There are many reports of people doing impossible feats if they have to safe their loved ones. Lifting cars and such stuff. Your body is capable of amazing things, but stops you from performing them so you don't injure yourself in the process. The so called hysterical strength can bypass that for a short time.
I'm rather irritated about the fact BlackBoxx mentioned. Strength or not there were pretty many zombies. Why didn't they react? Who knows...