firmness vs flexibility

firmness

noun
  • physical durability; rigidness (of material) 

  • security; steadfastness; good grip 

  • strictness; mental strength 

flexibility

noun
  • The quality of being flexible; suppleness; pliability. 

  • The quality of having options. 

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