apply oneself vs knuckle down

apply oneself

verb
  • Synonym of strive: to work hard, to complete an activity with good effort and focus. 

knuckle down

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

  • To submit; knuckle under. 

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