Monday 3 September 2007

Pic of the day: Most of us are not superheroes who can run and run without getting tired. You'd think this would be a bad thing. But is it?

Why do we tire?

I'll tell you up front, I don't really know the answer to this. But I know that I pondered this yesterday, as I returned from a walk that was longer than I am used to. Unsurprisingly, my legs started to get tired toward the end. It wasn't so that I was getting very stiff or weak, just a growing sense of discomfort. Well, slightly stiff at the very end perhaps. Anyway, what is this discomfort, what causes it and what is its purpose? This I pondered, and when I came home I started searching the Web for an answer.

There wasn't much to find. I am not surprised: When something is very common, people just accept it. "That's just the way it is" they say, and then they forget the whole thing. Or at worst, someone makes a guess and everyone latches on to it. Yay, we have an excuse for not having to think about it anymore! This seems to be what happened here. For more than a generation - perhaps several - the party line has been that lactic acid accumulates in the muscles, slightly damaging them and causing the burning feelings and the inability to function at peak efficiency. This is so obviously illogical that I could dismiss it out of hand, and for good measure science has actually proved the opposite. Lactic acid helps the muscles work harder for longer.

The reason why I could reject the lactic acid poisoning out of hand was that we tire even from moderate exercise, if we keep at it long enough. This was what happened to me yesterday. I kept a pulse of roughly twice my resting pulse, which in my case also happens to be around 2/3 of my max pulse. Neither my heart nor my lungs are stretched to their limits. Blood is flowing rapidly around in the body, bringing oxygen to the muscles and transporting carbon dioxied back, at worst traces of nitrous chemicals made from burning protein. (Normally we don't burn a lot of protein unless we eat a lot of protein.) If the muscles made lactic acid even during moderate exercise, the blood would flush it out and take it to other cells that could reassemble it. (If nobody else volunteers, liver cells take lactic acid greedily.) If the muscles didn't build up excess acid during the first hour, they sure won't do it the next. And yet at some point there we begin to tire. Actually, that point depends on how much you normally use your body, and on your age. But even young men tire, and boys become weary.

Another theory mentioned in one of the innumerable Google hits was heat. Again, this makes sense during intense exercise. And I certainly do grow hotter over time. But again, blood consists largely of water, a wonderful invention with amazing capacity for absorbing heat. It could hardly have been better suited if someone had specifically planned to use it in animals with a stable body temperature. Also, as you must have noticed, evaporating water from the skin leads the excess heat away from the body. During moderate exercise this process reaches an equilibrium, so that the heat lost equals the heat produced. It does not accumulate. I guess hard training can cause local hot spots, again because the blood simply doesn't flow that fast in a human body. (Remember that part of the system is capillaries so thin that the red blood cells have to travel in line rather than side by side. You just cannot force more speed there or things will break. In fact they sometimes do.)

One site mentioned that the muscles use up their local storage of glycogen, the "animal starch" made from condensed glucose. When the muscles need fast energy, these molecules dissolve into glucose again at great speed, flooding the cells with its fastest fuel. Keep it up, however, and the local reserves are used up. This actually makes sense, but the site added that this is mostly a problem when running marathon. Which I hardly do, although the body may comparatively think so compared to my quiet everyday life. Still, it was only half again as long as the usual trips, probably less than that. And in any case, the liver has stored enough glycogen for a day of normal use. The blood sugar level stays high (unless you have a certain condition where this regulation fails, and then you would notice it by feeling weak and lightheaded and nervous). Also, the muscles are able to burn fat unless they are under heavy load: Fat takes longer to process but gives twice as much energy per gram. I certainly have enough fat for days and days of walking.

You'd think that Evolution would favor people who don't get tired. You could hunt or forage a much larger radius, exploiting natural resources without having to move your camp quite so often. The people who don't tire would then gradually out-compete the ones that do, and inherit the Earth. Obviously this did not happen. Either there never was such a person, or they got eliminated.

Actually, there are such people. I wrote about one this summer. With just 40% more muscle mass than an average toddler, this little guy not only showed amazing feats of strength and agility. He also could keep running (and evidently did) without a break until he needed sleep or found something else to do. But there was one concern for his foster parents: He was always hungry. Even though he ate large meals often, he was simply not able to build fat reserves. His digestion could barely keep up with what his muscles used. Presumably he will be able to handle this better when he grows old enough to choose to sit still part of the time, but it won't be easy. Hey, even I am more or less vibrating on my chair with restless muscle movements. You can imagine someone who simply don't get tired. He would itch for activity.

During most of human history, there simply wasn't that much food. Once we switched to agriculture, the ability to travel long distances was fairly useless, since farms weren't that big. Even before that, there would be some point beyond which the long travel ate up most of the food you tried to bring home. Encouraging people to just keep going would be a fatal mistake. A kind and loving Evolution does not make that kind of mistakes toward its children. ^^

I still don't know HOW this warning system kicks in when you walk / run / swim further than you usually do. I am still curious. But I know that it is there for a reason. It reminds us - and eventually stops us - so we don't wander off the map on our own. A commendable function. An intelligent designer could hardly have done it better...