I think it is the circulatory system , muscular system, and the skeletal system.
A knee Joint must face forwards and must bend.
Energy must be added to a system to break a bond.
muscular system,nervous system, skeletal and respiratory system
It must pass though a medium from an another medium.
coalition government
Before going through the answer, I strongly recommend you do not try this on anybody. To harm the heart you must know its defenses. They include the sternum (breastbone) and cardiac fluid, while the nervous system controls the heart. To attack the heart you must disable the control. It can be done by applying sufficient force on the sternum. The human knee can provide that force. One only needs to bend his knee and even a small strike on the opponent's sternum can be enough to stun the heart. This lethal action is rarely used in kick boxing for a knock out because of the possibility to kill the opponent. So even though the knee is not sharp, it is enough to stop the heart and can even lead to death.
The ankle bends two ways, anterior (up) and posterior (down), so it depends. The bend the ankle down (point the toe), the gastrocnemius muscle (calf) must contract. To bend the ankle up (lift the toe) the tibialis anterior muscle must contract.
coalition -apex
the skeletal and muscular systems
it would bend towards the point of origin or where it starts.
You must lie down and bend over to kiss the Blarney Stone.
If the pad fits, you must acquit!