Fitness types toss the term "core" around with little explanation, leaving many people confused and believing that ab work, like crunches, is strengthening their core. But this is not the case; the core is not simply the muscles on the front of your body. Nope. The core is three-dimensional, and in general terms you can think of it as your torso. One of my favorite definitions of the core is that it runs from your nipples to your knees.
Essentially, the core stabilizes your pelvis, low back, and rib cage, and it is made up of many muscle groups: all four layers of your abdominal muscles, including the obliques and transversus; the muscles on either side of the spine; the little muscles that run vertebrae to vertebrae know as the multifidi; as well as your glutes, hip flexors, and inner thighs. In Pilates, we also include the pelvic floor as part of the core; these small deep muscles help stabilize the pelvis.
To see how these muscles work together
All these muscles work together to maintain torso stability, but I feel a working core is easiest to feel if you imagine sandwiching your spine with your deep abs and deep spinal muscles, like when holding plank. When you add leg movement to a plank, like a donkey kick, you are challenging your core even more. You should feel your core working during squats as well as boat pose in yoga. And if you don't feel your back and abs working together, imagine that sandwiching sensation and know that if you pull your abs toward your spine without moving the vertebrae, the muscles in your back will be working too.
Looking for more exercise to strengthen your core, now that you know crunches are not targeting all the elements of your core? Check out these core exercises.