 The Borg scale for perceived exertion has been widely used in sports and science to measure physical strain. It has been shown to be reliable at moderate and high altitudes, but not at extremely high altitudes. This suggests that there must be additional factors besides physical strain, that contribute to the feeling of being overexerted at extreme altitudes. Further research is needed to determine what these factors are. This article was authored by Thomas Cupper, N. Houston, Audrey Morrison, and others.