Once you step inside the AirFit studio at the Quad in the San Francisco Bay Area, you're suddenly transported to somewhere like Denver, Half Dome or Lake Tahoe -- the altitude many pro-athletes successfully train at. A complex air filtration system sucks some of the oxygen out of the room, so you breathe less of it.

While your heart rate goes up, the oxygen in the room goes down. As the air gets thinner, the body adapts to lower levels of oxygen by increasing red blood cells and hemoglobin, which changes muscle metabolism.