Hi Folks,
Look this up...
Commotio cordis (Latin, "agitation of the heart") is an often lethal disruption of heart rhythm that occurs as a result of a blow to the area directly over the heart (the precordial region), at a critical time during the cycle of a heart beat causing cardiac arrest.
This was discussed extensively on a UechiRyu forum in the past. It is common in modern UechiRyu schools to deliver strong punches to the chest and upper body during Sanchin Kitae, to demonstrate body conditioning. While (to my knowledge) deaths have not been reported because of commotio cordis in UechiRyu classes, it is a bit frightening to think that this has been documented in several sporting activities where a person (especially a younger participant) can be struck in the chest. Beside MA, we are talking baseball, football, basketball, etc.
My dojo does not practice chest-striking for students of any age during Sanchin Kitae.