If you have bad knees, look out for exercise machines which don’t place strain on the joints.
That means avoiding running, jumping and any other high impact exercise which forces you to bear weight on your knees.
However, low impact doesn’t mean low intensity – there are plenty of exercise machines that will save your joints but provide a challenging, full body workout.
Ellipticals, recumbent bikes, rowing machines, and resistance training machines are all good options.