Why is Pyeon Sang-Wook so strong?

When Pyeon Sang-wook is attacked from behind by the first monster, who was strong enough to substantially dent a steel door by ramming against it, he seems barely perturbed by its attempts to eat him. He then grabs it, rips it off of his back, holding it at arm’s length, off the floor, with one hand (bear in mind that it was previously an adult woman), tosses it to the ground, then smashes it through the masonry wall, breaking through the wall. Further, with nothing more than his hands and feet, he seems to have hurt it so badly that we never see it again. Bear in mind that other monsters have survived partial decapitation, falls from ten stories, or electrocution without inconvenience. Although I suppose it is possible that it was knocked out of the building with his last blow.

He seems to be reasonably tall and athletic, and most of his feats of strength, such as shaking off two hammer blows to the head with barely a stumble while still being able to drag two corpses at once, are at least theoretically possible for a human, but what he did with the monster was clearly superhuman. However, he did not seem to be in the process of turning into a monster. So why does he exhibit this sort of strength?