Core Stabilisation

I had a query from a girl who wanted to strengthen her core muscularity. She had back issues so sit-ups & crunches were out as any movement like that aggravated the neck, so I quickly snapped a few pics of a few static holds you can do that will strengthen the area & work to 'resist motion' which is a major function of the abs & core region that is overlooked.
If you are just starting I would simply move into the position then lower until you get a feel for the movements, but once you are confident you have them correctly mastered begin to hold each position for time. I would work up to 90 seconds or so, then begin to either add weight, lift a limb or use some other method to make the stance more difficult.
I've included 4 (5 if you include both sides of the side plank), which will work your static holding strength & anti-rotational strength. Do these exercises once or twice a week will aid you in increasing your deadlift, squat or any other exercise that needs a strong core.

Front plank







In the front plank the body should be straight & the spine should follow its natural curvature. The top of the arms should be at right angles when viewed from the front & sides. Do not lift your head, but let it follow the natural flow of the rest of the spine. Do not let you belly sink down (so you look like a seal). The idea is to use your stomach muscles to keep your belly in place (pink socks are optional)

Side plank





With the side plank you are using your obliques to stop your hips sagging into the ground. Your upper arm should be at 90°. An important point to note is the spine & neck should remain straight so the head shouldn’t droop or be pulled upwards, but the whole spine should be straight.

Bird dog





With the bird dog you are resisting rotation, so the idea is to keep the back flat & the curve of the spine natural. The head should not droop or be raised, but the neck should follow the natural curve of the rest of the spine. Your back & buttocks should be level, not twisted. You lift the opposite arm & leg, so in this example you’ve got the right arm & the left leg raised.

Shoulder Bridge





The shoulder bridge works the back muscles directly. In the pictures I should have the feet just a little closer so the bottom of the legs are at 90° to the floor. The idea is not to extend the back, but to hold the body in its normal position with the spine holding its normal shape, so don’t lift the hips too high, just enough to keep the back in its natural shape.

Final points

If you need any further explanation then feel free to leave a comment & I'll get back to you. If you are doing stretches for the back, do them after you have done these, not before.
Work up slowly & start off easy. Learn to do the moves right first, then go up in time. Stop as soon as you feel the form is slipping, do not hold a pose using the wrong form or you’ll do more harm than good.
Don’t start these while you have a bad back, be patient, let any back injuries heal, then start adding them into your day.
Twice a week is plenty.
Use a mirror to begin with to see if you’re doing them right, or get someone to check out that you are getting into the right position.
Get hold of a book & write down how long you hold each position for, the date you did it, etc. It is probably best to set a day or 2 aside each week (like say Monday & Thursday for example) & those are the days you do them, that way you’ll form a habit of doing them those days. Aim at doing a little bit more every week.