Updated: Dec 19, 2019
Trying to do a plank without understanding how to do it by engaging your core can be detrimental to your back. Here, we train as keeping the back in neutral position while inclining from the hip. And keeping the shoulders on top of the elbows.
I recommend to do that exercise next to a mirror so you can heck your alignments.
In doing any fitness exercise, it is always more valuable to do an easier version with the right muscles engaging instead of trying too hard and losing form (which will require te help of other muscles you don't want to load).
When lifting one leg from the floor, your aim will be to NOT move your back at all. Instead, feel your abs compensating the rotation and the pressure between elbows changing slightly.
How to set up a floor plank will come later... Stay tuned.