) was assessed using an on-line turbine spirometer (Microlab, MicroMedical, Kent, United Kingdom). Measurements were made according to the American Thoracic Society and European Respiratory Society recommendations (14). The forced vital capacity, forced expiratory volume in 1 second (FEV1), and peak expiratory flow rate were determined.
Maximal inspiratory mouth pressure is a global measure of inspiratory muscle strength. The MIP was measured at residual volume using a portable handheld mouth pressure meter (MicroRPM, MicroMedical Ltd, Kent, United Kingdom), with swimmers standing upright in the pool. We chose this position because in a prestudy determination of the test-retest reproducibility of resting MIP, involving 6 swimmers, we found that MIP was more reproducible when standing upright in water than when being supine in water (ICC = 0.89 and 0.81, respectively). During all measures of MIP, participants received visual feedback of pressure exerted during the test to maximize respiratory effort and were consistently instructed to inspire maximally. Because previous work has shown that there is a considerable learning effect for measures of MIP (2,10), all swimmers underwent repeated MIP familiarization trials before actual data collection.
During the experimental trials, 3 baseline measurements were taken after the athletes' self-chosen typical competition warm-up that involved swimming and dynamic stretching. Each swimmer was requested to perform the same warm-up for all trials. The highest MIP recorded was included in subsequent analysis. After baseline measures of MIP, swimmers completed, on separate days, 100-, 200-, and 400-m TTs in a 25-m indoor pool, the order of which was randomized. Each TT was performed in matched pairs, to introduce a competitive element and to ensure a near maximal effort. Performance was defined as the time taken to complete the given distance as measured by 2 experienced coaches. Within ∼45 to 60 seconds of completing each TT, a postexercise measure of MIP was made. The pre-post difference in MIP was used to determine the %IMF associated with each TT. The stroke rate for each swimmer during each trial was determined and peak heart rate (HR) for each TT was measured using short-range telemetry (Polar Electro, Oy, Finland). In addition, a rating of perceived dyspnea (RPD) scale from 1 to 11 was used (20), whereby swimmers were asked “How hard was your breathing during that swim?” The descriptors were 1 = breathing is not hard at all, 3 = breathing is a little hard, 5 = breathing is moderately hard, 7 = breathing is hard, 9 = breathing is really hard, and 11 = breathing is very, very hard.
Data are expressed as mean ± SD. The performance times, MIP, IMF, and HR responses were analyzed using a repeated-measures analysis of variance using a statistical software package (SPSSv14, Chicago, IL, USA). Paired t-tests were used for post hoc analysis if differences were observed between race distances. Paired t-tests were also used to determine prepost differences for MIP and %IMF for each individual distance. Relationships between measures were explored using Pearson Product-Moment Coefficients. In all tests, statistical significance was set at p ≤ 0.05.
The descriptive characteristics of the swimmers participating in the study are presented in Table 1. All pulmonary function measures were within the normal range.
Individual MIP responses pre- and post-TT are shown in Figure 1. The group mean ± SD MIP before and after each performance TT is presented in Table 2. Baseline MIP did not differ across distances (p > 0.05). A reduction in absolute MIP was observed after all trials, though this was only significant for 100 m (p < 0.05; Table 2). Absolute posttrial MIP did not differ between TT distances (Table 2). A small difference in MIP between 100 and 400 m (p < 0.05) existed when MIP was expressed as percentage of the baseline value for each trial, but this was not observed for other trial comparisons (p > 0.05, Figure 2). No other differences between trials existed.
There was no correlation between mean pre-TT MIP and %IMF (r = −0.28, p > 0.05), or between %IMF and performance time, for any TT (r = 0.25, 0.34, and 0.18 for 100, 200, and 400 m TT, respectively; p > 0.05).
The purpose of this present study was to determine if, and to what extent, different swimming events influence the degree of IMF in trained swimmers. The findings of this study suggest that IMF is minimally influenced by TT distance after all-out swims.
The observed IMF in the present study ranged from 4.6 to 8.2% (Figure 2), with a tendency for 100-m TTs to be associated with the greatest IMF. It is difficult to directly compare the magnitude of IMF observed in the present study with previous work, because no study has quantified IMF after all-out swimming trials. However, the observed IMF in the present study for 200-m TTs was substantially less than the ∼29% IMF reported previously by Lomax and McConnell (10) after a 200-m swim at 95% effort, and the 21 and 11% IMF reported by Jakovljevic and McConnell (7) at 90% of race pace, at 2 and 4 strokes per breath, respectively. Given the greater intensity of effort during the 200-m TTs in the present study (i.e., all-out, maximal vs. 90-95% effort), it was surprising to observe a lower IMF for this particular distance compared with other studies. In fact, neither shorter nor longer distances in the present study were associated with such significant IMF as compared with previous studies (7,10). We attribute this dissimilarity to subtle methodological differences between the studies, especially with respect to warm-up procedures and respiratory measurement of MIP that are likely to significantly affect measures of MIP. Specifically, both Lomax and Mconnell (10) and Jakovljevic and McConnell (7) used a specific respiratory muscle warm-up before their 200-m trials. Based on the findings of Volianitis et al. (18), it is very likely that this specific respiratory muscle warm-up inflated the pretrial MIP (18), and so when the posttrial MIP was expressed as a precent of the elevated pretrial MIP, the resulting %IMF was magnified significantly, at least compared with our data, which did not use a respiratory muscle warm-up protocol. Although acknowledging and accounting for the considerable learning effect of repeat measures of MIP (2) in the present study, we chose not to administer a respiratory warm-up because we wanted to determine the degree of IMF experienced by athletes who do not use such a technique before normal competition. Another factor that may have been somewhat influential was that our measures of MIP were performed in the upright position where as other studies (7,10), having determined that this position was associated with a ∼16% lower MIP at baseline, performed their measures in the ‘swimming-specific’ supine position. We also determined that, in our prestudy pilot work involving 6 swimmers performing repeated MIP maneuvers, the MIP was ∼6% different between upright and supine positions after a 100-m TT. This suggests that the differences between studies were consistently different, regardless of posture.
In comparison to athletes from other sports, such as running (9), rowing (19), and cycling (17), the observed IMF in trained swimmers also appears much lower in the present study, but this may be expected given the generally well-developed respiratory system that swimmers have as a result of the demands and environment in which training takes place (2,13). Despite lower IMF, the pre-exercise MIP of the swimmers in the present study was not exceptionally high, at least not compared with that of very well-trained athletes from swimming (13,20) or other on-land sports (10,17,19,20), though it should be noted, as aforementioned, that we did not prime the respiratory muscles before determining pretrial MIP, and these were younger swimmers than previously reported. However, it is possible that the endurance or fatigue-resistant qualities of their inspiratory muscles are indeed superior to that of endurance athletes from other sports, resulting in less IMF, regardless of baseline MIP. In support, we observed no relationship between the severity of %IMF and the baseline strength of the inspiratory muscles, in this cohort of male swimmers. This finding is contrary to previous work involving on-land exercise (12). Furthermore, there was no relationship between IMF and TT performance for any distance.
Previous studies involving well-trained athletes have studied events lasting ∼6 minutes (rowing ), 30-60 minutes (cycling ), and ∼3 hours (marathon running ). These events correspond to exercise intensities from ∼80 to 100% O2peak. Although there may be some evidence for a relationship to exist between exercise intensity and IMF for intensities up to 100% O2peak (8,17), until now no study has established if this holds true for much higher intensities. The 3 swimming distances used in the present study differed with regard to their energetic demands (aerobic vs. anaerobic contributions) and subsequent physiological responses (4,5,11). For example, short-duration events such as the 100 m would rely more heavily on anaerobic energy supply compared with longer duration events such as the 400 m (5). Likewise, the ventilatory responses and work of breathing during these swimming events is also likely to differ, and this could be influential in determining the degree of IMF observed. As anticipated, we observed a greater stroke rate in the shorter trials, and this may have contributed to greater IMF for this distance. More specifically, the accessory muscles used in breathing are also likely to be heavily engaged during the stroke action itself during the 100-m TT, and this was reflected in our global measure of MIP.
Although the current study focused on the effect of freestyle TT performances, on MIP only, it is possible that different swimming strokes may influence IMF. As previously alluded by Lomax and McConnell (10), the chest is typically fully submerged during freestyle swimming and body position, despite some lateral body roll when breathing, remains relatively horizontal to minimize nondesirable resistance and drag. In other strokes, in particular breaststroke (3) and butterfly (1), greater undulations in body position occur (trunk and hip extension and flexion) which may require greater recruitment of muscles involved in breathing and place the intercostals and diaphragm in a less efficient position during inspiration (and expiration). Also, during some strokes, the opportunity to breathe and the actual body position when breathing may differ. This is most obvious during backstroke, for example, when the swimmers face is not immersed. Some swimmers, in short-distance events may choose to avoid or limit breathing, though breathing during freestyle swimming at 200-m race pace has been found not to interfere with basic stroke parameters (15). Although the effect of different swimming strokes and consequent different breathing strategies on IMF during swimming, and on subsequent exercise performance, is unknown, 1 study has examined the effect of different breathing frequencies within a single stroke. Specifically, Jakovljevic and McConnell (7) reported that breathing more frequently during a 200-m freestyle effort at 90% race pace attenuated MIP by 10% suggesting that the chosen breathing strategy has functional consequences for the respiratory system during swimming.
In summary, the lack of difference between distances for posttrial MIP suggests that the distance of swimming events minimally influence the degree of IMF experienced in trained swimmers. The degree of IMF in the present study was substantially less compared with previous studies involving swimming and also less than that reported for other land-based endurance sports. Further research is required to determine if other factors, such as stroke choice and breathing pattern during all-out trials, influence IMF in swimming and whether IMF after swimming trials can be attenuated using specific respiratory training strategies.
This study demonstrates for the first time that substantial IMF occurs after all-out swim efforts across a range of standard competitive swimming distances. The fact that no substantial difference in IMF exists between distances suggests that individuals specializing in a range of swim distances would benefit from specific intervention strategies that are designed to enhance inspiratory muscle function. Specifically, coaches and sports scientists would be wise to consider prescribing inspiratory muscle training techniques in an effort to reduce the inevitable IMF associated with maximal effort swimming exercise.
There was no financial assistance with this project. The authors would like to thank the swimmers and the coaches for their willingness to participate.
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