impede vs keep back

impede

verb
  • To get in the way of; to hinder. 

keep back

verb
  • To restrict or restrain. 

  • To hold back; to refuse to give or share. 

  • To prevent (a pupil) from advancing in a course. 

  • To stay back; to refrain from approaching. 

How often have the words impede and keep back occurred in a corpus of books? (source: Google Ngram Viewer )