Not surprisingly, as runners train and spend more time running, physiological changes are occurring all over their bodies. Many of them are things that we are very aware of - for example, the muscles in our legs get bigger, and our resting heart rates become slower. Over time, we also notice that our endurance increases, and that we're able to go for longer periods of time. You may not realize though, that this is partially due to the fact that our motor neurons are actually changing. Our motor neurons actually have quite a bit of plasticity (ability to change) in response to changes in use. Both the motor neurons and the muscles that they activate are able to change their size, shape, and biochemical features in response to differences in activity over a period of time.
Generally, these motor pools (combinations of motor neurons and muscles) can be broken up into three categories: slow, fast fatigable, and fast fatigue-resistant. I think the use of examples from Top Gear will be a useful teaching tool. Also because Top Gear is one of my favorite things in the whole wide world.
Fast Fatigable motor pools are capable of generating a LOT of force. Like, a lot. They're at use in power lifters, linebackers, and other athletes that are hitting or shoving things with a lot of violence.
Not surprisingly, all of that POWAAAAH (power) uses up quite a bit of energy. These neurons very quickly burn through their fuel sources. This is because they have a high level of activity of the enzyme glyceraldehyde 3-phosphate dehydrogenase (or GAPDH), and simply burn through their energy stores really in a very short period of time. Think of them as a very very fast loud muscle car, with really lousy gas mileage.
Fast fatigue-resistant motor pools generate less force than the fast fatigable pools, but they're able to last for longer periods of time. In these pools, the GAPDH is slightly less efficient, which means it burns through energy more slowly. This means that they aren't capable of generating quite as much force as the fast fatigable pools, but are able to sustain a decent amount of force over a longer period of time.
Think of them as something like this Minsk that was used to drive across Vietnam. It's pretty tough. It was able to handle hills, it coasted through wet sand just fine (until it got too wet, but that's not part of the analogy), it was a good tough machine.
Slow motor pools are the final category of motor pools that we will discuss here. These neurons aren't capable of generating nearly as much force as the other types of motor pools. They burn through energy relatively slowly, as their GAPDH is not as active. They aren't terribly useful for tasks that require a lot of force, or quick bursts of energy.
However, this confers an advantage to these neuron types, in that their energy stores actually as much longer. These neurons are easily the most fatigue-reistant. Because less energy is broken down at a time, the energy stores in the muscles last longer. Even as energy sources are used, energy that is consumed (say - in the form of Gu), has enough time to get into the muscles to be ready for use as a fuel source.
As we run longer distances, over time the motor neurons controlling our muscles will gradually adopt biochemical and physiological properties more like the fast fatigue-resistant and slow motor pools, and less like the fast fatigable. This is why, although sprinters can generally go much faster than even distance runners, the distance runners will have enough fuel afterwards to allow them to keep going. And going. And going. It's so cool - not only are your muscles getting bigger and stronger, but their actual biochemistry is changing to adapt specifically help you with distance running!!! SCIENCE!
References: http://www.ncbi.nlm.nih.gov/books/NBK11021/
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