Effects of Different Types of Respiratory Muscle Training on Exercise Performance in Runners

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1 MILITARY MEDICINE, 177, 5:559, 2012 Effects of Different Types of Respiratory Muscle Training on Exercise Performance in Runners Hiromi Uemura, MS*; Claes E.G. Lundgren, MD, PhD*; Andrew D. Ray, BS, MS, PhD*; David R. Pendergast, EdD ABSTRACT To compare two different types of respiratory muscle training on exercise performance, a protocol was devised consisting of a combination of a 4-week, 12-session resistive respiratory muscle training (RRMT) followed by a 4-week, 12-session voluntary isocapnic hyperpnea training (VIHT) and conducted in experienced runners (4 men, 4 women). Measurements before and 5 days after training included: pulmonary function (spirometry), maximal inspiratory and expiratory mouth pressures, respiratory endurance time, maximal oxygen uptake (V O2 max), running time to voluntary exhaustion at 80% V O2 max, blood lactate concentration, and minute ventilation. There were no statistically significant differences in pulmonary functions and V O2 max post-rrmt and post-viht compared to pre-rmt. Following RRMT the inspiratory muscle strength had improved by 23.8 ± 30% and 18.7 ± 21.4% at rest and immediately after the running test, respectively. RRMT did not increase the time intense voluntary isocapnic ventilation could be maintained during rest while VIHT increased it (237 ± 207.8%). The duration of the endurance run was extended 17.7 ± 6.5% after RRMT and 45.5 ± 14.3% after VIHT. INTRODUCTION Respiratory limitations to physical performance during prolonged submaximal 1 3 and maximal 4 exercise have recently been demonstrated. The work and energy cost of breathing increases from 3 5% to 13 16% of total O 2 uptake as exercise intensity increases from low to maximal intensities. 5 When the respiratory muscles increase their effort, or become more fatigued, their O 2 consumption increases as does their blood flow at the expense of reduced perfusion of locomotor muscles, 4,6 thus reducing exercise performance. 4,7 Lactate accumulation in the respiratory and locomotor muscles lead to hyperventilation and indirectly to fatigue and reduced exercise tolerance. 4 Earlier studies have shown that respiratory muscle training (RMT) improves respiratory muscle performance and prolongs exercise endurance by 24 to 86% at constant submaximal rates of 60 to 85% of maximal O 2 uptake. This has been demonstrated for running, 8 10 rowing, 11 cycling, 3,12 18 and swimming Controversy over the effects of RMT continues as some studies failed to show significant improvements in exercise performance. 10,23,24 One of these studies 24 did not allow enough recovery time after RMT and fatigue likely obscured the gains in exercise endurance. 8 Another recent article 23 failed to demonstrate a benefit of RMT during a graded maximal effort, which is well known not to reflect gains from RMT. 8 It has been suggested that influences of RMT on exercise performance are psychological. 24 However, several studies with positive results of RMT have used control and placebo groups. 8,11,14 *Center for Research & Education in Special Environments, University at Buffalo, 124 Sherman Hall, Buffalo, NY Department of Rehabilitation Sciences, University at Buffalo, 515 Kimball Towers, Buffalo, NY Methodological factors might influence the benefits of RMT. Different training methods such as voluntary isocapnic hyperpnea training (VIHT), resistive respiratory muscle training (RRMT), or inspiratory muscle training 13,25 may have different outcomes. So, for instance, is VIHT more effective than RRMT for enhancing running endurance 8 or swimming at the surface, while RRMT is superior for improving divers swimming endurance at depth The present study employed two modes of RMT in runners: RRMT which enhances respiratory muscle strength 25 followed by VIHT which improves respiratory muscle endurance. 12 The hypotheses of the present study were: (a) RRMT would improve respiratory muscle strength and running endurance in a constant-load running test; (b) VIHT following RRMT would further enhance respiratory muscle endurance and endurance running performance to exhaustion; (c) combined 4-week RRMT followed by 4-week VIHT would result in enhancement of endurance exercise capacity previously shown in our laboratory for each alone. 8,20 METHODS The study protocol was approved by the Human Subjects Institutional Review Board of the University and the subjects signed an informed consent form. SUBJECTS Eight healthy, nonsmoking experienced distance runners (4 men, 4 women) completed the study. They were 34 ± 5 years old, 179 ± 4 and 160 ± 2 cm tall, and weighed 79 ± 9.4 and 56.5 ± 3.6 kg (men and women, respectively). They were experienced in long-distance competitive running and maintained a constant level of training of 35 to 55 miles per week at a pace of 70 to 80% of maximal O 2 uptake for the duration of the study. Their fitness levels were as follows: mean maximal O 2 uptake for males 51 ± 7 ml/min/kg, range MILITARY MEDICINE, Vol. 177, May

2 42 to 60 ml/min/kg and for females 43 ± 7 ml/min/kg, range38to53ml/min/kg. PROTOCOL Before the beginning of each RMT training period and approximately 4 to 5 days after the completion of each type of training (to allow respiratory muscle recovery after the training) the following baseline tests were performed: pulmonary function, maximal inspiratory and expiratory mouth pressures (P I max and P E max), respiratory endurance time (RET), progressive maximal treadmill running test to voluntary exhaustion, and a constant-load endurance running test to voluntary exhaustion at 80% of maximal O 2 uptake. Pulmonary Function Tests Pulmonary function was tested with a computerized spirometer (Morgan Spiroflow Spirometer, Model #131, P. K. Morgan, Rainham, Gillingham, Kent, UK) in accordance with American Thoracic Society standards. Seated, the subjects breathed room air from a mouthpiece and wore a nose clip. Each test was repeated three times with the highest value used for data analysis. Maximal Inspiratory and Expiratory Mouth Pressures Measurements of P I max and P E max were measured from residual volume and total lung capacity, respectively, with a sensitive pressure transducer (Tycos, Arden, NC) while the subject wore a nose clip. They sustained each inspiratory and expiratory effort for at least 1 second. The maneuvers were performed three times with the highest values used for data analysis. Respiratory Endurance Test This was a timed, isocapnic breathing test, used to assess inspiratory and expiratory muscle endurance. It was conducted with a nose clip and a mouthpiece connected to inspiration and expiration valves and a rebreathing bag to maintain isocapnia. Bag volume was initially set at 50% of each subject s slow vital capacity (SVC). The paced breathing frequency was selected by dividing 60% of each subject s best maximal voluntary ventilation (MVV) in 15 seconds by the tidal volume used in the MVV test. The time the subject was able to maintain the target ventilation is denoted by RET. Progressive Maximal Treadmill Running Test To determine each individual s maximal O 2 uptake, subjects performed a progressive running test on a treadmill (Quinton Instruments, Model # 24-72, Seattle, WA). After a 2-minute warm up at 0% inclination the speed was increased to 6.5 mph at a grade of 0% and subsequently the grade was increased 2.5% every 2 minutes until voluntary exhaustion. Standard open-circuit gas collections and calculations were performed for the last minute of each grade increment. Heart rate, blood pressure, and breathing frequency were recorded during steady states. Blood lactate concentrations (BLCs) from finger pricks pre- and post-running were measured 6 to 7 minutes post-exercise with a lactate analyzer (Accusport, Boehringer Mannheim, Indianapolis, IN). Following the American College of Sports Medicine criteria the O 2 uptake was considered maximal if two of the following criteria were met: the respiratory exchange ratio reached more than 1.15, the O 2 uptake showed less than 150 ml/min increment, and BLC was above 8 mm. Constant-Load Endurance Running Test Each subject performed the constant-load endurance running test on a treadmill until volitional fatigue at 80% of his/her maximal O 2 uptake. Gas collection was performed 5 minutes after the beginning of the run, and every 15 minutes until exhaustion when a final 1-minute collection was made. Immediately before and after the run, P I max and P E max were recorded followed by the RET within a minute. The same measurements and equipment as in the maximal test were used. These tests are known to be highly reproducible. 3,8,13 Respiratory Muscle Training After the baseline tests, subjects began 4 weeks of RRMT. Following completion of RRMT and testing, they performed the 4-week VIHT. Both RRMT and VIHT employed the same in-house built training equipment, which consisted of a laptop computer, mouthpiece equipped with spring-loaded valves, a rebreathing bag, and a pressure sensor. The pressure sensor was connected to the laptop computer for documentation of the trainee s breathing frequency, and the data were downloaded to a floppy disc. Both RRMT and VIHT were conducted for 30 minutes a day, 3 days a week for 4 weeks, for a total of 24 sessions. One session was supervised by the investigator who also checked protocol adherence by reviewing the computer recordings weekly. Resistive Respiratory Muscle Training For RRMT the rebreathing bag was not used. The opening pressures of the unidirectional breathing valves were adjusted to approximately 60% of each individual subject s maximal inspiratory and expiratory pressures. RRMT consisted of repeated vital capacity breaths, each lasting 10 seconds while on the mouthpiece and wearing a nose clip; these loaded breaths were separated by 30-second resting periods. Each training session lasted 30 minutes. A timepiece countdown was displayed on the computer screen, followed by a beep at the end of each resting period. After each beep a new 10-second vital capacity maneuver was performed. In the resting period, the subject removed the mouthpiece and nose clip, breathed normally, and waited for the next timed cycle. In the beginning of each weekly training session, maximal inspiratory and expiratory pressure measurements 560 MILITARY MEDICINE, Vol. 177, May 2012

3 were performed under supervision and the load pressures of the valves were adjusted to 60% of the measured pressures. Voluntary Isocapnic Hyperpnea Training For VIHT the mouthpiece was connected to the rebreathing bag, valve spring pressures minimized, and a nose clip was worn. The first part of each inspiration came from the rebreathing bag, which was filled with part of the previous expiration, and then the inspiration valve opened and fresh air was inspired for the remaining volume; when the bag was filled to its capacity by next expiration the remainder of the expired gas was discharged to ambient air through the expiration valve. Thus, the subject could achieve a large ventilatory minute volume while maintaining normoxia and isocapnia. The breathing frequency was paced by auditory and visual metronome signals. It was initially set at 30 breaths/min (b/min) and the rebreathing volume at 40% of each subject s MVV divided by the breathing frequency. The subjects maintained this ventilation for 30 minutes. The breathing frequency was increased by 1 to 2 b/min for each training session. When the breathing frequency approached an uncomfortable level the bag volume was adjusted up in daily increments of 0.1 to 0.2 L and the breathing frequency was reduced so as to maintain the same ventilation as in the last session. This breathing pattern was used for the following training session, after which a daily increase in breathing frequency was again employed. Statistical Analyses All statistics were calculated using SigmaStat 3.5 (Systat Software, San Jose, California). Descriptive data were expressed as means ± SD. Statistical significance was accepted at p The data were tested for normality before statistical analysis. A one-way analysis of variance for repeated measures over the 4-week RRMT and 4-week VIHT periods was applied to individual variables in order to determine the probability of differences in all parameters. If the data were not normally distributed, nonparametric statistics were used and Friedman repeated measures analysis of variance on ranks was performed to compare differences in pre-rrmt vs. post-rrmt, pre-viht vs. post-viht, and pre-rrmt vs. post-viht parameters. The Tukey post hoc test was used for multiple comparisons among all testing periods. On the other hand, if the data were normally distributed, parametric statistics were used. The Holm Sidak test was used for identifying specific differences if the statistics were significant. because of health problems or work schedule changes that resulted in conflicts with testing and training. Resistive Respiratory Muscle Training Inspiratory and expiratory spring-load pressures for RRMT were increased weekly by about 10 cmh 2 O from 34 to 70 cmh 2 O and 53 to 87 cmh 2 O, respectively over the 4 weeks of RRMT. The best-fit regressions of each individual s data were linear with r 2 values ranging from 0.88 to 0.98). The best-fit regressions of average data were: inspiratory maximal pressure = 8.5 weeks + 69 cmh 2 O(r 2 = 0.96) and expiratory maximal pressure = 9.52 weeks + 91 cmh 2 O (r 2 = 0.98) (Fig. 1). Voluntary Isocapnic Hyperpnea Training Expiratory minute volume increased every week, starting at 72 ± 32 L/min and eventually reaching 104 ± 36 L/min. The best-fit regressions of each individual s data were linear with r 2 values ranging from 0.86 to The best-fit regression of expiratory minute volume as a function of weeks of training was: expiratory minute volume = 7.9 times weeks L/ min (r 2 = 0.98). Inspiratory and expiratory maximal pressures did not change significantly during VIHT, and were 86 ± 31 cmh 2 O to 81 ± 30 cmh 2 O for the former (p = 0.68) and 114 ± 43 to 114 ± 42 cmh 2 O for the latter over the 4 weeks of VIHT (p = 0.52) (Fig. 2). Pulmonary and Respiratory Muscle Function All baseline pulmonary function values, as assessed by forced expiratory volume (FEV) in 1 second, forced vital capacity (FVC), SVC, and MVV in 15 seconds were within normal limits. Pulmonary function data pre and post all training are summarized in Table I. There were no significant differences in FEV in 1 second and FVC after the 4-week RRMT or the VIHT ( p = 1.00 and 0.97, respectively). RESULTS Respiratory Muscle Training Eight runners completed 4 weeks of RRMT and then 4 weeks of VIHT, and testing pre- and post-training. Four other subjects (2 men and 2 women) resigned from the study due to injuries that occurred during their normal physical training or FIGURE 1. The average (±SD) values of mouth pressures (P I and P E ) observed at the start and after each week of RRMT are shown. The * indicates a significant increase above the initial value ( p 0.05). MILITARY MEDICINE, Vol. 177, May

4 FIGURE 2. The average (±SD) gas flow (L/min) rebreathed from the rebreathing bag during VIHT as recorded at the start and after each week of RRMT is shown. The * indicates a significant increase above the initial value (p 0.05). However, compared to pre-rrmt, there was a significant difference in post-viht SVC (4.91 ± 1.27 vs ± 1.40 L after; 3.43 ± 2.63%, p = 0.013) and MVV 15 ( ± 70 before vs ± 54 L/min after; 20.0 ± 22.8%, p = 0.040). Comparing pre-rrmt and post-rrmt, inspiratory and expiratory maximal pressures had increased significantly (the former pressure: 81 ± 23 vs. 94 ± 21 cmh 2 O; 19 ± 23%, p = 0.025) and the latter: ± 26.4 vs ± 19.8 cmh 2 O; 12.3± 14.4%, p = (Table II). After VIHT, there were no further improvements in those pressures (p =0.967). The duration of the RET was not significantly affected by RRMT (5 ± 2.7 minute) compared to pre-rrmt (4.9 ± 2.8 minute) (p = 0.49) (Table II), however, after VIHT it had increased by a substantial 237% to 16.5 ± 7.7 minute; p = 0.001) (Fig. 3). Progressive Maximal Treadmill Test The maximal values measured during the progressive treadmill test were not different from each other based on nonparametric statistical analysis for pre-rmt, post-rrmt, and post-viht, and the average values were: maximal O 2 uptake FIGURE 3. The average (±SD) values for RET pre-rmt, and post- RRMT and VIHT. The * indicates a significant increase above the initial value and + above the RRMT value (p 0.05). 48 ± 8 ml/kg/min (p = 0.90), maximal expired minute volume 125 ± 37 L/min (p = 0.72), maximal breathing frequency 55 ± 8/min (p = 0.76), maximal tidal volume 2.4 ± 2.4 L, p = 0.08), maximal heart rate 192 ± 12 b/min, p = 0.47) and maximal BLC 7.8 ± 2.4 mm, (p = 0.13). Constant-Load Endurance Running Test Compared to pre-rrmt, the constant-load endurance running time after the 4-week RRMT increased 17.7 ± 6.5% (p = 0.011) while it was increased 45.5 ± 14.3% after VIHT ( p = 0.011) (Fig. 4). The O 2 uptake measurements during the constant-load endurance running tests, measured at the same successive points of time were not different during the pre-training runs and averaged 40 ± 6 ml/kg/min (p = 0.78) (i.e., 83% of maximal O 2 uptake). The O 2 uptake during the endurance run was not affected by RRMT or VIHT (p = 0.86). The constant-load endurance running tests were performed at the same percentage of maximal O 2 uptake, and there were no differences in heart rate (nonparametric test) at the beginning of the run, pre-training, post-rrmt, or post-viht (mean = 173 ± 11 b/min, p = 0.92), and heart rate was not significantly affected by either RRMT or VIHT TABLE I. Pulmonary Function Variable Pre-RRMT Post-RRMT p Post-VIHT p FEV 1 (L) 4.22 ± ± ± FVC (L) 4.74 ± ± ± SVC (L) 4.91 ± ± ± MVV 15 (L/min) ± ± ± Values are means ± SD. Indicates a significant difference (p 0.05) from pre-rrmt. FEV 1 = forced expiratory volume in 1 second. FVC = forced vital capacity. SVC = slow vital capacity. MVV 15 = maximal voluntary ventilation in 15 seconds. 562 MILITARY MEDICINE, Vol. 177, May 2012

5 TABLE II. Respiratory Muscle Function at Rest Variable Pre-RRMT Post-RRMT p Post-VIHT p P I max (cmh 2 O) ± ± ± P E max (cmh 2 O) ± ± ± RET (minutes) 4.36 ± ± ± Values are means ± SD. P I max = maximal inspiratory mouth pressure. P E max = maximal expiratory mouth pressure. RET = respiratory endurance test. at any time point in the run (p ranged from 0.24 to 0.74), although RRMT and VIHT runs lasted longer (17.7% and 45.5%, respectively). Expired minute ventilation is plotted as a function of running time in Fig. 5; there were no significant differences in ventilation after 5 minutes of running. However, during pretraining runs ventilation increased with time (p = 0.036), but 20 minutes into post-rrmt and post-viht runs it was 8% lower (p = 0.02). At the end of the run, post-rrmt and post-viht ventilation levels were not different from each other (p = 0.68) in spite of the post-viht run time being 27% longer than the post-rrmt run. Tidal volumes were 2.35 ± 0.68 L, 2.27 ± 0.67 L, and 2.22 ± 0.66 L at 5, 20, and 25 minutes pre-training, respectively and they were significantly higher post-rrmt and post- VIHT (p = 0.03), but were not different from each other (2.55 ± 0.86 L, 2.44 ± 0.78 L, and 2.38 ± 0.72 L, for 5, 20, 30, and 38 minutes, respectively; p = 0.39). The breathing frequency was 42 b/min, 48 b/min, and 55 b/min; 5, 20, and minutes into the runs pre-rmt (p = 0.06) and significantly lower during post-rrmt and VIHT (p = 0.32) runs and didnotdifferfromeachother(36±5b/min,43±7b/min, and 44 ± 11 b/min for 5, 20 and 30, and 38 minutes, respectively; p =0.77). The venous blood lactate during the pre-rmt constantload endurance running tests increased from 3.63 ± 1.34 mm at 5 minutes to 6.81 ± 3.74 mm after 20 minutes and increased further to 6.47 ± 3.91 mm after minutes when the subject stopped (nonparametric test, p = <0.01). In spite of the longer running times, post-rrmt and VIHT lactate concentrations were not different from pre-training or from each other (p = 0.77). In view of the maximal lactate and longer running time, the overall rate of lactate accumulation (production consumption) was significantly lower after both RRMT (0.2 mm/min) and VIHT (0.2 mm/min) compared to pre-rmt (0.3 mm/min) (p = <0.01). As reported earlier, RRMT has been found to significantly increase both inspiratory 19 and expiratory maximal pressures 12 and, in the present study, they remained above pretraining values post-viht (Fig. 6). Inspiratory and expiratory maximal pressures were significantly decreased (i.e., 20%) after the pre-training, the post-rrmt, and post-viht endurance runs (p = 0.26). After both RRMT and VIHT the postrun inspiratory maximal pressures were significantly higher than after the pre-training runs (Fig. 6) (p = 0.04). The postexercise values for expiratory maximal pressures were also significantly higher after RRMT and VIHT, compared to pretraining (95 ± 26 cmh 2 O) values (p = 0.03). As noted earlier, resting RET was not significantly increased by RRMT (7%), but very much improved by VIHT (237%). Pre- to post-run RET decreased by 35% (p = <0.01) FIGURE 4. Running times (y-axis) vs. experimental conditions. The * indicates a significantly longer time than before RMT and + a longer time than after RRMT (p = 0.01). FIGURE 5. The average (±SD) expired ventilation values for the endurance run for pre-rmt, post-rrmt, and VIHT. The * indicates a significant increase above the initial value and + above the RRMT value (p 0.05). MILITARY MEDICINE, Vol. 177, May

6 In agreement with previous studies, 8,12,26 VIHT in the present study did not improve the inspiratory and expiratory maximal pressures. However, VIHT following RRMT was not connected with any loss of the improved pressures that resulted from the preceding RRMT. FIGURE 6. Maximal inspiratory (P I max) and expiratory (P E max) mouth pressures are plotted as a function of endurance running time. Pre-RRMT data for P I max ( )andp E max (o), post-rrmt for P I max ( )andp E max (D), and post-viht for P I max ( )andp E max ( ). The * indicates significantly lower than pre-exercise and the + significantly greater than pre-rmt and pre-viht (p 0.05). before-rmt, but after RRMT only by 18% and after VIHT by 70%; however, the absolute values were significantly higher for VIHT (5.1 ± 2.1 minutes, p = <0.01) and RRMT (4.0 ± 2.9 minutes, p = <0.01) than pre-training (3.2 ± 2.2 minutes). DISCUSSION The present study demonstrates that both strength (RRMT) and endurance (VIHT) training of respiratory muscles enhance submaximal running endurance (18 and 48%, respectively). However, preceding VIHT with RRMT did not improve the overall increase in run times previously reported for VIHT only (50%, 8), probably because the modest effect of RRMT on exercise at 1.0 atm was masked by the pronounced effect of VIHT. This is interesting in light of earlier studies that have shown RRMT to be superior to VIHT for enhancing swimming endurance at depth where the diver has to overcome higher flow-resistive forces in lungs and breathing gear than a runner or swimmer is faced with at the surface. 20 By contrast, VIHT is considerably more effective than RRMT for increasing exercise endurance at the surface. Apparently, the greater respiratory muscle strength reflected by maximal static inspiratory and expiratory pressures after RRMT than after VIHT (Fig. 6) was less protective against respiratory fatigue than the conditioning conferred by VIHT. Respiratory Muscle Performance Maximal Mouth Pressures Following RRMT and VIHT In present study, inspiratory and expiratory maximal pressures were increased by 23.8% (p = 0.05) and 13.5% (p > 0.05), respectively, after RRMT. These improvements were similar to those of previous reports that used similar valve opening pressures Respiratory Muscle Endurance Following RRMT, RET was not significantly improved (only 6.8%), which is less than previously shown for RRMT (30.7%; 19, 20). Previous studies that used inspiratory muscle training have also shown improved respiratory endurance (27 to 128%). 10,13,27 VIHT in the present study improved RET (242%), which is in agreement with earlier studies that used similar training and measurements. 8,12,26 A notable exception is a study that combined inspiratory strength and endurance RMT 24 and found no improvement in exercise endurance or respiratory endurance and only an 8% improvement in inspiratory maximal pressure. That study may inadvertently have done performance measurements while the subjects suffered from respiratory muscle fatigue because the protocol did not allow subjects to recover from their intense training but tested them immediately post-rmt. 8 Studies which, like the present one, tested subjects 3 to 5 days post-rmt have shown significant improvements in exercise endurance. 8,12,26 Maximal Exercise Performance The influence of RRMT and VIHT 3,8,12,18 20,23,27 on maximal O 2 uptake, heart rate, stroke volume, and cardiac output in previous studies found no changes in maximal or the submaximal values and the present findings are consistent with them. Endurance Exercise Performance Although open-ended submaximal exercise tests as used in the present study have been criticized, 24,28,29 previous studies of running 8 and cycling 3,13 documented such tests to be highly reproducible when repeated. By design, no control group was used in this study, which calls for a discussion of potential errors. Previous studies 3,18 have shown that control groups do not improve respiratory muscle performance or exercise performance. In addition, studies in our laboratory that evaluated the effects of the same VIHT with a placebo group 8 or RRMT with a control group failed to demonstrate improved respiratory muscle performance or exercise performance in the placebo and controls groups, while the improvements in the VIHT and RRMT groups were substantial. Finally, the magnitude of the improvements and resultant statistical differences would argue that the improvements were valid, especially as they are in agreement with many other studies. 3,8 22 The improvements in exercise endurance in the present study agree with many previous studies. 3,8,12,13,18,26 By contrast, a previous study in competitive runners using only inspiratory muscle training protocol, 11 while demonstrating 564 MILITARY MEDICINE, Vol. 177, May 2012

7 significant improvements in both strength and endurance of respiratory muscles, failed to show improved running endurance apparently because they used too high an exercise intensity. It should be pointed out that overall, the literature supports the notion that the effects of RMT on exercise endurance decrease as the intensity of exercise increases, i.e., at <85% maximal O 2 uptake there is a 33 to 86% improvement in endurance, whereas at 85 to 95% (time trial performance) there is only a 5% improvement 8,14,16,24 and none in maximal exercise performance 8,12,19,21 23 where the limit is set by circulatory factors before respiratory muscle fatigue becomes critical, a view first expressed by Boutellier et al. 12 Changes in Breathing Pattern and Ventilation Although expired minute volume, tidal volume, and breathing frequency were similar in the early stages of the endurance running tests, ventilation and breathing frequency during the pre-rmt runs increased, and tidal volume decreased toward the end of the run. Such changes have been associated with respiratory compensation for metabolic acidosis. 18 Following RRMT and VIHT in the present study ventilation was reduced and did not demonstrate this respiratory compensation. In addition, tidal volume was sustained throughout the run and breathing frequency did not increase. This is in agreement with previous reports for running 8,12,13 and swimming In spite of the reduced ventilation shown in this and in a previous study, arterial O 2 saturation was earlier reported to be sustained. 8 These studies suggest that RRMT and VIHT attenuated the respiratory compensation for metabolic acidosis (lower expired minute ventilation) and prolonged exercise before the respiratory muscles fatiguing. Several investigators have demonstrated a reduction in BLC during submaximal exercise following RRMT as well as VIHT, 8,19,26 and the present findings support these observations, in spite of the runs lasting 17.7 and 45.5%, respectively, longer. The lower lactate would reduce the need for respiratory compensation (hyperventilation) for metabolic acidosis and eventually lower the energy cost of breathing and consequently the respiratory muscle blood flow, which may allow improved perfusion and O2 supply to locomotor muscles resulting in longer running times. 2,8 Reduced respiratory muscle fatigue can, in reason, be inferred from the values of inspiratory and expiratory maximal pressures and RET measured immediately after exhausting runs following RRMT and VIHT, which were higher than after runs performed before RMT. The notion of a reduction in respiratory muscle fatigue is also supported by the observation in this and previous studies 8,19 22 that V T was sustained throughout the endurance run after RRMT and VIHT, but not during the pre-training run. CONCLUSION The present results demonstrated that a 12-session, 4-week RRMT improved respiratory muscle strength and endurance as well as the ability to sustain running at 80% of maximal O 2 uptake before exhaustion. A 4-week VIHT, following RRMT, preserved the gains in respiratory muscle strength, and further improved both respiratory endurance and endurance running time. Combining 4-week RRMT followed by 4-week VIHT did not bring additional enhancement in respiratory muscle endurance or endurance exercise performance compared to 4 weeks of VIHT alone, using similar methods. 8 The present study demonstrated a reduction in lung ventilation, and eliminated hyperventilation while maintaining tidal volume toward the end of the endurance run and, when coupled with increased maximal mouth pressures and respiratory endurance, suggests reduced respiratory muscle fatigue. It may be of practical interest for devising an RMT program for troops engaged in both underwater swimming and terrestrial exertion such as running that RRMT has earlier been found particularly beneficial for the former activity and this effect would not be lost if VIHT is subsequently used to enhance the latter. ACKNOWLEDGMENTS The authors thank the subjects who cheerfully dealt with the demands put on them by this study. We also gratefully recognize the technical assistance of A. Barth, C. Senf, E. Stimson, M. Fletcher, and N. Niedermayer. The study was supported, in part, by a grant from the Naval Sea Systems Command via the Naval Experimental Diving Unit (#N C-0014). REFERENCES 1. Babcock MA, Pegelow DF, Harms CA, Dempsey JA: Effects of respiratory muscle unloading on exercise-induced diaphragm fatigue. J Appl Physiol 2002; 93: Dempsey JA, Harms CA, Ainsworth DM: Respiratory muscle perfusion and energetic during exercise. 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