keep (back) vs obstruct

keep (back)

obstruct

verb
  • To get in the way of so as to hide from sight. 

  • To impede, retard, or interfere with; hinder. 

  • To block or fill (a passage) with obstacles or an obstacle. See synonyms at block. 

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