Answered by Shaykh Ali Laraki al-Husaini
If one wants to train in an Asian Martial Art is it then permissible to bow, like it is done in Karate for instance?
This is a matter that has concern me as a karate student and teacher. What is haram is to do ruku’. The Japanese bowing is not ruku’ because the angle of bowing is less pronounced than the one needed for ruku’. I have also found in a commentary of the Risala (al-Fawakih ad-Dawani by an-Nafrawi) that some ulama are of the opinion that bowing (inhina’) is allowed if it does not reach the level of ruku’.
And Allah knows best.
 Besides being an Islamic (Maliki) scholar Shaykh ‘Ali Laraki al-Husaini (hafidhahullāh) is also a Sensei, a black belt in Shotokan Karate and Judo.
This Q&A is also available in Danish here
source : http://www.muwatta.com/bowing-in-martial-arts/