My favorite exercise instructor is Jane Fonda. Her DVD is Firm and Burn. It's low impact aerobic and definitely is done with safety in mind for the body's muscle groups, bones, tendons and ligaments. There are so many fitness instructors who are really good and knowledgeable -- with credentials. But there are many others who have no credentials -- or perhaps have some credentials, but not enough practical experience and yet they put themselves up on You Tube, for instance, to get subscribers. We can really injure ourselves if we choose an instructor who does not know what they are doing.