Yeah, our emotions change our whole physiology. It's kind of fascinating. You can actually experiment with this a little.

First, you can think of the happiest moment in your life. Go back to a time when you felt really good. Don't just think about the moment...remember it. Put yourself as deeply into that memory as you can. When you've spent several minutes putting yourself into that memory, open your eyes and take immediate notice of your posture and how your body is positioned. You'll most likely find your face is relaxed (possibly even smiling). You'll notice that your shoulders are pulled back and relaxed. Your heart is beating slower. Etc.

You can do the same thing with a bad memory. Think of a time that made you sad, and put yourself in that moment in the same way you did with the happy moment. Spend several minutes really remembering the situation. When you take notice of your physical presence, you'll find things like your facial muscles are tense. Your shoulders are slumped. Your body is tense. Etc.

It's an involuntary reaction to our emotions that create real, physical movements and gestures.

The interesting thing about this is that you can work it in reverse. You can create the emotion by positioning your body in the way your body is when you experience that emotion. For example, if you put your shoulders back, put a huge smile on your face, and breathe deep, and you do this for long enough, you will actually start to feel happy or confident or both.