constrain vs impede

constrain

verb
  • To keep within close bounds; to confine. 

  • To force physically, by strong persuasion or pressuring; to compel; to oblige. 

  • To reduce a result in response to limited resources. 

impede

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

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