repetition vs strengthening

repetition

noun
  • The act or an instance of repeating or being repeated. 

  • The act of performing a single, controlled exercise motion. A group of repetitions is a set. 

verb
  • To petition again. 

strengthening

noun
  • The process by which something is strengthened. 

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