I would like to add that there is very little added benefit to holding a plank in the push up or single arm side hand on floor position. But there is a much greater risk of injury to the wrists from the positioning alone and added risk if stability is lost and the person must adjust suddenly. Especially in the side plank on one hand position.
I liken this to the exercise of standing on a fit ball. The benefit has not been demonstrated to be any greater than that of using a BOSU or inflated discs. But the number people who have blown out their knees doing it, would lead me to think that having clients perform this position for balance is a bad idea.
Some have argued that the straight arm/hand position is more difficult. And it is, but I can achieve this increase in difficulty with other methods that don’t risk injury to nearly the extent.