We greatly underestimate the work our bodies can do. Physiologists used to believe that we feel exhausted when we physically cannot go any further. It used to believe that fatigue was nature's way of telling you to stop and this made sense in theory.
There was one physiologist, however, who believed this was not the case. In 1924, Archibald Hill proposed the idea that when the body was working hard, the brain would start to send messages to the muscles to slow down in order to store energy to survive challenges we might deal with. Going back to our caveman hunter-gatherers ancestors, they may have been walking around for 2 or 3 days looking for food. Their caveman brains would start sending signals to conserve energy in case a saber-toothed tiger attacked them.
Recently, an ultra-distance marathoner, sports scientist, and countryman, Tim Noakes, started to test this theory. He came to the conclusion that the brain will create this sense of fatigue that has very little to do with the actual muscle's ability to continue to work.
These findings have massive implications for our physical energy and our willpower. Willpower is like a muscle which means it can also feel tired and the same principles apply. When you get home from a long and stressful day, you can feel as tired as if you have run a marathon. When the brain starts to expand mental energy, it will start sending you messages to rest. It will make you feel more mentally drained so as to keep some resources in reserve just in case.
How does this manifest? Unhealthy food may seem more tempting, exercise may seem more daunting. You need to realize that your body can still do one more at this time. One more what?
One more set at the gym, one more kilometer on the run, one more hour on the bike, and one more wave when you are surfing. Your body is stronger than you think. Your will is stronger than you think and your mind is stronger than you think.
Not many people can say they have truly defied their own limitations in their lifetime. So go out and test your limits - you will be amazed as to how much more resilient and tough you are.