Venusian aikido

Venusian aikido (sometimes called Venusian karate) was an unarmed combat style developed on the planet Venus, used frequently by the Third Doctor to defend himself and others. It was created by the Venusians, which meant it optimally needed 5 arms and 5 legs. (NA: First Frontier) (However, according to one source, Venusian aikido didn't exist until the Doctor himself invented it. (EDA: Interference - Book One))

It could be used to immobilise an opponent by utilising pressure points (DW: The Mind of Evil, Inferno) as well as perform joint locks, throws, and kicks. (DW: The Claws of Axos) It tended to use the attacker's own movements against them. (ST: Freedom) These were effective against many different humanoid species, but were useless against non-human duplicates and seemed to have no effect on Ogrons or Sontarans. (DW: The Claws of Axos, Day of the Daleks, The Time Warrior)

The Third Doctor used it to disable Professor Stahlman. He claimed that, if the move was held for a period, it would permanently paralyse the target. (DW: Inferno)

The Third Doctor attempted to use Venusian aikido against Primords. When this didn't work he moved on to Martian karate, but this had no effect either. (DWN: Inferno)

The Eighth Doctor, despite his amnesia, was able to remember a Venusian aikido lesson and use it against Mo. (EDA: The Eight Doctors)

Behind the scenes

 * Barry Letts was opposed to the idea of an aggressive Doctor. Therefore, Terrance Dicks suggested aikido as it has no attacking moves. (DOC: Can You Hear the Earth Scream?)