knuckle down vs shape up

knuckle down

verb
  • To get to work; to focus on a task. 

  • To submit; knuckle under. 

shape up

verb
  • To take shape; to transform into or become. 

  • To improve; to correct one's bad habits or behavior. 

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