I believe it is important for your instructor to have a general fitness, physiology,
or medical degree. Then, the trainer must be fully certified in Pilates. Fully certified means completing courses in 7 distinct disciplines ( see carousel):
And for each, there are 3 levels, basic, intermediate, and advanced.
Finally, there is a special course on Injury and Special populations, that addresses differing injuries and categories of people, for example, diabetes, elderly, pregnant, and so on. Worth noting is that the studies include practical experience so a trainer with the complete certification will have had three years or more teaching, not just learning. Perhaps not surprisingly, there are not many instructors who have completed this full program, but you need just one!
Also, I think it is very important to find a Pilates instructor who is a very good listener. One who performs not just an initial assessment, but adjusts the program as you and she move forward. If you have a sense that you are related to as a person as well as a client – perhaps even a full partner -- in the journey to pain-free energized living, this is a sign of a great Pilates teacher.