Master Sgt. Robert R. Snyder Jr., Air Forces Central force protection liaison officer, wears an elevation training mask to improve his cardio and ability to endure higher altitudes. (U.S. Air Force photo by Tech. Sgt. Raymond Hoy)
<a rel="nofollow" class="external text" href="https://www.flickr.com/people/39513508@N06">US Air Force</a> from USA
<a rel="nofollow" class="external text" href="https://www.flickr.com/photos/usairforce/8694213482/">Ultra-marathon</a>