Imagine standing on one foot.  Better yet, try this yourself.  Do you struggle to keep your foot flat?  Does your arch roll in and your knees buckle, or does your weight shift to the outer edge of your foot?  Have you noticed the ball of your hands curling up off the ground when you hold a plank, or do a push-up?  Or do you feel your quads burn, instead of your abdomen, during “core” exercises?