http://www.copperjoint.com/ankle-injuries-and-12-low-impact-exercises/

Low impact exercises are best for people who are just starting with working out and those who have bad joints in their hips, knees or ankles. These types of exercises do not put much toll on the joints of your lower body but at the same time still give you ample exercise minutes and burn calories. It is best for people who have had ankle injuries because low impact workouts are easier on the joints. These exercises can still give you the heart-pumping workout but reduces your chances of getting injuries. Plus, it also reduces the risk of cardiovascular disease.