The best replacement for step up is squat.

The best alternatives to step up are rated as following:
★★★★★  squat
★★★★★  step down
★★★★  lunge
★★★★  step out
★★  first aid
★★  step up 2
★★  box jump
★★  step-up
★  street dance
★  leg press
★  split squat
★  stomp the yard
★  ramp horse trailer
★  abcd
  master the boards
  you got served

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Speaking about step up, one probably shouldn't forget about channing tatum and jenna dewan.

Other search terms related to step up and their ratings are:
★★★★★ channing tatum and jenna dewan
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step up
squat
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