Do you do your 5min BBA plank all in one go?
I do that one - helps a lot to keep my back in a good mood.
I am not expert, but I believe that those shown above would not strengthen the back. From what I experience, the muscles facing the floor are the ones tensed. So one would do a plank "facing up" to strengthen the back.
Well, that's a good one. I have experienced a lot of both. Pull-ups are the one I do to strengthen my back muscles. But I also need to have my abs tightened to lower the pressure on my back by a mechanical result.
I think you would find that you tense more muscles than you think when you are doing a plank, both back and front.
@coach - it depends *how* you use your muscles to maintain the positions. Plenty of people can hold off the floor but have dormant abs, flaccid glutes, winged shoulders and so on and so forth - depending on their own strengths/habits/inadequacies.
the subtle difference is that a plank strengthens muscles that help in supporting the back. So, as Thierry says, it keeps the back in a good mood
and that is always a good idea.