The sotithorax, nicknamed the "life jacket" by Empyraen Army Hoplites, is quite the versatile piece of kit. 

Made of tough, flexible alloys and fibro plastics, it carries a core of liquid mithril which, when exposed to kinetic force or heat, will stiffen and provide a little extra protection to the wearer. 

Though the heavy Hoplite helmet and armour panels worn by soldiers are useful in their ablative and protective properties, a Hoplite prizes his or her life jacket above all else

As the upper control panel hints, rebreather cabling is concealed on the underside of the sotithorax meaning that, for the sake of comfort, a Hoplite is obliged to wear their padded undershirt whenever they enter battle, regardless of the heat they may encounter. 

This is, perhaps, the reason Hoplites are known as grasos or pongos.