The Berserkers were the Viking Shock troops. No armor, only Bear skins...The English created the word "Berserk" that means Bear shirt. The ancient texts referenced them being strong beyond measure and that fire or iron could not touch them.
In the modern world, it means fearless audacity to challenge the norms of fitness, to be superior, to share great energy, and to push yourself past the comfort zone. Berserkers have no comfort zone.