Exercise is hard. It’s even harder if your muscles can't get the oxygen they need. This physiologic challenge is known as hypoxia. When muscles are hypoxic, they often can’t work as hard. Atop a mountain in Colorado, atmospheric pressure is much less than it is at sea level. Even though the percent of oxygen molecules found in the air is the same, there is less driving force for that oxygen to get into the body.

If an individual has a physical condition, such as congestive heart failure or COPD, which impedes the body’s ability to capture and transport oxygen to exercising muscle, similar hypoxia could occur. Sports scientists and doctors can simulate these conditions in healthy participants by restricting the amount of oxygen they breathe through special masks or low-oxygen chambers.

New research shows that these two causes of hypoxia have different effects on exercise performance. Researchers combined 21 previously published studies which evaluated exercise capacity under the simulated normobaric hypoxia (NH) and high-altitude hypobaric hypoxia (HH) in athletic men. These studies included two different exercise methods: running and cycling.

As expected, participants’ maximal exercise capacity was impaired under both NH and HH conditions. However those in HH performed slightly better than those in NH — perhaps because of lower air resistance making it easier to breathe. And between those who ran or cycled, those who ran had lower blood-oxygen levels and lower heart rates. There was no difference between running and cycling seen in NH. 

These insights may allow athletes and doctors to better predict how they or their patients will respond to exercise in low oxygen environments.