The conclusion of a four-part series on physiological extremes encountered by Olympic athletes.
Retired cross-country ski legend Bjorn Daehlie's record tally of 12 Winter Olympic medals was matched last week by fellow Norwegian Ole Einar Bjoerndalen, who will attempt to dethrone Daehlie in the biathlon relays later this week in Sochi.
There was much less fanfare two years ago, when Daehlie lost a more obscure title. Until then, he had also been hailed as the greatest oxygen-user of all time – a laboratory marvel whose off-the-charts ability to suck oxygen into his lungs, transfer it into his bloodstream, and deliver it to hard-working muscles played no small part in fuelling his repeated trips to the podium.
Oxygen is the universal currency of athletic effort. Your muscles need it for the chemical reaction that converts food energy into motion, and those who use it best rise to the top. But for elite athletes, getting more isn't as simple as taking a deeper breath. Scientists are still trying to figure out exactly what separates oxygen super-users like Daehlie from the rest of us, and athletes are climbing mountains – literally, in some cases – in search of any oxygen boost they can get.
Tests in the 1990s indicated that Daehlie had a maximal oxygen uptake, or VO2max, of 96 millilitres of oxygen per kilogram of body mass per minute. (An 18-year-old cyclist named Oskar Svendsen, also from Norway, eclipsed that mark with a reading of 97.5 in 2012.) That reflects the rate at which oxygen enters his lungs, diffuses into the bloodstream, gets pumped throughout the body by the heart, and then extracted from the blood by the muscle cells that need it.
Identifying where the bottleneck occurs in this process is one of the most contentious topics in exercise physiology, with various lines of evidence pointing to the lungs, the heart, the muscles and even the brain. One possibility is that oxygen can't diffuse from the lungs into the bloodstream fast enough during intense exercise, causing oxygen levels in the blood to drop. Indeed, a characteristic "desaturation" pattern is often seen in trained endurance athletes when they exercise.
"These are the most fit people in the world, but their blood gas looks like someone who might present at an ICU," says Dr. Bill Sheel, a physiologist at the University of British Columbia. Sheel and his colleagues recently travelled to the highlands of Kenya to test the lung function of some of that country's famous distance runners. These runners often live at elevations of 2,000 metres or higher, giving rise to speculation that their lungs might have adapted to the thin air over many generations to deliver oxygen more efficiently, giving them an advantage when they race at sea level.
The results, which will be published in Medicine & Science in Sports & Exercise in April, show that the Kenyan test subjects were just as likely to exhibit oxygen desaturation during exercise as runners from other countries, and their respiratory system had to work just as hard. In other words, whatever makes the Kenyans such great runners, it isn't because they have better lungs.
That doesn't mean it's impossible to improve your lungs. Breathing takes effort, since you have three to five kilograms of respiratory muscle that can fatigue like any other muscle. "If you can make that process more efficient, you can divert more blood and oxygen away from the respiratory muscles to the working muscles," Sheel says.
Sheel and others have studied the effects of "respiratory muscle training" – training the breathing muscles by inhaling repeatedly through a special tube that adds resistance to the flow of air. While the evidence remains mixed, a meta-analysis of 21 studies by Sheel and his colleagues last year found that this form of training does seem to improve performance in endurance sports like running, swimming and cycling.
Another potential oxygen bottleneck is the blood itself. In this case, there's no doubt that living at altitude confers an advantage. If you travel from sea level to high elevations, your body responds by increasing the numbers of red blood cells available to ferry oxygen from the lungs to the muscles. The effect lasts for several weeks after you return to lower elevations, which is why Canadian cross-country skiers headed to Seiser Alm in Italy and biathletes headed to Seefeld in Austria, high in the Alps, to get a final oxygen transport boost in the weeks leading up to Sochi.
Still, not all scientists are convinced that VO2max represents a purely physical limit. After all, the record for breath-holding belongs to French free diver Stéphane Mifsud, who stayed underwater unaided for 11 minutes and 35 seconds in 2009 – a mind-boggling achievement that he attributes in part to learning to ignore the overwhelming distress signals that force most of us to gasp long before we're actually out of oxygen.
A study of Swedish cross-country skiers published last month in the International Journal of Sports Physiology and Performance used an alternate testing protocol to measure VO2max, in which subjects chose their own pace instead of having it set for them. Engaging their brains in this way produced higher VO2max results than the standard test – a puzzling result that suggests VO2max isn't necessarily a concrete upper limit.
This echoes the results of a 2012 study from the University of Cape Town that used a "reverse" testing protocol – starting fast and getting slower instead of starting slow and accelerating – to produce higher-than-max levels of oxygen use. Scientists still aren't sure how this is possible, but they're eager to put the findings to use. Dr. Fernando Beltrami, lead author of the 2012 study, is planning a follow-up study using his reverse-testing protocol as a method of training runners to use more oxygen, and he's already testing the technique with an athlete who is preparing for a 100-kilometre race in Patagonia.
All of this points to a revision of how scientists think about how we use oxygen, and what our ultimate limits are. "People are slowly moving away from the old dogmas," Beltrami says, "or at least opening themselves to the possibility that there might be situations where the theories do not apply."
None of this is lost on Svendsen, the young cyclist who now holds the distinction of having the highest recorded VO2max. "The figures are not important," he told reporters after news of his test values spread. "I've been beaten by many with lower oxygen uptake than me over the years." Still, Svendsen did win the world junior time-trial championships a few weeks after his VO2max test. Being an oxygen super-user certainly doesn't guarantee that he'll achieve as much as Daehlie did – but it's a good start.
Alex Hutchinson blogs about exercise research at sweatscience.runnersworld.com.