Elevation training masks have experienced a boom in popularity among athletes searching for the benefits of training at high elevations without traveling to the mountains. The masks limit the flow of air to the nose and mouth in an attempt to simulate the effect of training at higher altitudes. Training Mask, LLC, manufacturer of Training Mask 2.0, claims its mask offers a number of benefits, including increasing V02 max—a key biological marker for aerobic exercise capacity and endurance performance. But does it? American Council on Exercise (ACE) commissioned an independent study by researchers at the University of Wisconsin-La Crosse to find out.