Just got back rom my personal trainer session. We've been doing planks for a couple of months 1 min 3x. now were doing 1 arm plank push-ups. Loads of fun.
I started Nov 1st so it would be easy to keep track of where I was. The jump from 45 seconds to a minute was tough. When it goes up to 1.5 mins, it may kill me. And that's barely scratching the surface.
I discussed this program with my PT. If it's not done properly, sore backs will be the result. One rule - elbows under shoulders. Another rule - don't stop breathing.