The question is, are the presence of these genes in non-runners considered? Not every person with these different genetic elements ends up being a runner, does he?

Is this causative or descriptive? In other words, different runners are seen to have these different genetic elements. But did those elements "guide" them into running? Or do we say, if you don't have such and such an element, you won't be a long distance runner?

This reminds me of the claim than there are genetic differences in homosexuals, or different brain cell conglomorates, I should say. We know these differences were observed in the corpses of dead adult homosexuals. What we don't know is what their brains were like at birth. We cannot know without sacrificing them for science!

We cannot know what influence behavior actually has on cellular composition or arrangement. I'm not versed in this lingo, but, from what I've read, there is a philosophical point continually by-passed. The descriptive is not causative. The causitive can be described, yes, but describing its effects is not the same as describing the cause.

Ignoring the first poster as much as possible, the ACTN3 gene is interesting. I was talking with these Australians about it earlier in the year.

There are two common alleles of this fast-twitch muscle protein, one works and the other doesn't. Both have worldwide distribution and presumably are at least as old as the move out of Africa - tens of thousands of years minimum.
Originally ythis Australian group thought it was a case of relaxed seelction, but the pattern is all wrong. With relaxed selection, as with some of the olfactory geens, you get lots of different inactivating mutations, not just one common and ancient one. And for the most part any relaxation of selection has been recent - people weren't flipping channels and eating Doritos 50,000 years ago.
So the two alleles must be in some sort of balance. The Aussies think that the working form helps you sprint, and they have good evidence for that. They have weaker evidence that the null mutation somehow helps endurance.

There are basically two ways this can work. Either heterozygoites have a higher fitness than either homozygotem, or there is frequency-dependent selection in which each allele becomes advantageous when rare. In the second case, there exist social niches of some kind for both fast and not-fast people.

So the article says:
'Instead of glucose, slow-twitch muscles are fueled by oxygen, and are responsible for sustained exercise.'

This makes no sense to me. You get energy from fuel+oxidizer, so (ignoring intermediate steps) glucose *plus* oxygen. Are the fast-twitch muscles using some stored energy (in the form of oxidizer) that doesn't require oxygen from the blood (until later, to recharge)? That would sound more likely.

Re: gcochran's comments, I think the frequency-dependent selection theory is interesting. If fast-twitch muscles are less energy-efficient than slow-twitch ones, then there would be some disadvantage to them, and perhaps the hunting advantage would diminish if everyone in a tribe had the fast-twitch homozygote. But of course it's all speculation...

I believe the author was engaging in shorthand--fast twitch fibers do better at anaerobic oxidation of glucose to lactate. They also have a higher tolerance of lactic acid, although that might not be affected by this particular gene.

Slow twitch fibers, however, have more mitochondria (this is easily seen under microscopy--really fascinating, actually) which aid in the oxidative degradation of glucose.

David