http://youtu.be/_poFwEbY8YE
Oh good, another fitness clip with a sense of humour. Is this the hardest plank position possible? It certainly is challenging to do well. The basic set up is as for holding a plank: nice long back, abs felt to be working down between the hips, upper abs not noticeably contracted and absolutely no sensation at all in the back. It will also help to slightly drive the elbows into the floor and gently squeeze the armpits; this helps stabilise the shoulders. Make sure you look straight down. As the opposite arm and leg are lifted, there is no shifting about, neither breath nor jaw are gripped, the back remains long and open.
This cross stabilisation of the body is very good for the back – even though we can’t feel it working at all. It helps strengthen the deep stabilisers of the core. However, the exercise is advanced and if back ache is an issue, then this exercise will only make it worse because the body will stabilise itself using the muscle that are meant to move us about and the deepest stabilising muscles will not activate. It will reinforce using the wrong muscles, which a major part of developing back ache. So if back ache is an issue, then a good place to start is superman on the Swiss ball, kneeling on the Swiss ball, then mastering the straightforwards plank.