Originally Posted by Darryl Shaw
Fenthis, it could be that your abs are already too strong for the plank to be of any use to you. If this is the case try making the exercise more difficult by placing your feet on a swiss ball or try bracing yourself with your hands on a medicine ball. If they still feel too easy try balancing on a swiss ball and a medicine ball at the same time. Also it might help if you pre-exhaust your rectus abdominus by doing a few sets of crunches first.
I strongly doubted this because he said he was feeling it as a back workout.. which it should NEVER be even if your abs were strong enough.
What happens with something like ab wheel rollouts where if you're not strong enough to do them is that when you get out far enough your abs fail to sustain your body above the ground. What happens then is that you start to lose the hollow shape and go to straight to slightly arched which starts putting pressure/working on your lower back. Failure to keep the abs contracted in hollow position for any exercise this happens.
This is why so many people complain of back pain when doing situps... they're doing it wrong. Abs are not getting involved, and the hip flexors are being used which exerts pressure on their origin which is the lumbar spine. Recipe for lower back pain.