Thawing and pounding chicken ?

The other day I was making chicken but I got a bit lazy and didn't move the chicken s from the freezer to the fridge early enough for them to thaw completely. It was just about there but the thicker parts were still a bit icy. I pounded them out anyways but when I put them onto the skillet/pan...it shrank up ALOT and ended up being rubbery. Is this because I was pounding a partially frozen chicken ? Was it cooking unevenly? Can you over pound it?