After open heart surgery i want to punch my heavy bag for excercise. i know it will put a lot of pressure on my sternum. it has been twenty weeks ?

OK: Your sternum has healed to almost full strength. You can start a punching bag routine. You will probably develop some chest wall or muscle pain as you start the exercise - because you haven't used your muscles to this extent since the operation. Listen to your body and start gradually and workup to the level that you performed prior to surgery.

Probably: I tell my patients that they can start gradually putting more stress on the breastbone after giving the bone two months to heal. Check with your doctor, but as long as you have healed well, it should work out.