Sam doesn't display any martial arts skills until the episode 13 of the 2nd season. Why so long? Sam's partial amnesia faded to allow him to do advanced math by the 3rd episode. It seems like a physical activity like bike riding, martial arts would be return quickly.
However, in the 2nd season episode "Americanization of Machiko" Sam is involved in a high stakes fight yet he resorts slugging it out.
I would have to re-watch to see if it was a retcon or part of his Swiss Cheese brain.
shareOne of the things that turned me off of this show was he always seemed to "remember" just the right skills needed to get the writers out of the hole they'd written themselves into.
shareJust a case of the character developing as the show went along, I guess.
You can write it down to his Swiss Cheese amnesia forgetting that he knew martial arts, if you want an "in show" answer.