How do i edit an existing animation and make one aspect of it not move?

hello, i have played around with it for the last couple days and tried to add tracks to the bones. But although i add tracks, the arm still moves up and down for some reason. The original animation is a guy running forwards with his arms swinging. All I want to do is have his left hand held out infront of him still for the whole animation. When i add tracks to the bones i want to not move, it still moves.