Option 1: Use a flat surface that ends abruptly

You can do this exercise on any surface that will allow you to grip it with your toes and let your heels fall downward.

Simply push up with your toes

Go back to the starting position. You can come down slowly to make the exercise tougher. 

Option 2: Use Leg Press Machine

Start with foot angled toward yourself.

Push forward with your toes only.