There have been studies stating that the reason we are the weakest of the primate species is our heightened abilities of Self Preservation and the ability that humans have to Second Guess our own abilities and decisions. Take those away and our bodies would be able to react much quicker and with greater strength. The stories you see and hear of Super Human strength is when pure adrenaline takes over and all doubt and second guessing wither away when. Zeds lose those abilities, thus they move much quicker as they aren't afraid of shattering bones as they run for their favourite meal.