I really want to run Voyageur on Saturday [good timing, Steve. That's the day it's being held!], because the weather's going to be almost ideal - given July's temps this year - but there's just no way I could do it. Though neither have signed up yet, a Chris Gardner/Chris Lundstrom duel could set up a new course record.
In some ways, your body decides what races you should do for you. One example is differing muscle fiber types. You've probably heard of "fast-twitch" and "slow-twitch," but there are three types, each of which are fueled differently. Slow twitch fibers can use sugars and fatty acids aerobically for fuel and they're what you use for endurance sports. Different muscles have different percentages of slow twitch fibers, so what's usually measured is the gastrocnemius, which you use a lot in running and which is very variable (the soleus, underneath the gastrocnemius is almost entirely slow twitch; the triceps brachiae in contrast are almost entirely fast twitch - useful for swatting mosquitos!) The best ultrarunners tend to have 95-98% slow twitch fibers in their calves. I have 46%, which means I could probably hang with them if they only used 50% of their legs and hopped the entire distance.
There are two types of fast twitch fibers and I always forget the terminology, but the important point is how they're fueled differently. One type can burn glucose for fuel, which can be supplied through the blood stream, though they can only get 1/18th of the energy possible by burning it in the presence of oxygen as in the slow twitch fibers. The glucose gets turned into pyruvate, which is in turn converted to lactate and alanine and gets exported to the liver, which can then deal with it. It's inefficient, but it's what you need if you try to run hard for any length of time.
The other fast twitch fiber has no equipment for using sugars or fatty acids. It uses a storage molecule called phosphocreatine, which gets completely exhausted in 5-30 seconds. If you're running 50 meters all-out, this is your fuel. If you're running much more than that, it's pretty useless.
Having half fast-twitch and half slow-twitch fibers means I'm best suited to run 1-5 miles, which turns out to be the distances with which I've had the most success.
Ultrarunners (I still don't consider myself one) are forever saying I just have to run slower and burn fat, rather than sugar. In one sense, that was true. In another, that meant I'd be ignoring half of my trainable muscle fibers. By using just slow-twitch fibers, I end up walking. And I've walked too many ultras already.
Another question that gets raised is: if you do lots of ultrarunning, don't you develop more slow-twitch muscle fibers? The answer turns out to be - no (however, the slow-twitch fibers you have get larger, have more mitochondria, become more efficient and develop better cappilarization), but you do get better at it anyway.
I've always wondered whether it's better to go with one's strengths or to shore up one's weaknesses. For me, endurance and stamina are always weaknesses; ultrarunning didn't really eliminate the problem. Now that I've decided to return to training for short races (a strength), I see I have a lot of little weaknesses I can work on. The answer to my question turned out to be: go with your strengths and shore up the weaknesses associated with that strength. As usual, the answer wasn't one or the other, but both.
Added: It's commonly said that the fast-twitch fibers that utilize glucose can be trained to act like slow-twitch fibers. This is misleading. They never use fatty acids for fuel, nor do they completely oxidize glucose; training for long distances causes them to adapt by using glucose and exporting waste products more efficiently.
4 days ago