decelerate vs step up

decelerate

verb
  • To reduce the rate of advancement of something, such as a disease. 

  • To go slower. 

  • To reduce the velocity of something. 

step up

verb
  • To increase, augment, or advance, especially by one or more steps. 

  • To assume or take responsibility. 

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