Hi, my sincere apology for only getting back to you now. Firstly what is the problem with the knee? How long has it been painful? What are your limitations and why? The orbitrek is great as it has cardio vascular benefits without the jarring. What level of fittness do you want? Your training is best to be in line with what your goals are. Firstly once you are clear on what your exact goals are its alot easier to training accordingly. There are so many different levels of fitness for so many different activities. Clarity is key. The best way to get fit is doing interval training. Invterval meaning increasing and decreasing level and intensity for a period of time. The golden rule is always work 20% out of your comfort zone in order to improve fitness and get the most amount of benefit. Hope this makes things a bit clearer. Bridget