The nervous system limits endurance performance by signaling to continue or cease effort.

“We have to ask the question, what is the limiting factor on performance? What prevents us from enduring? What prevents us from moving forward? What are the factors that say, you know what, no more, I'm not going to continue this run? There are five main categories of things that allow us to engage in effort. I don't want to completely write off things like the immune system and other systems of the body, but nerve, muscle, blood, heart, and lungs are the five that I want to focus on today because that's where most of the data are.”