concur vs knuckle under

concur

verb
  • To meet in the same point; to combine or conjoin; to contribute or help towards a common object or effect. 

  • To converge. 

  • To agree (in action or opinion); to have a common opinion; to coincide; to correspond. 

knuckle under

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

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