What people often miss here, is that in order for us to feel strong and burly, we do fight with ourselves. We clench our fists and teeth, we get tense. And just like with arm flexing, we put our own force against ourselves.
I understand, this is a natural reflex. It was built in us as a part of survival mechanism. We do need physical readiness when we feel danger. However, this mechanism was not designed to be used all day every day, like many people do.