These are some great responses! Full of knowledge and science!
In addition to maintaining the integrity in the shoulder girdle and maintaining proper form no matter which method you decide to use, one of my immediate concerns with planking on the palms is wrist integrity. I really like that Debbie addressed that! I think planking on your hands opens the door to injury (mostly with side planks).
I agree with Debbie in that neutral wrist position would probably be best for this exercise. If I were instructing a client on a side plank, I would definitely be sure to instruct my client to utilize Debbie’s method for maintaining that neutral wrist, or I would give my client a good padded base of support and instruct that client to make a strong fist and dig into that base of support while maintaining structural integrity through the wrist. No doubt these are great exercises though!
Good question with some great answers!