If you have suffered a serious knee injury in the past have had recent knee surgery, activities that in-volve a lot of running are going to be counterproductive for you. However, if you can find an effective low impact workout that is gentle on your joints, your previous injuries should be of no concern.
If you have bad knees, one of the most important things to keep in mind is that running is your adversary. When you are running, every stride you take puts the equivalent pressure of three times your body weight on each of your knees. If you have bad knees, you should do your best to avoid running, jogging, jumping, lower-body weight training activities (such as squats), and most high impact sports.