I think you are asking the wrong question. If you feel pilates is helping patients in some way, how do you think it is happening? Which exercises? Taught by who? Individual variance from therapist and patient would be huge so you'd either have to do a very general questionnaire here with a large sample size, OR, a small study that is very specific. The concept of core is purely that, a concept. It cannot specifically be measured as such. Llet's look at your question from different angles to work out what you really want to know here.