The knee is a common attack in Capoeira. Although knee attacks can be seen in other martial arts such as Muay Thai, in Capoeira these strikes are more rare. In Muay Thai there are many examples of knees being used as a way to attack while standing or while grappling with your opponent. Capoeira usually uses knees in a more situational way. For example, if somebody ducts under your kick it’s common to use knees as a way to attack the face. Knees are generally used when the opponent’s head lower to the ground, which makes sense because most kicks in Capoeira are ducted under.
If the person you are playing with ducks their head close to your knee, the strike to knee is almost a way of saying, “look at how silly your response was. You ducked under a kick only to expose yourself to greater danger”. It is not clear why knees are used to attack the face and not other parts of the body such as the abdomen. This could have something to do with the nature of Capoeira. There is very limited grappling, which would make the use of a close range knee ineffective. Strikes to the head with the knee are one of the reasons why capoeiristas have to be careful as they move on the ground. Just because you’re moving on the ground doesn’t mean that you are invulnerable to attack. You always want to have at least one hand protecting your face and make sure to be aware of the other person positioning.