Viking shields worked better when wielded as weapons
Viking warriors likely felt that the best defense was a good offense. This isn’t a comment on berserkers running into battle without protective chain mail, but the way any viking would have best employed their shield in battle. While shields were generally defensive devices as you might expect, experiments with recreated shields and swords found that they were more effective at protecting someone using it to aggressively against a foe, rather than passively blocking attacks from opponents. In this sense, Viking shields were a secondary weapon, not just a portable obstruction.
Viking shields were usually made of wooden planks, cut into a circle around three-feet in diameter. A circular hole was cut from the center of this disk, which was covered with a dome-shaped boss made of iron on the front side. On the back a wooden bar the length of the shield crossed over this opening, so that the holder’s hand could grip the handle at its center, protected from the front by the iron. Wooden shields would splinter or shatter rather easily, so many were covered either in linen or leather on the front side. This would add weight to the object, but help it from coming apart when being hit by an ax or sword.
Pushing and parrying
Most of this is pretty intuitive, with the centered-hand placement allowing for good maneuverability and control over a potentially 15-pound object. However, some archaeologists noticed some odd dents in the iron bosses that didn’t seem to match up with being hit by a slicing sword or ax, suggesting they were being used for something other than absorbing attacks. This prompted combat archaeologist Rolf Warming to build and test some Viking armaments, including shields and swords based on excavated weapons. In even mock combat, it became clear that while these shields could cover a lot of the body, they weren’t absorbing blows that could be denting iron in the observed patterns. What’s more, they also allowed a dangerous amount of contact to the head as swords bent and bounced over the shield’s edge.
Warming experimented with other uses of the shield, and found that using it to aggressively shove and parry an attacker made them much more effective. If attacks were actively met and deflected, the shield-holder could more easily counter attack with their own weapon. Using the shield more like a weapon also helped explain the dented bosses. That damage was likely due to thrusts made by the shield holder moving in on an opponent, rather than vice versa. There would obviously be limits to how far a warrior could successfully extend their reach with a shield before exposing themselves, but the maneuverable grip of these objects certainly helped with actively redirecting foes weapons in order to counterattack.
Source: New Powers of Viking Shields Discovered by Paul Seaburn, Mysterious Universe