By the time they reached Base Camp, at just over 16,000 feet, the members of a British military expedition found that their arterial oxygen saturation was 20 percent lower than it had been at sea level — except for the members who had been assigned inspiratory muscle training for four weeks leading up to the expedition:
When the IMT group got to Base Camp, they had desaturated by only 14 percent, a significant six-percent advantage over the control group that persisted as they kept climbing to the advanced base camp at over 18,000 feet.
Humans have between seven and 11 pounds of respiratory muscle, primarily the diaphragm and intercostal muscles around the ribcage, which consumes energy and fatigues just like the hamstrings or biceps. The idea of training this muscle—and particularly the muscles required for inhaling—originated with patients suffering from breathing-related conditions like chronic obstructive pulmonary disease.
The basic IMT protocol those patients followed hasn’t changed: you take a deep breath through a tube with variable resistance that makes it harder to inhale. Repeat 30 times a day, ramping up the resistance as your muscles get stronger.
For over a decade, researchers have been studying whether IMT can boost endurance performance at sea level. The evidence remains mixed, but a meta-analysis of 21 studies in 2013 concluded that it probably offers a small boost, particularly in breathing-constrained sports like swimming. At altitude, though, the situation is different: breathing takes a significantly higher proportion of your overall energy, consuming 20 to 30 percent more oxygen by 9,000 feet, so the breathing muscles fatigue more quickly.
The idea that IMT might be useful at altitude was first tested in a 2007 Kansas State study that found improvements in exercise at a simulated elevation of around 10,000 feet. After four weeks of IMT, blood oxygen levels during exercise were higher, and the strengthened respiratory muscles were able to handle the demands of breathing in thin air more easily, reducing their total oxygen usage. Ratings of effort and breathing discomfort were also reduced. Curiously, actual performance in a time-to-exhaustion trial was unchanged. Read: the subjects didn’t have better endurance, they just felt better.
More recently, Lomax, the author of the Makalu study, has followed up with a lab study of her own, also at a simulated altitude of around 10,000 feet. While the results haven’t been published yet, she found that four weeks of IMT produced higher arterial oxygen levels, reduced overall oxygen demand, increased breathing efficiency, and reduced breathing discomfort during exercise. As in the Kansas State study, the benefits were apparent only during exercise at altitude, not at sea level.
The training tool of choice is an incentive spirometer, like the PowerBreathe.