Why is it hard to slow down or speed up in the second half of a long run?

Question

I want to know what is the phenomenon, or if it exists, that makes you stay at the same pace after about 2 hours of running. Changing speeds whether its to slow down or speed up feels slightly painful at first. Several times in marathons I get stuck in problem and end up running very slow to try to get rid of the pain with no success. A few times I've run at speeds that are faster than normal or slower than normal where that pace became the only comfortable pace after a few hours. Today even trying to walk up an incline my legs almost automatically continue to move at a similar cadence and slowing down didnt feel good.

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Actually once I managed to run alot faster than normal for 20 miles and I want to repeat this. The pace was a normal short 6 mile pace for me, so Im wondering why or how it was possible. Couldnt repeat it though. Today it was similar but at a slower pace where I couldn't speed up or slow down easily. If the phenomenon exists I'd like to manipulate it to assist in running a fast marathon or half or to help do an ultra.

Answer 1

I don't think there is a name for this phenomena, but would say it's related to several complex issues that endurance runners have when they get far enough into a long run.

First off, our bodies and especially our brain combined with it's central nervous system are quite complex. They all actually adapt and become more efficient at doing things in a certain manner. I think your use case is a great example of this. It's likely your body is extremely efficient at running at a certain pace. When you combine that, and fatigue, fully determining what you are experiencing becomes difficult.

If I were to guess, your body is experiencing fatigue and depending on your nutrition intake, possibly starting to "bonk". But to help you out a little more on determining what it may be, here's a good article.

You're likely hurting no matter what speed you go because of the fatigue. But your body has trained enough at that middle pace to know how to deal with it the best. Speeding up or slowing down simply puts the body in a state that it is not as efficient in. Likely sending your brain signals to tell it to stop, or get back to the pace it knows how to handle.

The article I posted also gives several hints at how you may better train your body to adapt to that phenomena.