Stand on a low box, stack of plates, or similar. Step off forward with one leg straight and the toes pulled back, reaching the heel to the floor by bending the support knee and letting the heel lift. Control the motion to just lightly tap the heel on the floor and then return to the top.
 
Support and assist yourself as needed by holding onto a pipe or rack.
 
Purpose
The Petersen Step-up is an exercise to strengthen the VMO for knee stability and health.
 
Programming
This is a good preparatory exercise for use in a warm-up. 1-2 sets of 8-12 total reps are appropriate.
 
Variations
The common method of execution is more accurately called a step-down; they can be done as a step up, starting with the heel on the floor, as an introductory version.