From Halopedia, the Halo wiki
Force-multiplying circuits are components of the MJOLNIR Powered Assault Armor system used by the SPARTANs. These circuits are located throughout the armor, boosting the force applied by the user. They make hand-to-hand combat easier, especially against stronger Covenant species, such as the Sangheili and the Jiralhanae but also make the armor difficult to adjust to; a slight motion can be translated into a potentially harmful one if not conducted correctly. For this reason it is only possible for SPARTANs to use the MJOLNIR armor, as their strengthened bodies are able to withstand the increased power and speed of movement, which, in un-augmented human bodies, results in broken bones or death, due to spasms caused by the pain of broken bones. Shrugging was considered difficult with the circuits, with a degree of concentration needed to do it.
List of appearances