As shown by the following study, the answer is a definite YES.
Leith DE & Bradley M (J Appl Physiol 1976 Oct;41(4):508-16 Ventilatory muscle strength and endurance training.) studied respiratory mechanics in young volunteers before and after 5-wk training programs limited to the ventilatory muscles. Four strength trainers (S) performed repeated static maximum inspiratory and expiratory maneuvers against obstructed airways. Four endurance trainers (E) performed voluntary normocarbic hyperpnea to exhaustion. Subjects spent 30-45 min each day in these exercises, 5 days a week. Four control subjects (C) did no training. We attempted to minimize the effect of learning. S increased pressure maximums by about 55%, but vital capacity and total lung capacity by only about 4%. Initially all subjects could sustain hyperpnea at about 81% of their control 15-s maximum voluntary ventilation (MVV) for 15 min; E increased this to about 96% and increased their MVV by 14% as well. No other statistically significant changes were recognized in any group. We conclude that ventilatory muscle strength or endurance can be specifically increased by appropriate ventilatory muscle training programs.
The next question is "Does it make a difference in performance?"
Boutellier U et al (Eur J Appl Physiol Occup Physiol 1992;65(4):347-53 The respiratory system as an exercise limiting factor in normal trained subjects.) had previously looked at the effect of specific respiratory muscle training for submaximal exercise (64% peak oxygen consumption) in normal sedentary subjects. These subjects were able to increase breathing endurance by almost 300% and cycle endurance by 50% after isolated respiratory training. They then studied normal, endurance trained subjects to see if they would also benefit from respiratory training. Breathing and cycle endurance as well as maximal oxygen consumption (VO2max) and anaerobic threshold were measured in eight subjects. Subsequently, the subjects trained their respiratory muscles for 4 weeks - 30 min daily. Otherwise they continued their habitual endurance training. After respiratory training, the performance tests made at the beginning of the study were repeated. Respiratory training increased breathing endurance from 6.1 (SD 1.8) min to about 40 min. Cycle endurance at the anaerobic threshold [77 (SD 6) %VO2max] was improved from 22.8 (SD 8.3) min to 31.5 (SD 12.6) min while VO2max and the anaerobic threshold remained essentially the same. Therefore, the endurance of respiratory muscles can be improved remarkably even in trained subjects. Respiratory muscle fatigue induced hyperventilation which limited cycle performance at the anaerobic threshold. After respiratory training, minute ventilation for a given exercise intensity was reduced and cycle performance at the anaerobic threshold was prolonged. They concluded that the respiratory system was a potentially exercise limiting factor in normal, endurance trained subjects.
Boutellier U (Med Sci Sports Exerc 1998 Jul;30(7):1169-72 Respiratory muscle fitness and exercise endurance in healthy humans.) looked at the effects of four weeks of isolated respiratory training (30 min normocapnic hyperpnea, 5 d.wk-1) and demonstrated this significantly increased the endurance time of respiratory muscles and the endurance time of constant-load bicycle tests in sedentary as well as physically active subjects once respiratory muscles had recovered from the training. Minute ventilation and blood lactate concentration were reduced during post-training exercise. Furthermore, respiratory trained subjects had lost the sensation of breathlessness. Maximal oxygen consumption was not affected by respiratory training. They concluded that the mechanism by which respiratory training improves overall physical performance is as yet unknown.
This supported the supposition that respiratory training is of benefit for sedentary individuals, decreasing their sensation of breathlessness with exercise, and probably was of some additional benefit to more active individuals who exercise on a regular basis. However the definition of "endurance trained" in the first article is not clear from this abstract and the fact that their anaerobic threshhold was at 77%VO2 max suggests that these were less than the "elite" trained athletes studied in the following two articles.
Inbar O et al (Specific inspiratory muscle training in well-trained endurance athletes. Med Sci Sports Exerc 2000 Jul;32(7):1233-7) looked at the hypothesis that specific inspiratory muscle training (SIMT) would result in improvement in respiratory muscle function and thereupon in aerobic capacity in well-trained endurance athletes. METHODS: Twenty well-trained endurance athletes volunteered to the study and were randomized into two groups: 10 athletes comprised the training group and received SIMT, and 10 athletes were assigned to a control group and received sham training. Inspiratory training was performed using a threshold inspiratory muscle trainer, for 0.5 h x d(-1) six times a week for 10 wk. Subjects in the control group received sham training with the same device, but with no resistance. RESULTS: Inspiratory muscle strength (PImax) increased significantly from 142.2 +/- 24.8 to 177.2 +/- 32.9 cm H2O (P < 0.005) in the training group but remained unchanged in the control group. Inspiratory muscle endurance (PmPeak) also increased significantly, from 121.6 +/- 13.7 to 154.4 +/- 22.1 cm H2O (P < 0.005), in the training group, but not in the control group. The improvement in the inspiratory muscle performance in the training group was not associated with improvement in peak VEmax, VO2max breathing reserve (BR). or arterial O2 saturation (%SaO2), measured during or at the peak of the exercise test. CONCLUSIONS: It may be concluded that 10 wk of SIMT can increase the inspiratory muscle performance in well-trained athletes. However, this increase was NOT associated with improvement in aerobic capacity, as determined by VO2max, or in arterial O2 desaturation during maximal graded exercise challenge.
These conclusions were confirmed by another study by Fairbarn MS et al (Int J Sports Med 1991 Feb;12(1):66-70 Improved respiratory muscle endurance (RME) of highly trained cyclists and the effects on maximal exercise performance.) who set out to determine whether the RME of highly trained cyclists could be enhanced and if so, to determine the effects of improved RME on their maximal exercise performance. Ten male cyclists (maximal oxygen consumption (VO2max) greater than 60 ml/kg-1) began the study by performing 3 tests. These were VO2max, RME measured as maximal sustainable ventilatory capacity (MSVC) and maximal exercise endurance (tlim) measured by an endurance cycling test to exhaustion at 90% of their maximal power output. Five subjects then completed 4 weeks of isocapnic hyperpnea training (16 session) and 5 subjects were controls. Following this training interval, each subject repeated the initial tests. After the RME training, the MSVC increased from 155 +/- 11 to 174 +/- 12 l/min (p = 0.004) for the training subjects while there was no change in the controls (155 +/- 26 and 150 +/- 34 l/min). There were no statistically significant changes for any of the 10 subjects in either the maximal exercise performance (VO2max = 66.1 +/- 4.7 to 66.5 +/- 4.8 ml.kg-1) or the maximal exercise endurance (tlim = 335 +/- 79 to 385 +/- 158 sec). They concluded that 4 weeks of respiratory muscle endurance training increased respiratory muscle endurance but had NO effect on the maximal cycling performance of highly trained cyclists.
But the final results may not yet be in. Dr Alison McConnell (Sport & Exercise Physiology, Brunel University, UK) per personal communication on 03 Oct, 2000, writes: "As a research scientist that has been working on inspiratory muscle training (IMT) and fatigue for many years, I was pleased to see your exposition on the subject. However, I was disappointed that you were not able to provide a comprehensive review of the subject area and may therefore have gained (and communicated) a false impression of the efficacy of IMT."
"The paper that you cite by Inbar is an interesting one, because the authors actually looked at all the wrong things with respect to factors that would be likely to change following this type of training. A paper from my research group will shortly be published in Med. Sci. Sports Exerc. indicting that inspiratory muscle training improves rowing performance in hightly trained rowers by some 2% more than a placebo group. The performance model used in this study was a 'real world' test of performance consisting of a 6 minute all out rowing effort, as well as a 5000m time trial. The coach to the GB Men's eight (Olympic Gold medalists in Sydney) was so impressed by this, and other data from my group, that I was invited to prepare an IMT programme for the squad which they adhered to in the 3 months building up to the Games."
The problem with many of the early studies in this field (e.g. Fairbarn) is that there experimental design and execution was weak and the data are therefore difficult to interpret. I am convinced that ALL athletes can benefit from IMT (and I have the data to support my view). The mechanism is not linked to VO2max, but is related to lactate turn-over and possibly blood flow redistribution away from the trained diaphragm in favour of locomotor muscles (look at papers by Dempsey, Harms and Whitter on blood flow distribution in response to changes in respiratory work). If you want more information about my research please feel free to email me: firstname.lastname@example.org. You might also take a look at other research published by Christina Spengler who works with Boutellier."