It is amazing the difference between instructors. I have posted before, my adoration of a particular spin instructor, who also races tandem with DH. A good instructor in anything will have mastery in the subject, will understand clearly, contraindicated movements/maneuvers as safety should always be paramount to a good physical workout. The group instructors at my gym do have degrees in physical ed or health ed. But even that does not guarantee a knowledgable instructor in form and safety. Unfortunately, by the time someone "listens" to their body, it could be at the expense of their knees, etc. I have found that those who ride on the road and teach, offer a better class, than those who only teach aerobics, Zumba, TBC, etc. Just MHO.