knuckle under vs stand up to

knuckle under

verb
  • To yield or cooperate when pressured or forced to do so. 

stand up to

verb
  • To object to or interfere with the actions of (someone seen as bullying, pushy, or controlling). 

  • To withstand, to weather, to survive in spite of. 

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