"Loosen up", "tighten up"

"To loosen up" means "to become not nervous". Could the phrase "to tighten up" be a good opposite and mean "to become nervous"?

Solution 1:

In sports, particularly baseball, tighten up has the meaning of "become tense". The Dickson Baseball Dictionary has this to say:

To Tighten Up: To become less effective; to choke. First use: 1928. "With runners in scoring position, the average hitter 'tightens up', he stands stiff at the plate, his muscles tighten and his whole motion is jerky. When he does that he's gone. (Babe Ruth's Own Baseball Book)".

While not stated, nervousness is implied in the word "choke".