agree with points by adam and chris. my thought, as always, is if you do any sort of resitence training, even to the point where the resistance is in far greater excess than you are capable of moving (ie, isometrics), then you want to keep the movement (or lack of) as close to the motion of a real punch or kick, or whatever.

And I also believe that you want to END your workout by doing the real thing (working the bag, for example,) so you re-instill the speed and muscle memory that can only come BY doing the real thing.