The story is told of Rav Soloveitchik that once someone asked him about the halachic acceptability of karate. Here is a paraphrase of the conversation that ensued.
Rav Soloveitchik: What is karate?
Questioner: It's a martial art where you learn how to fight people in a non-violent way.
RS: What could possibly be the problem with that?
Q: Well, before beginning a karate match you have to bow down to your opponent as a gesture of respect.
RS: That's stupid.
Q: I'm not asking whether it's stupid, I'm asking whether it's assur.
RS: It's assur to be stupid.