I used to be a cop and use to read cop stuff and watch videos of the same. I saw an interview of an officer once who should have died after a violent confrontation. He should have been dead before the paramedics arrived. He should have been dead before he got to the hospital. But he had a strong will to live and he survived to talk about afterwards.
Contrast that to another cop who was afraid of getting shot because he just knew he would die if he did. He even told people that. Well, one day he did get shot and sure enough, he died. The problem is that it was a flesh wound in the arm that would have been cleaned and bandaged and then he would have been sent home. But he died. He killed himself with his own mind.
So people know that the mind has a strong influence on the body but what a lot of people don't realize is that the body can also have a strong if not stronger influence on the mind. You can be happy and in a good mood but make yourself depressed just by changing the way you move. Put your head down, slump your shoulders, shuffle your as you walk, and put a frown on your face, and it won't be long before your good mood is out the window.
Conversely, you can be in a bad mood or depressed but if you force yourself to walk with your head held high with a smile on your face and a spring in your step you can change your mood all by changing the way you use your body.
How does relate to working out? It's simple. If you allow your body to behave as if you're worn out, you won't perform near as well as you will if you force yourself to act as if you're not near so tired. And it doesn't have to be anything major. Just little things will do.
A few weeks ago, I was doing a particularly brutal bodyweight circuit. When I do pushups, I make it a point to pop my knees under my chest and just pop right up when done. However on the 6th circuit of that particular workout, I put my knee on the ground and stood up like a tired old man. I realized what I did and knew that it just wasn't right. The next circuit I struggled to get through as my body told my mind that I was worn out. On the 7th circuit, I forced myself to pop right up like normal and finished 3 more circuits much better than I did right there in the middle.
If you've ever worked out with others whether at the gym or in sports in high school or college or whatever, you've no doubt seen people bend over with their hands on their knees. I've done and if you've ever exercised at all, chances are you have also. I don't allow myself to do that anymore no matter what. For one, I've read somewhere that a lot of back injuries during exercise actually occur when people assume that particular position for rest. And two, your body is sending a signal to your mind that you're too tired. Even when you feel like doing it, force yourself to stand up straight and walk around and you'll be surprised how much better your next set will go than if you assume the position. Try it. You'll be amazed.
And the next time your alarm goes off and all you want to do is bury your head under the covers and hide from the coming day, just throw them back and hop out of bed like you're ready to conquer the world. Your day will go much better--guaranteed. Just don't wake your wife up in the process.