The c curve is a movement of the spine that strengthens the deep abdominal while stretching the muscles of the back. The c curve is a basic movement in pilates that is used in many different exercises.
The classic c curve is always initiated by the abdominal. Try a c curve by sitting up tall with your legs slightly bent in front of you. Imagine someone punching you in the lower stomach, and allow your spine to round by scooping in your deep abdominals. Your upper back, neck, and head may naturally follow this motion and round forward. So you initiate the c curve with the lower back (lumbar spine), then you add the upper back (thoracic spine), and finally you add the head and neck (cervical spine). Now your whole spine is making a capital c. This movement should feel like a big stretch for your whole spine and all the muscles that surround it.
Here's a little more specific information about the three natural curves of the spine and how they participate in the c curve movement.
Pilates lumbar c curve
The lumbar c curve movement is always initiated by your lower abdominals. This is the most difficult spinal movement to initiate because the lumbar spine has thick vertebrae that are meant to stabilize and hold the weight of the body. When you're standing or lying, the natural curve of your lumbar spine is in slight extension (like neutral spine), so when performing a lumbar c curve, you must pay much attention to pulling in your abdominals from the lowest part of your abdomen and attempting to reverse the natural curve of your low spine. You can accomplish this only by deep and strong low abdominal engagement.
The lumbar c curve in pilates.
Pilates thoracic c curve
The upper back (thoracic region) naturally curves forward in a c shape. When performing a thoracic c curve, think of pulling your ribs in and allowing your shoulders to round forward. Doing so creates a nice stretch in the upper back.
The thoracic c curve naturally follows the lumbar c curve, but it is easy to do the thoracic c curve without actually starting from the lower back. In other words, it's easy for people to round their upper back because the back naturally rounds in that direction. Initiating the rounding from the lower back is more difficult and takes low abdominal work. The idea in pilates is generally to try to do more work from the belly and to move the spine starting from the lower back and then adding in the upper back afterward.
The thoracic c curve in pilates.
Pilates cervical c curve
The cervical c curve is a way to visualize the correct way to lift your head off the mat during an abdominal exercise. If you know the right way to lift your head up and understand proper neck alignment, you won't overstrain your neck when doing the abdominal-related exercises in pilates.
Lie on your back with your hands interlaced behind your head to support the neck. Lift your head off the mat by lengthening the back of the neck and by imagining that you're squeezing a tangerine under your chin to bring the head up (kind of like nodding your head yes as you lift it off the mat). Don't lead up with your chin. Once your head is off the mat, you have created your cervical c curve; the c shape begins at the top of your head and ends at the base of your sternum (or rib cage). You must lift your head high enough to form the shape of the c. Think of your abdominal muscles lifting up the weight of the head, not the neck muscles. If you're very tight in your neck or very weak in your tummy, you may not be able to make a complete c shape. But if you keep doing the work, you will!
The cervical c curve in pilates.