As judo and karate both originate in Japan, probably one way to refer those athletes who practice these martial arts would be using the Japanese expressions as follows:
- "karateka" (空手家?) [karate practitioner]
- "judoka" (柔道家) [Judo practitioner]
But I'm quite confident most native speakers have not even heard those terms once!
I know more people who do karate / judo are called:
- "karate / judo practitioners"
- "somebody does karate / judo," [which is a description rather than a name like: "wrestler: who practices wrestling!"]
I would like to ask if one really wants to use an English expression how they should normally refer to someone who practices these sports in everyday idiomatic speech and common English?
- Karate / judo player
- Karate / judo athlete