If the 200m can be painful to run, the 400m is roundly considered the hardest track event -- it has even been dubbed the "man killer."

Humans are simply not made to sustain near-maximum speeds for more than 30 or 35 seconds. At that point, runners really begin to feel the effects of oxygen debt -- the muscles literally running out of their oxygen fuel -- and the painful accumulation of lactic acid in their muscles. The fast-twitch muscles that are the bread and butter of the sprinter here becomes the source of torment -- they rapidly burn through oxygen and energy and leave the hamstrings on fire.

Like 200m sprinters, 400m runners need to be very strong and skilled at taking turns, but they also need to have an abnormally (some might say dangerously) high pain threshold. They train to get accustomed to the kinds of pain they'll endure in 400m races, so they can keep their strides relaxed and smooth and maintain their technique even while they feel like they're going through re-entry.

Physical strength is therefore only part of it; 400m runners at elite levels require real mental toughness. It takes courage to push themselves beyond sensible limits and discipline to run at a pace that will allow them to keep up with the race without burning out early.

Aside from the distance covered, the biggest difference between the 400m and its 200m cousin is at the start. You'll notice the starting points in the 400m have about twice the stagger of the 200m because it has an extra turn.