So happy you enjoyed! Filming for PA in December 2016, so if you have any "wants", please feel free to message me here or my email: cathleen@synergypilates.com
PART ONE: Hello Mastaneh, thank you for posing this very debatable question. The degree of concavity for the lumbar curve is variable from person to person. Some individuals have a very pronounced lumbar curve, while others seem rather "flat" in the lumbar vertebrae. In GENERAL, I ask individuals to lie supine with their knees bent and "feel", without touching where their lumbar spine lies. (this is in the group class setting) Always, the feedback ranges from some individuals feeling their spines flat on the floor, while others have quite a pronounced sensation of curve or concavity.
PART TWO: So, in the generic group mat class setting, I will say that whatever the person feels from the above instructions in the previous paragraph, that is their personal neutral lumbar curve THAT DAY. It may change as time goes on for many different reasons (taking Pilates included). I never say that the lumbar curve should be the same for everyone, or something like "you should have a space under your lower back", "you SHOULD have an inch of space under your lower back", or those types of statements. I hope this answers your question. Feel free to contact me personally at: cathleen@synergypilates.com should you have more questions or need any other clarity.