When it comes to training the core, it's a good idea to review what actually constitutes this key region of the body. Often thought of as simply the internal and external obliques and rectus abdominis, the core actually comprises all of the muscles linking the upper and lower extremities, including the muscles of the back (such as erector spinae, quadratus lumborum and paraspinals) psoas major, and even the glutes.
How frequently one should train the core depends on who you ask, but it is my opinion that you should include an element of core training each time you train. It is important to remember that the body works as one synergistic unit, so you cannot isolate muscle groups anyway, but the key thing is to ensure that you train the appropriate core focused movement patterns for you on a weekly basis.
Obviously the exercises you perform will be dependent on your level of core strength and stability, but whatever stage you are at, you should be thinking about covering the following movement patterns.
Anti-Extension/ Stability Exercises
Anti -Rotation Exercises
Lateral Stabilisation Exercises
Trunk Flexion Exercises
Trunk Extension Exercises
Trunk Rotation Exercises
Hip flexion with neutral spine exercises
When starting a new training regime having been out of training for a while, or if you know your core to be weak, the following patterns should be focused on first:
examples of progressions: forward ball rollouts > stiff arm pulldowns > prone stability > front plank
example of progressions: horse stance vertical > horse stance horizontal > tall kneeling anti-rotation press > half kneeling anti -rotation press > standing anti-rotation press
example of progressions: side plank > T stabilisation > single arm farmers walk > single arm waiter walks > windmills
Once you are able to resist rotation and extension effectively and therefore stabilise and protect your spine, the following patterns can be added to your training program:
example of progressions: Reverse crunches > Ab pulldown > Turkish Get Up
example of progressions: back extension > Prone cobras > dynamic cobras > prone med ball throws
example of progressions: tall kneeling high to low woodchops > half kneeling high to low woodchops > standing high to low woodchops
example of progressions: swiss ball jacknife > hanging knee raises > swiss ball pike > Single leg jacknife
Obviously, it is not necessary to train every single movement pattern every time you train, and it is also important to remember that your core muscles will activate during all of your compound lifts (deadlifts, squats, lunges, bench/ overhead presses etc.). Pick 2 or 3 core specific exercises to focus on each time you train and monitor your progress before progressing to a harder version. Moving on too quickly, and before your body is ready, will likely result in compensation from other muscles and negate the purpose of the exercise.