Hey, you know what's no fun at all? A shattered sternum.
A Coup de Grace to the chest will probably do just that. Jumping directly onto a person's rib cage with your heels is probably going to deliver enough force to crack their chest open like an egg. Seeing as the ribcage is home to some of your favourite organs, such as the heart and lungs, this is not particularly good news for the recipient.
If the original trauma doesn't literally crush the organs from the get-go, the resultant sharp edges created by a broken bone are perfectly placed to rip through something important with one wrong move.The prevalence of death from sternum fractures is as much as 45%, but you don't even need to break anything to die horribly.
Commotio cordis is a condition caused by a blow to the area directly above the heart. The shock of the impact will cause the old ticker to simply stop beating. Defibrillation needs to begin within three minutes and your chances of survival could be as low as 1 in 5.