(The Guardian) From club runners to Olympians, every athlete has a limit. Scientifically, this limit is defined as the body's lactate threshold and when you exercise beyond it, running rapidly becomes unpleasant. We've all experienced that burning feeling – heart pounding, lungs gasping for air – as your muscles begin to fatigue, eventually locking up altogether as your body shuts down. However, there is one man whose physiological performance defies all convention: Dean Karnazes is an ultrarunner from California and, at times, it seems as if he can run forever.
Karnazes has completed some of the toughest endurance events on the planet, from a marathon to the South Pole in temperatures of -25C to the legendary Marathon des Sables, but in his entire life he has never experienced any form of muscle burn or cramp, even during runs exceeding 100 miles. It means his only limits are in the mind.