Stigmata—marks on tops of feet from fixed firm pose?

I have just completed my 30 day challenge! I have seen some great benefits, however, there is one problem. I have these patches of dark skin on the top of my feet in the exact same spot. It’s definitely not from shoes, I suspect it’s from fixed firm pose. Does anyone have any tips to help reduce this side effect during the pose, i.e., pose modifications? 2 other girls girl in my class have it too.

Thanks in advance!