Fatigue is that feeling that you can’t possibly push your body to go any farther. An example is when you have run 4 miles and you just can’t go any farther.
Studies have shown that there is no physical symptom of fatigue. It does not occur because you are physically at a breaking point. Instead fatigue starts within the brain.
Interestingly, the biggest factor in when fatigue begins is when you over exceed your expectations. For instance, if you thought anticipated throwing 100 pitches and suddenly you’re asked to throw 110. You will feel a lot more tired at 110 pitches if you only expected to throw 100 than if you had expected to go 130. Your brain holds off on feeling tired until the anticipated end of an exercise and will feel fatigue when it nears that point despite your body being physically capable of continuing.
To combat this, we have to train our brains to both expect high duration outings and the ability to properly respond to things out of our control. This is why we throw simulated bullpens, why we mix up exercise routines, and continue to train mental toughness.