Another question would be what part of the human body is considered the weakest? It makes sense to say that whatever part that may be the weakest will sustain damage the easiest, not taking into consideration any specific position whilst hitting the surface.
My suggestion would be that the organs and the brain would take the most damage as they continue to move downwards due to sudden massive decelleration of the body when impact occurs.
Internal damage probably occurs long before any muscle injury or "bruising" and would greatly cut the distance required to sustain damage.
Would the height at which you sustain damage from when hitting for example pavement be similar to the height when hitting water? At what speed does water turn into stone?