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> I find it difficult to imagine unless you corner the animal and have enough manpower to not be run over. It would be easy to imagine, if you had experience o
by ordu 7y ago
> I find it difficult to imagine unless you corner the animal and have enough manpower to not be run over.
It would be easy to imagine, if you had experience of collapsing from fatigue. Try to get on the roof of a skyscraper by stairs, you'll get all the needed experience. For more realism, you could run for a several floors, then rest for a few minutes, and to repeat it as needed. For even more realism find a person who is better than you at marathons, give him/her a stick and tell him/her to move slowly up by the stairs (1-2 steps per second, I believe, will do) and to beat you with the stick when he catches up with you. At some point you will prefer stick to moving further. It could be done without stairs on the plains, but it would take more time, up to a few hours.
I couldn't give you experience, but I could give you some theory. Moving animal needs to absorb oxygen, to oxidize glucose into ATP. If animal tries to move faster than some limit based on the ability to absorb and utilize oxygen, then some of glucose metabolizes only halfway to H2O+CO2 and lactic acid. Muscles start to accumulate lactic acid. If concentration of lactic acid in muscles is high enough, animal feels it painful to strain muscles. The more concentration is the less strain is needed to feel pain, the higher level of pain at the same strain level. At some point even the slightest straining feels painful. Cells in muscles could die from too much acidity, but it hard to do because of pain. (Though a heart could be damaged this way, for this one needs to force his heart to work at ~180+ beats per minute for a ~10+ minutes, it doesn't feels like pain in the heart, so be careful when pushing your body to its limits).
To get rid of lactic acid animal needs oxygen. Muscles could metabolize lactic acid to get energy, or the liver could do it. But in any case it needs oxygen, animal need to stop and to rest, or at least to run at lesser power, to have enough spare oxygen in a bloodstream.
If animal have no time to stop and rest for long enough time, then lactic acid concentration continue to grow. It feels like overwhelming fatigue, pain in muscles, and all you want is to take a break. If you try to work your muscles through it, at some point they just stop working. But it is hard to get to this point, because of a pain and a loss of motivation to continue.
- jeremysalwen 7y agoFWIW the specific biological mechanism for fatigue you described is not accurate: https://www.nytimes.com/2006/05/16/health/nutrition/16run.html https://www.nytimes.com/2006/05/16/health/nutrition/16run.ht... but your basic point that you will eventually get tired enough that you can't move any further is true,
- ordu 7y agoI cannot accurate describe biological mechanism because I'm not a chemist and not a biologist. But I do not see how my explanation contradicts your link. Concentration of lactic acid is building up when there are not enough oxygen in the muscles. Lactic acid could be used as a fuel by muscles or a liver, but to do that oxygen is needed. Without it concentration just builds up to the fatal levels. It means, that to get rid of lactic acid, one needs to lower his power output, to move into aerobic zone when body have plenty of available oxygen, more than it needs, and to work there for some time. One could stop completely when tired, then lactic acid through a bloodstream will reach liver, where it will be metabolized into something more useful. I believe in this case the result will be ATP, which will be ejected into a bloodstream and then used by all the variety of cells in the body. So you are going into a debt, when your power output is higher than your aerobic abilities allow. While it is a free debt, interest rate is zero, there is a limit how much lactic acid your body could contain without harm to itself. It is oxygen debt: the more lactic acid you have, the more oxygen you owe to your body. To repay this debt you need to lower your power, to run slower than you could run indefinitely (i.e. without build up of lactic acid). Of course it just one of the mechanism for fatigue. The other one, for example, is the lack of fuel. It is the worst kind -- you could stop and rest for an hour, but it doesn't help. You need to eat and to wait while glucose reaches muscles. Though with the training it comes easier, body learns how to digest food while running, and how to feed muscles, while they are working. If your body have fats, than it can be trained to use them. Mine body doesn't have fats, so if I plan to cycle for a several hours, I need to eat continuously while cycling or after a few hours I'll lose ability to use my legs.