I hear that A LOT! Sadly I find most people don’t even know what their core is? Usually it’s abs people want. Better abs is what these people are looking for and that’s mostly diet.
IF you are truly looking to train your core here are the muscles and exercises you should be doing;
Trapezius for posture
Latissimus Dorsi for stabilization
Gluteus Maximus Gluteus Medius for sprinting and balance
Rectis Abdominis, Transverses Abdominis for bending and twisting and acting as a girdle keeping you tight and aligned
Hip Adductors, Keep you stable and aligned HIP FLEX
Quadratus Lumborum, lower back
Spinal Eredgors Runs along your spine stand up straight and rotate
THAT’S your core and that’s how you work it!