This post have been moved. Visit the new link No more cheating at workouts
You mentioned you have weak knees? Is it due to an injury? While workingo n strengthening your knees, it may be useful to know that stepper/jogging type exercises (weight bearing) are generally harder on the knees compared to the elliptical trainer or bicycle (non weight bearing). So you may find that you are able to push yourself harder in these machines.
Thanks for the tips. I am quite addicted to the stepper unfortunately but I will try to use other machines more :)