There is disagreement about whether "berserk" referred to "bare shirt", going shirtless, or "bear shirt", wearing a bear skin into battle, in the same way the ulfsarks wore wolf coats. These warriors would enter a rage in combat, where it is said their skin turned aside weapons. Some modern scholars suggest that the berserker rage may have been induced by consuming hallucinogenic mushrooms.
"A third I know, if great is my need Of fetters to hold my foe; Blunt do I make mine enemy's blade, Nor bites his sword or staff." The Poetic Eddas
Originally Berserkergang was known as Regenerate (according to its icon name) meaning that at one point, the Ulfsarks (or possibly other units, if not even other civilizations) were given the ability to regenerate lost health.