While there's nothing wrong with doing a modified version of traditional plank, celebrity trainer Teddy Bass sees many women clasping their hands into a fist, or "prayer mode," while balancing on their forearms, which isn't good for achieving great posture like his longtime client Cameron Diaz.
The reason? Locking your hands into a fist causes the upper back to round and the shoulders to hunch in, when ideally, you want to stay open and flat with the shoulders stacked directly over the elbows. Heed Teddy's advice in order to make the most out of this common exercise and achieve that long, lean physique you're going for.
I can't agree with this at all. Try it yourself. If you keep your mind on form, the motion of the shoulders which brings your lower arms to the angle to allow the hands to clasp doesn't have to have any effect on the spine or shoulder girdle. It just changes the force distribution on the muscles. In fact, in yoga there is a version which takes the arms to even greater internal rotation: a genie plank. You hold the the crook of the elbows with your hands. It greatly increases the workload in the posterior deltoid, and strengthening that muscle is great for posture.