STEP UPS
For a low-impact cardio workout, turn to an aerobic step bench.
Step up onto the step with your right foot. Tap your left foot on the top of the step and then lower.
As you step up, your knee should be directly over your ankle to ensure you’re protecting your knees.
Repeat 10 times for a great calorie burn.
Read more: http://www.womenshealthmag.co.uk/fitness/sports-injuries/3195/5-best-cardio-exercises-workout-bad-knee-injury#ixzz3yv6Wf3L5