Amby Burfoot won the 1968 Boston Marathon and nothing else of note. Only 500 ran that year, it was an Olympic year, so few fast runners did it and the weather was terrible even by Boston standards, so few ran aggressively. Amby, being from Boston, used his familiarity with the course to pull out the upset win. Years later, when muscle fiber biopsies on runners were first done, Amby was found to have more than 98% slow twitch fibers, which was the highest ever recorded. He wasn't surprised - he couldn't do most sports; for example, in tennis, he could never get to the ball.
You might be an ultrarunner if you fail at every other sport.
I was found to have 48% slow twitch, typical for those who race 1-3 miles, which happens to be the range at which I do run best. No one needs to have the test to find what distance is best, however. Everyone tends to find it out by trial and error and one usually runs best the distances one enjoys racing the most. If you get a list of world records and divided them by your own times, you'll see that there's almost always one distance or a small range of distances at which one does well. People almost never start running by running ultradistances, so it's hard to know if one will be good at it without trying, but if they find that the longer the distance, the better they do, trying longer distances makes sense.
If you watch a 10K, you'll see many runners run the last mile faster than all the others, as they switch to anaerobic running. Anaerobic running requires burning sugars, whereas ultrarunning burns fat and few are expert at both. If you're getting passed at the end of races from 5K to 1/2 marathon and think "I'd beat them if only the race were longer!" you might be an ultrarunner.
The faster one runs, the more sugar one burns and the less fat. Ultrarunners are adept at burning a higher amount of fat at faster rates than others, something which can be trained to some extent, but which appears to be partly genetic as well. The "wall" is a phenomenon that ultrarunners rarely see - when one's muscle glycogen is gone, one involuntarily slows; this point can be extended to as much as 5 or 6 hours. If you can run 7 hours or more without an energy crash, you might be an ultrarunner.
And you'll probably be passing me.
This covers flat surfaces; trails are next. Some ultrarunners (Kouros, Horn), excel on flat, but not trail. There's other qualities trail runners need.
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