Seriously, get on a treadmill and start trying to run/sprint forwards and backwards on it with the power OFF.
Running forward while "pushing" against the console = sled pushing
Running/walking backwards with hands on side rails = sled pulling
Maybe not as equally effective but I does hurt to breathe when I do a couple of these "sprints" so it must be doing something.
Tried it before a few times and no damage to the machines occured (in case you're worried). The worst that can happen is someone coming along, seeing what you are doing, and give you teh WTF look... had it happen to me once when this cute blonde came up smiling and told me, "You know the machine is off right?".