The problem with the figures in previous posts is that whilst they "look" accurate they are not!
If you are going to take acceleration (due to gravity) it is only aplicable in a free fall situation. If you were to jump 2m in the air the acceleration would determine your speed when you "hit the floor"! As you were running when you "tripped" the acceleration would be reduced.
Your body mass did not stop when your knee came into contact with the step, but slowed as your body crumpled up.
Your arms would be flailing round and therefore alter the effective body mass.
As you came into contact with the step your shin absorbed some of the energy of the impact by compressing and bending.
Unfortunately the many forces involved quickly become so complex that an accurate figure is not attainable! If you doubt this you only have to look at slow motion video of bodily contacts, for example in the film "Rocky"! The body can change shape dramatically for short periods of time!
Still hurts though!!