Doctor's oxygen level graphs (Images Courtesy of Skeptics Stack Exchange)
Doctor's Opinions
For eight years, running a mile in under four minutes appeared impossible to most doctors creating a mental barrier. As seen in the graph to the left it was believed that running a mile in this time caused oxygen deprivation resulting in overheating and fatigue. Very few doctors thought this was possible. This was believed until about a year before the mile was ran, when people came within seconds of a mile in under four minutes.
There are several times in history where doctors claimed that things that were possible weren't, on example being climbing Everest without oxegen.
“Doctors and scientists said that breaking the four-minute mile was impossible. Thus, when I got up from the track after collapsing at the finish line, I figured I was dead,"
-Roger Bannister