Dean Karnazes

Dean Karnazes


Most runners have to stop when they reach their lactate threshold, but Dean Karnazes' muscles never tire. He can run for three days and nights without stopping.


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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.

When running, you break down glucose for energy, producing lactate as a byproduct and an additional source of fuel that can also be converted back into energy. However, when you exceed your lactate threshold, your body is no longer able to convert the lactate as rapidly as it is being produced, leading to a buildup of acidity in the muscles. It is your body’s way of telling you when to stop but Karnazes never receives such signals.

He says: “To be honest, what eventually happens is that I get sleepy. I’ve run through three nights without sleep and the third night of sleepless running was a bit psychotic. I actually experienced bouts of ‘sleep running’, where I was falling asleep while in motion, and I just willed myself to keep going.”


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