You can’t just be a bookworm for the year before the exam. You need to use other body parts, too. Exercise helps all parts of the body, including the brain. More exercise, more oxygen to the brain. More oxygen to the brain, clearer thinking.
What is the meaning of “just” here? I think that it is used to emphasize what you are saying. Is it right?
It emphasizes the word (bookworm), that means to be only a bookworm nothing more nor less.
Wish that helps