Respiratory training: why your breathing muscles matter for endurance

Inspiring thoughts: will training your breathing muscles boost your performance? Is it OK to give you one more thing to worry about in connection with your training? You are already working diligently to optimise the capacities of your cardiovascular and neuromuscular systems as well as your sport-specific skills. Isn’t that enough to promote peak performances?

In addition, the lungs do not respond to training in any meaningful way: unlike the muscles, they don’t get bigger or quicker no matter how strenuously athletes work. Doesn’t that mean that we can just forget about those airy structures as we plan our training programmes? Well, yes – and no.

In fact, we can forget about the lungs themselves as we plan our weekly schedules, but it may not be wise to ignore the key muscles outside the lungs which cause air to swirl in and out of our pulmonary structures. The diaphragm, a dome-shaped muscle located just below the lungs and heart, which separates the thoracic and abdominal cavities, and the intercostal muscles, which run between the ribs, are the so-called respiratory muscles – sinews which create pressure gradients within the thoracic cavity and thus allow air to move and blood to be oxygenated.

When the diaphragm contracts, for example, it moves downwards, in effect expanding the size of the thoracic cavity; this lowers pressure within the thorax and air cascades into the lungs; as the diaphragm relaxes it springs back up, diminishing the size of the thoracic cavity, increasing thoracic pressure and causing air to flow up and out through your trachea and mouth. Like any muscles in the body, the diaphragm and intercostals can become tired, and when this happens there is a drop-off in breathing capacity, exercise will feel harder, and performance may suffer.

In one scientific investigation, nine athletes tired out their respiratory muscles – but not the rest of their bodies – by engaging in 150 minutes of sustained maximal ventilation while remaining seated. The physiologists carrying out this research were certain that a fatigued state had been attained, because over the 150-minute period the amount of air breathed in per minute and the rate of oxygen uptake associated with the heavy breathing both declined.

Interestingly enough, the 150 minutes of heavy breathing had no significant effect on vital capacity (the total volume of air that can be voluntarily moved in one breath, from full inspiration to maximum expiration, or vice versa), maximum voluntary ventilation (the maximal amount of air that can be breathed in over a one-minute period) or forced expiratory volume (the maximal amount of air which can be forced out of the lungs in one second). But in a high-intensity running test, which followed the deep breathing, the athletes were able to run at high speed for only 6.5 minutes, compared with their usual 7.6 minutes.

Effects of respiratory muscle fatigue

Indeed, after inducing respiratory-muscle fatigue the athletes stopped running at lower ventilation rates, lower heart rates and lower oxygen uptakes than before. And the researchers concluded, quite logically, that reduced respiratory muscle endurance (or increased respiratory muscle fatigue) could hamper athletic performance during relatively high-intensity efforts.

In follow-up work, carried out at the State University of New York in Buffalo, 10 individuals fatigued their respiratory muscles by breathing at high rates against inspiratory resistance and then attempted to cycle for as long as possible at 90% of maximal capacity. Following the induction of respiratory muscle fatigue, the subjects were able to exercise for only 238 seconds, compared with 311 seconds in the non-fatigued state.

In addition, as you might expect, breathing – and thus the overall exercise test – felt much harder when the respiratory muscles were tired. OK, then – let’s suppose you do nothing to tire out your diaphragm and intercostals during the 24 hours before a major physical challenge. Would you still need to train these muscles in special ways? Certainly, you have reason to be attracted to an ‘escape clause’ at this point, especially since I have not shown that respiratory muscles actually do get tired during exercise, only that if you do something really foolish, like put your respiratory muscles under enormous strain just before exercise, your subsequent physical capacity will diminish.

The diaphragm recovers slowly even after brief exertion

True, we are talking about the marathon, and most athletes do not put their respiratory systems under stress for over three hours without a break. However, when 12 fit individuals (average VO2max = 61 ml/kg-min) exercised to exhaustion at 95% VO2max (a process which took just 14 minutes) and also at 85% VO2max (which took 31 minutes), their diaphragms were significantly fatigued post-exercise and took 70 minutes to recover. In another study, when subjects exercised at high intensities for just 8-10 minutes, diaphragm strength was reduced by 15-30%. The organisers of this latter study believed that the diaphragmatic fall-off was due partly to good old-fashioned muscle fatigue but also to blood-flow redistribution, with the muscles involved in locomotion effectively ‘robbing’ the diaphragm of blood, and thus oxygen. This research also showed that it took the diaphragm an hour or more to recover from significant fatigue, even with an exercise duration of 10 minutes or less. So the picture seems clear: the respiratory muscles can become fatigued during strenuous or prolonged exercise, and significant respiratory muscle fatigue has the potential to impair performance. What’s an athlete to do? The obvious answer is to train the respiratory muscles. Most athletes are a little sceptical about this concept: after all, don’t you give your diaphragm and intercostals a challenging workout every time you train strenuously? All the huffing and puffing you do would certainly attest to the fact that your respiratory muscles are working overtime.

In fact, you do train your respiratory muscles in the course of your regular training, but it’s clear that something beyond normal training might be needed to spur the respiratory muscles on to new heights of fatigue resistance. After all, as mentioned, even well-trained athletes experience significant respiratory muscle fatigue after as little as 10 minutes of hard work. Clearly, their normal training was not enough to keep their respiratory muscles humming in an optimal way during a challenging exertion. About 10 years ago, eight well-trained athletes tried something well beyond the norm: in an investigation carried out at the University of Zurich in Switzerland, the athletes trained their respiratory muscles alone for four weeks by breathing at rates of 85-160 litres per minute for 30 minutes every day. In case you were wondering, the normal human ventilation rate at rest is about 12 litres per minute, and maximum voluntary ventilation (MVV) among seasoned athletes is frequently 180-190 litres per minute, indicating that the athletes were breathing at up to 89% of MVV during their special respiratory training. Perhaps surprisingly to you, athletes do not call upon MVV to reach VO2max, which is often attained at about 135 litres of air per minute; thus, the Swiss athletes were often well above the breathing patterns associated with very intense exercise. (Incidentally, some sceptics have argued that since VO2max is reached at 135 litres of inspired air per minute but maximal ventilation is around 180 litres per minute, this constitutes evidence that the respiratory system does not limit high-level performance; after all, it is stressed to only 75% of maximum when an athlete hits his/her maximal rate of oxygen consumption. That’s decent thinking, but what we are concerned about here is not the overall capacity of the respiratory system but the fact that the respiratory muscles can become fatigued during exercise and thus might begin to limit performance potential.)

Apart from the heavy breathing they carried out in their special sessions, the Swiss subjects trained in their usual way over the four-week experimental period. After four weeks, VO2max and anaerobic threshold were unchanged, but the athletes improved their endurance times at their anaerobic thresholds from 22.8 to 31.5 minutes – a very nice 38% upgrade. An interesting finding in this study was that after the four weeks of breathing training, minute ventilation (the amount of air breathed in per minute) at a given intensity of exercise was lower than it had been before the training. This would indicate that a specific intensity of exercise was easier for the respiratory system to support following respiratory training; this greater ease may have allowed athletes to perceive their efforts at anaerobic threshold as being more comfortable and thus more sustainable.

More blood for leg muscles

In a more detailed, follow-up study from the same laboratory, 20 active individuals trained their respiratory muscles (using the kind of high-MVV work described above) for 30 minutes per day, five days per week for four weeks. Again, the work was beneficial, boosting cycling endurance time by 27%, from 20.9 to 26.6 minutes. Interestingly enough, blood-lactate levels were lower in the respiratory-trained athletes after both high-intensity and endurance-type exercise, an effect which the researchers attributed to improved lactate uptake by the trained respiratory muscles. This is certainly plausible, but there is another intriguing possibility: the well-trained respiratory muscles might have allowed the leg muscles to have more blood! If that idea seems strange, bear in mind that during high-intensity exercise the respiratory muscles demand a significant amount of ‘cardiac output’ – the blood sent out to the body by the heart. With only so much blood to go around, if the respiratory muscles take more, less is available for the leg muscles. However, if the respiratory muscles become stronger and more efficient as a result of respiratory muscle training, they will need less energy, oxygen and blood to support a specific exercise intensity. This will, in effect, ‘free up’ blood and oxygen for the leg muscles, an effect which could account for the 27% increase in endurance observed in the Swiss follow-up, as well as the reduced overall blood-lactate levels; given a greater infusion of blood, the leg muscles could ‘gobble up’ more lactate as it passed by.

If you are still not convinced, consider the double-blind, placebo-controlled study recently completed at Brunel and Birmingham Universities, in which eight competitive cyclists carried out inspiratory muscle training (IMT) over a six-week period, while eight controls completed a ‘sham’ version of this training. The subjects completed 20k and 40k time trials before and after the six weeks of respiratory training. The IMT consisted of 30 rapid inspiratory efforts, completed twice a day for six weeks, against a pressure threshold load equal to about 50% of maximal inspiratory mouth pressure. The ‘sham’ version involved 60 slow protracted breaths completed once daily, using a resistance of just 15% of maximal inspiratory mouth pressure – a form of training that has been shown to produce negligible changes in respiratory muscle function. An inspiratory muscle-training device called POWERbreathe was used to maintain the proper pressure thresholds, and the pressure loads were adjusted in the IMT group so that no more than 30 inspiratory efforts could be completed successfully at one time. To ensure compliance with the training protocols, the number of inspiratory efforts completed by subjects during the six-week period was monitored using a thermistor suspended within the main body of the POWERbreathe device, which sensed sudden drops in air temperature associated with changes in air flow.

Over the six-week period, the IMT group made significant improvements in pulmonary and respiratory muscle function, as evidenced by a 28% gain in maximal inspiratory mouth pressure and a 22% gain in the amount of air which could be breathed into the lungs in 30 seconds. Needless to say, the sham-trained cyclists failed to improve. After six weeks, the IMT cyclists were also 65 seconds faster than the controls in the 20k time trial and 114 seconds quicker in the 40k effort, although their times had been equal before the study began. Immediately after the pre-study time trials, both groups of cyclists exhibited sizeable (and equivalent) reductions in respiratory muscle function, an indication that significant respiratory muscle fatigue had occurred during the trials. However, after six weeks the IMT group experienced much smaller drop-offs in inspiratory muscle function after the trials than the controls, while their respiratory functioning returned to normal much more quickly. The bottom line of this research is that respiratory muscle training looks like an attractive option for endurance athletes.

If you are an endurance athlete and are interested in carrying out IMT as part of your own training, it is relatively easy to purchase a respiratory muscle training device. You may want to take a close look at the PowerLung, for example, a device which can be purchased online (at around £110) at www.peakcentre.ca/powerlung.htm. An advantage of this device is that it features threshold resistance cells with independent adjustment controls; once a certain number of reps can be completed without trouble at a given level of resistance, the resistance can be increased quite easily.

You might also want to consider the Sports Breather, a device said to be used by more than 37,000 athletes, emphysema patients, asthmatics and people who suffer from panic attacks. Finally, it would make sense to examine the POWERbreathe, the device used in the English study mentioned above, which is described vividly online at www.powerbreathe.com. You may also contact POWERbreathe’s distributor, Leisure Systems International Ltd, at +44 (0) 1926 816177.

Owen Anderson