A downed opponent (also called a grounded opponent) is a combatant that is on the ground, as compared to a combatant that is in a standing position. This commonly implies that the downed combatant is lying on the ground, but can technically refer to any position in which anything except the soles of the combatants feet are touching the ground. In many combat sports featuring striking, such as Boxing or Taekwondo, it is illegal to strike a downed opponent. The referee will promptly bring the downed opponent to his or her feet to resume the bout standing. The only one combat sport which allows strikes when the opponent is down is Mixed Martial Arts. Majority of MMA organization follows the common No Knees and Kicks to the Head of a grounded Opponent, Fighters are allowed to strike their opponent to their body. Hand Striking and Elbows to the head are considered legal.
There are many MMA organization that still follows the roots of MMA which is No Holds Barred Fighting. Brazil and Japan are the two main countries that allows Soccer Kicks to the face/head when the opponent is down as well as knees to the head of a downed opponent.
Pride Fighting Championship was the famous MMA organization in the world and they were the first Big organization which adopted soccer kicks, knees and stomps to the face/head of downed opponent.