"This condition is attributed to poor conditions in the setter or hatcher. When humidity is set too high, many chicks hatch with an overly large belly, causing them to struggle to leave the shell. As the overheated chick thrusts with its folded legs against the shell, it will - in its bid to be free of its shell as quickly as possible - often thrust too violently, causing damage to its hocks in the process."
I seem to have given too much info and then not enough??? love you Kat!