i take a LOT of pictures of my chicks for my records and education on their development. but i don't think i have EVER taken a picture of the underside of their feet!
i'll have to do that next time. but i don't think it'll be today.
it's possible you might get more responses if you were able to post of pic of yours.
Most of mine fall asleep if I hold them that way and rub their tummy. I have several pics that I have taken of different ones. They are ones I have hatched also, if that makes a difference.
They won't willing let me do it now that they are grown.......