Apples have them, and so do we — but, what exactly does it mean to have a strong or weak core? Hint: "Strong" doesn't necessarily mean being able to do a hundred crunches, nor does it mean holding a plank for 20 minutes (although that is seriously impressive).
First, it's important to understand exactly what your core is. It's more than just your abs; "core" basically implies everything between your shoulders and your pelvis. One simple way to check in with your core strength is by testing your push-up form. Although you may typically think of push-ups as exercise for your upper body, your core needs to be engaged throughout the move as well, to help keep your spine stabilized. That's why your instructor is always spouting pearls of workout wisdom such as "keep your core tight!" and "don't sink or raise your hips!" Go ahead: Grab a friend, and have her or him watch your push-up. Do it a few times for good measure (hey, might as well sneak in a mini workout). Then, instead of a traditional push-up, Women’s Health says to try the Functional Movement Systems (FMS) version. To do this, lie flat on your stomach with your palms directly under your shoulders, and tuck your toes. Now, press up through the palms of your hands until you're in the top of a push-up position.
What your friend (or you, if you're taking a video of yourself instead) should look for is whether your body is moving as a whole. Your entire body should be tight and tense, and your spine should be in a straight diagonal — from neck to heels. And, it should stay this way throughout the entire movement. If your hips begin to sag, or if your push-up looks more like a wiggling worm, try incorporating more core-stabilizing exercises (such as a plank with shoulder taps) into your routine.