Here's something to think about when it comes time to set your goals for 2018: What if running a marathon was easy? Okay, maybe not easybut certainlymore of a psychological challenge and less of a physical challengethan you probably think. There is some evidence that physical fatigue is a psychological phenomenon rather than something determined by VO2max and lactate threshold. This is theCentral Governormodel of fatigue, and it argues that the fatigue you feel is the fatigue your brain creates. Some of this research has been donewith marathon runnersand it supports the idea that your brain regulates your physical output to prevent (what your brain believes to be) physical harm. This model also explains why we're all capable ofmoments of hysterical strengthin emergency situations (as the crisis overrides the normal settings of the central governor). The model isfar from perfectbut it does suggest that at least some of the physical fatigue we feel is an illusion that takes place in our heads rather than in our muscles. Which is exactly why theUS Navy Sealshave a mantra that says when your mind is telling you you’re done, you’re really only 40 percent done.