Adding collision to hands

I have a character that is ditting behind a deck, The desk is independant and fixed, so that doesnt move. When i add physics to hands they detach from the arm what am i doing wrong. My goal is not to have the hands drop past the top of the desk like they are resting on it. I was to see what it is like to try the same with the arm too, put they are no longer attached to the shoulder socked and drop to the gound.