let up vs loosen up

let up

verb
  • To cease or stop (doing something intense). 

  • To cease or stop. 

loosen up

verb
  • To relax; to act less seriously. 

  • To become loose; to loosen; to relax (a muscle, etc.). 

How often have the words let up and loosen up occurred in a corpus of books? (source: Google Ngram Viewer )