Charles Garfield used to coach the Russian Olympic weight-lifting team. He noticed that when the team members had lifted as much as they could, to the point of exhaustion, they would invariably grimace and frown and screw up their faces at the painful effort. In an experiment, he encouraged the weight lifters to smile when they got to that point of exhaustion. This simple change and seemingly minor difference resulted in the weight lifters being able to add another 2-3 more repetitions to their performance.
So this also applies to communication, conversations and public speaking. When you grimace, frown or feel flat while doing it, you are sending your brain the message, “This is really hard, I am tired, I don’t want to be here. I should stop.” The brain then responds by sending stress chemicals such as cortisol into your bloodstream. And this creates a vicious circle: the more stressed you are, the more difficult the task becomes.
Conversely, when you smile, your brain gets the message. It releases good chemicals including serotonin and endorphins, and your brain says “It’s not so bad after all. I can do this.”