Athletes who participate in high-intensity, intermittent-type sports, such as soccer and rugby, have long appreciated the role of high-intensity interval bouts of running as part of a comprehensive training program for enhancing the predominant energy systems that are specific to such sports (1,6). Edwards et al. (4) suggested that the inclusion of respiratory muscle conditioning within a whole-body exercise training program might alter the perception of effort during the exercise training bouts and consequently augment the training volume and quality and, in turn, enhance the whole-body exercise capacity. Although both chronic (training) and acute (warm-up) loaded ventilatory activity applied to the inspiratory muscles (IM) have been shown to improve the tolerance of intense intermittent exercise, this has been demonstrated only in a single trial promptly after the intervention (10,19,21). Little is known about the practical use of the inspiratory muscle loadings (IMLs) for augmenting the training volume of a long-term interval running program and the resultant effects on high-intensity intermittent running performance.
Mechanisms underlying the improvement in the tolerance of high-intensity intermittent running resulting from the specific IM training and IM warm-up are not identical. Improvements in intermittent exercise performance immediately subsequent to an IM warm-up have been largely attributed to (r2 > 0.8) the temporary attenuation of the sensation of breathlessness near the point of volitional exhaustion (19). This factor appears to be less crucial in the enhancement of the exercise tolerance after long-term specific IM training (21). Attenuation of the IM metaboreflex-induced peripheral fatigue resulting from the biochemical and structural adaptations to specific IM training may also play a part in augmenting the tolerance of such exercise (12,21,27). Such findings suggest that the ergogenic effects of the IM warm-up and the IM training on the tolerance of intense intermittent exercise could be synergized if the chronic and acute loaded ventilatory activities are applied to the IM in an integrated manner. This study examined the hypothesis that the application of a 4-week IM training before a 6-week interval running program in athletes, plus the execution of a specific IM warm-up regimen before each workout of the program, would augment the repeatability of high-intensity running bouts during an interval training program. This in turn would result in additional improvements in the maximum performance of the Yo-Yo intermittent recovery test in comparison with interval training alone.
Experimental Approach to the Problem
In this study, subjects were randomly assigned to 2 groups. The physical characteristics of subjects between the 2 groups were not different (Table 1). For the IML group, subjects participated in a 6-week interval training protocol composed of high-intensity intermittent runs after a 4-week IM training. Before each interval training session, specific IM warm-up was performed as part of whole-body warm-up regimen. For the control group, subjects participated in the 6-week interval training program only. To explore the hypothesis, the training volume of the 6-week interval training program and the improvement in the tolerance of high-intensity intermittent running after the interval training program were compared between the IML and control groups.
Eighteen male nonprofessional athletes who had received training at local soccer or rugby clubs for at least 4 years were recruited to this study. All subjects were used to training 2-3 h·d−1 for 4-5 d·wk−1 to compete in either first or second division competitions. After being fully informed of the experimental procedures, which were approved by a local ethics committee, subjects gave their written consent. The sample size (n) of 9 in each group was computed based on the formula n = σ2(−z0 + z1)2/(μ0 − μ1)2 (23), where μ0 (exercise performance under control condition), μ1 (expected improvement in exercise performance), and σ (SD) were estimated from published data on the change in the tolerance of intermittent bouts of running after IM training (21). The statistical power was 0.9 and alpha level was 0.05. z0 is the critical value for effect size under the null distribution and z1 is the critical value associated with the alternative distribution.
Before the experiments, a familiarization trial of the Yo-Yo test was undertaken to familiarize the subject with the testing procedure and with the sensation of exercising to exhaustion. In all tests and workouts, the subject was required to perform a standardized whole-body warm-up exercise, which comprised a 5-minute treadmill run at a self-selected moderate speed, a 10-minute period of stretching, and a 5-minute overground running at a self-selected pace.
Inspiratory Muscle Function
Inspiratory muscle function was assessed by measuring the peak inspiratory mouth pressure at zero flow (P0 in cmH2O). This was measured by performing maximal inhalations at residual volume against an occluded rubber-scuba-type mouthpiece with a 1-mm orifice. During the maximal inhalation, the change in the mouth pressure was detected by a differential pressure transducer coupled with a signal conditioner (Collins, Braintree, MA, USA). The measurement was repeated at least 5 times until the result became stable. The highest value was recorded. In this study, each subject performed the same IM function test before and after the IM training period. In the IML group, the IM function immediately after the specific IM warm-up exercise was also assessed after IM training.
Yo-Yo Intermittent Recovery Test
To assess the change in the tolerance of intense intermittent bouts of running, the Yo-Yo intermittent recovery test (level 1) was performed before IM training and after interval training. The exercise tolerance was assessed by the number of repetitions of the 20-m shuttle completed in the Yo-Yo test (repetitions [reps]). Details of the testing protocol have been described previously (9). The test was conducted twice on each pre- and post-training occasion, respectively, to assess the repeatability of the maximal exercise performance. For the number of repetitions completed in the pre-training Yo-Yo tests, the coefficient of variation (CV) and intraclass correlation coefficient (ICC) were 2.7% and 0.95, and 3.9% and 0.91, respectively, in the control and IML groups. In the post-training trials, the CV and ICC were 3.8% and 0.91, and 4.3% and 0.98, respectively. These values demonstrate highly reliable test-retest results for the Yo-Yo tests in both groups.
During the Yo-Yo test, the ratings of perceived breathlessness (RPB) and the ratings of perceived exertion (RPE) were recorded before the exercise, then at the subsequent 10-second recovery of every fourth exercise bout starting from the 13th level of the Yo-Yo test and at exhaustion. The RPB and RPE were assessed with the aid of Borg Category Ratio (CR-10) and Borg Category Scales (6-20), respectively (22). To assess the respiratory response to the Yo-Yo test, minute ventilation (E), O2 consumption (o2), ventilatory equivalent for oxygen (E/o2), and mean inspiratory flow (VT/ti) were recorded with a portable cardiopulmonary measuring instrument (MetaMax; Cortex, Leipzig, Germany) in an additional trial. This was to avoid interference from the instrumentation on the intensity of breathlessness and exercise tolerance during the test. Details on the testing venues and conditions have been reported previously (19).
During the Yo-Yo test, whole-body metabolic stress was examined by measuring plasma ammonia ([NH3]pl), uric acid ([UA]pl), and blood lactate ([La−]b) accumulations. A pre-exercise blood sample was collected before the standardized warm-up exercise. To assess post-exercise peak values of plasma [NH3]pl and [UA]pl, blood samples were taken immediately and 1 hour after the exercise, respectively. The blood sample taken immediately after exercise was also used for [La−]b analysis. At each blood sampling, 2-3 ml venous blood was drawn from the antecubital vein using a venous puncture with the subject in a sitting position. A 25 μL blood sample was drawn for analyzing [La−]b using the YSI 1500 Sport Analyzer (Yellow Springs, Dayton, OH, USA). The remaining portion of the blood sample was immediately centrifuged, and the plasma was separated for the [NH3]pl and [UA]pl assays with the Vitros DT60 II Chemistry System (Johnson & Johnson Clinical Diagnostics, Rochester, NY, USA). For examining the change in whole-body metabolic stress after the interval training, the post-training Yo-Yo test was further repeated with repetitions of a 20-m shuttle run identical to the pre-training value (ISO).
Six-Week Interval Training
Table 2 shows the protocol of the 6-week high-intensity interval running training program recommended by Fox and Mathews (6) for enhancing individual's exercise capacity both aerobically and anaerobically. The program consisted of 3-4 workouts per week. Each workout comprised 1-3 sets with different repetitions of selected distances of 100, 200, 400, 600, 800, and 2,400 m in each set. The ratio of work to recovery duration in repetition of different events was 1:3 in distances ranging from 100 to 400 m, 1:2 for the 600-m distance, and 1:1 for the 800-m distance. The running training was performed on a high-speed treadmill (h/p/cosmos Pulsar lt 3p 4.0, Cosmos, Nussdorf-Traunstein, Germany) with gradient at 0%. The initial speed for each distance was set according to the subject's maximal speed achieved in the pre-IM training Yo-Yo test (100 and 200 m: 95-100%; 400-600 m: 90-95%; 800 m: 80-85%; and 2,400 m: 75-80% Yo-Yo maximum speed). After the initial trial of each distance in the training program, running speeds in each of the subsequent training distances were adjusted voluntarily on a trial-by-trial basis, such that volitional exhaustion (>90% maximum heart rate [HRmax]) was attained at the end of the set. During recovery intervals, the subject walked briskly on the treadmill at 5 km·h−1. In this study, all subjects complied with the training protocol. The comparison of training volume between groups was based on the percentage of total number of repetitions in each distance with running speed ≥130% of the initial value. The criterion of 130% was the best compared with other values for discrimination of individual's sustainability of exercise bouts at high intensity.
Inspiratory Muscle Training
The protocol of IM training, which has been shown to improve IM strength, was applied (15,21). Briefly, the IML group performed 30 inspiratory efforts twice per day, 6 days a week for 4 weeks. Each effort required the subject to inspire against a pressure-threshold load equivalent to 50% P0 by using the POWERbreathe device (Gaiam Ltd., Warwickshire, United Kingdom). During the training, the subject was instructed to initiate every breath from the residual volume in a powerful manner. The inspiratory effort was continued until the inspiratory capacity for the preset loading limited further excursion of the thorax. For training progression, the inspiratory load was increased by 10-15 cm H2O once the subject had adapted (i.e., the subject was able to complete 30 maneuvers without a break). In the present study, adherence to the IM training was close to 100%.
Inspiratory Muscle Warm-up
A loaded ventilatory protocol, which was designed specifically for IM warm-up, was used (7,19). Briefly, the subject performed 2 sets of 30 inspiratory efforts for work against an inspiratory pressure-threshold load equivalent to 40% post-IM training P0 by using the POWERbreathe device. The performance of the inspiratory efforts was similar to that during the IM training, and it was performed in between the stretching exercise and the free running exercise of the standardized whole-body warm-up exercise.
For data analysis, all the data recorded in the second of the repeated Yo-Yo tests before and after training were selected. Kolmogorov-Smirnov test and Levene's test of equality of error variances had been applied and revealed that the data were normally distributed in groups, and the error variances of dependent variables were equal across groups. A series of 2-factor analysis of variance were applied to analyze the between-group effects (IML and control) and within-group effects (pre- and post-training, training weeks, distances) on most of the dependent variables. Post-hoc analyses using Newman-Keuls were performed when interaction effects were significant. Relationships between variables were determined using simple regression. All tests for statistical significance were standardized at an alpha level of p ≤ 0.05, and all results were expressed as the mean ± SD.
Inspiratory Muscle Function
The maximal inspiratory pressure (P0) for the IML group was improved substantially after IM training (Pre: 163.0 ± 29.8; Post: 195.9 ± 23.5 cm H2O, p < 0.01), whereas it remained unchanged in the control group (Pre: 163.6 ± 16.4; Post: 163.4 ± 20.1 cm H2O, p > 0.05). In the IML group, the specific IM warm-up resulted in a small but significant reduction in the post-IM training P0 (188.2 ± 20.4 cm H2O, p < 0.05).
Figure 1 shows the change in the percentage of total training bouts per week with running speed ≥130% of the initial values for each subject during the 6-week interval program. The trend of the percentage increased from week 1 to week 3 is similar in both groups. However, the greater proportional increases during weeks 3-6 in subjects in the IML group are significantly higher than the corresponding values of control group (p < 0.05). Furthermore, in comparison with the control group, the number of repetitions in each distance for which the running speed ≥130% initial value was greater in the IML group, with significant differences observed for 200-, 600-, and 800-m distances (Figure 2).
Intense Intermittent Run to Exhaustion
The number of pre-training repetitions was not different between the 2 groups (control: 42.7 ± 4.1 reps, IML: 40.3 ± 5.0 reps; p > 0.05), and both groups improved significantly after training, although the extent of the improvement in the IML group was greater (control: 49.8 ± 4.4 reps, IML: 52.7 ± 6.4 reps; p < 0.05).
Perceptual responses for the control and IML groups during the pre- and post-training Yo-Yo tests are shown in Table 3. In comparison with the pre-training RPB at exhaustion, the post-training RPB was unchanged in the control group (p > 0.05), but it was significantly lower in the IML group (p < 0.05). Similar changes were also observed in the rate of increase in the RPB, which is expressed as the slope of the linear relationship of the increase in RPB for every fourth exercise interval (RPB/4i) during the Yo-Yo test (control [Pre vs. Post]: 0.22 ± 0.03 vs. 0.20 ± 0.02, p > 0.05; IML: 0.26 ± 0.02 vs. 0.20 ± 0.02, p < 0.05). Furthermore, the change in the RPB/4i in the IML group was correlated to that in the repetitions completed in the Yo-Yo test (r = −0.80, p < 0.05) when the 2 variables were expressed as percentage of pre-training values. For the maximal RPE, a significant reduction (p < 0.05) was observed in both groups after training and the reduction was not moderated by IML (p > 0.05). Such a change was also revealed in the rate of increase in the RPE (RPE/4i) (control [Pre vs. Post]: 0.32 ± 0.05 vs. 0.27 ± 0.02; IML: 0.33 ± 0.03 vs. 0.24 ± 0.04, p < 0.05). The change in the RPE/4i in both groups was correlated to that in the repetitions in the Yo-Yo test (control: r = −0.76, IML: r = −0.68, p < 0.05).
Marked accumulations of [NH3]pl, [UA]pl, and [La−]b after the Yo-Yo test were observed in the control and IML groups before and after training (Table 4). In the post-training ISO trial, the metabolite accumulations were reduced from pre-training values in the both groups, whereas the reductions in the [NH3]pl and [La−]b, but not the [UA]pl, in the IML groups are comparatively in greater extents.
The respiratory response to the bout of 20-m shuttle runs and the subsequent 10-second recovery of the Yo-Yo test in pre- and post-training trials, which are the average of the responses recorded at the time points corresponding to 80, 90, and 100% of the pre-training repetitions, are shown in Table 5. With the exception of o2, the E, E/o2, and VT/ti were lower in the post-training trial in both groups. However, the reductions in the E and VT/ti in the IML group were greater in comparison with the control group (p < 0.05).
The results of this study support our hypothesis that the application of a 4-week IM training before a 6-week interval running program, plus the execution of a specific IM warm-up regimen before each workout of the program, would lead to an improved ability to repeat high-intensity running bouts during an interval training program. The training strategy outlined in this study results in a greater improvement in the tolerance of high-intensity intermittent running bouts in comparison with an identical interval program, which does not include chronic or acute loaded inspiratory activity.
In the present study, IM training was undertaken by following a well-established training maneuver that has been shown to be effective for improving the static and dynamic strength of the muscles (15). Such a maneuver has been further demonstrated to improve performance capacity in different types of exercise, including high-intensity intermittent running bouts (16,17,21). After the 4-week IM training, IM function (as indicated by P0) was improved by approximately 21% in the IML group. However, when the subjects performed the IM warm-up activity with the load of 40% of the post-IM training P0 applied to the IM, a tiny but significant reduction in the IM function (∼5% post-IM training P0) after the warm-up activity was observed. This finding was in contrast to that reported in previous studies (10,19,24). It may be because the stimulus of the IM training was strength orientated (13). The resultant increase in the strength and the endurance capacity of the IM may not occur concurrently and may not be in proportion. Hawkes et al. (7) noted that potentiation and low-frequency fatigue in the IM after a deliberate IM warm-up activity may occur simultaneously. Because a 40% maximal force output of untrained IM is considered to approximate the upper limit for inducing fatigue in the IM muscles (18), the application of the 40% of the post-IM training P0 in the IM warm-up activity might have overloaded the muscles to an extent that IM fatigue became dominant. Although the specific IM warm-up did not augment IM function, other associated transient physiological and perceptual responses might have occurred. It has been suggested that specific IM warm-up may lessen the demand of IM force generation in each breath by reducing the degree of antagonist co-contraction during inspiration (25). It may also cause a temporary alteration in the “memorized” association between breathlessness and respiratory load (26). Such acute neural adaptations to the IM warm-up, independent of the IM function, are considered essential in attenuation of the breathless sensation evoked during high-intensity intermittent run (19).
For the interval training, it was performed by following a well-designed protocol that could stress primarily the lactic acid and oxygen systems that are predominant when performing high-intensity intermittent-type sports (6). To quantify training volume based on the running speed, the high-intensity intermittent runs were performed in a controlled laboratory environment rather than on the outdoor running track. In comparison with the control group, the application of the IM training and IM warm-up in the IML group augmented the repeatability of high-intensity running bouts of the subjects during the 6-week interval program. The total number of repetitions across the 5 distances performed at speeds ≥130% of the initial speeds was 26.8 ± 20.6% greater than that in the control group. The augmented tolerance of high-intensity running bouts can be attributed to an adaptive response to the specific IML rather than a result of psychological phenomena. This is based on the evidence in previous studies that psychological effect of either IM training or IM warm-up on exercise tolerance is minimal even in a single trial promptly after the interventions (16,19,21). Furthermore, the minor group differences in the training volume in the first 2 weeks in comparison with that in the later 4 weeks of the 6-week program are indicative of least psychological influence.
After the 6-week training period, the maximum number of repetitions performed in the Yo-Yo test in the control group improved by 16.9 ± 5.5%. The reduction in whole-body metabolic stress during the post-training Yo-Yo test indicated that aerobic energy utilization might have being enhanced. These findings are in accordance with the previous notion of the enhancement of running speed at anaerobic threshold after high-intensity (>90% HRmax) interval training (2). During the post-training Yo-Yo test, E, VT/ti, and E/o2 were reduced from baseline values with no change in o2. Such a response is a typical ventilatory adaptation to endurance training (14). However, although E was reduced during the post-training Yo-Yo test, the sensation of breathlessness, which has been shown to limit the tolerance of intense intermittent exercise (20,22), was not alleviated significantly.
In the IML group, the 6-week interval program, in combination with chronic and acute IML, led to a greater improvement in the maximum repetitions (30.7 ± 4.7%) during the Yo-Yo test. This was concomitant with a greater reduction in E, VT/ti and RPB/4i compared with the control group. A reduction in the VT/ti, which is commonly used to indicate the central inspiratory drive (11), has been shown to be a physiological adaptation to IM training (21). On the basis of the negative relationship between IM strength and inspiratory motor drive (8), the greater reduction in the inspiratory drive during the post-training Yo-Yo test was partly attributable to the reduction in the fraction of maximal IM tension generated with each breath. The intensity of the sensation of breathlessness is an analog of the magnitude of the central respiratory motor drive (5), the sensation of breathlessness revealed by the RPB/4i during the Yo-Yo test was 23.8 ± 8.4% lower than the pre-training value. In accordance with the previous findings (20,22), the reduction in the RPB/4i accounts for 64% of the variance in the improvement in the maximum performance of the Yo-Yo test in the IML group. Although the greater level of performance of the IML group in the Yo-Yo test is largely attributed to the greater relative attenuation in the sensation of breathlessness, the greater relative alleviation in whole-body metabolic stress might have also played a role in it. The greater reduction of metabolic stress, with no change in the total o2 during the Yo-Yo test in the IML group, suggests that aerobic energy utilization in the locomotor muscles has been further improved. This may be partly attributed to the amelioration of the vasoconstriction within the locomotor muscles, which is a consequence of the metaboreflex elicited from the respiratory muscles when the work of breathing becomes severe (3), from the 4-week IM training program (12,27). It may be also because of the further adaptation in the locomotor muscles as a consequence of an augmented volume of training completed during the 6-week interval training program. Such a scenario is supported by the reduction in the RPE/4i from the baseline value during the post-training Yo-Yo test where the change in the RPE/4i and that in the maximum repetitions performed in the Yo-Yo test were correlated in the IML group.
This study demonstrates that the application of a 4-week IM training before a 6-week interval training program plus the execution of a specific IM warm-up regimen before each workout of the program will augment the training volume of the interval program by approximately 27%. The resultant improvement in the maximum performance of the Yo-Yo intermittent recovery test was approximately 14% greater in comparison with the interval program alone (Cohen's effect size = 2.4). Based on these findings, it is recommended that chronic and acute IML is included within high-intensity interval programs for athletes who participate in intermittent-type sports. For the IM warm-up regimen post-IM training, a slight reduction in the load assignment, which currently equates to 40% of the improved maximal inspiratory pressure, may optimize the warm-up effect on IM function.
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