Under Armor, or Underarmor

Medieval armor was made of either chain mail, or later plate.  It could be heavy, but men who trained to wear it could move and ride around it in with as much ease as a football or hockey player can move in their body armor.  However, one thing that was essential, as it is for modern athletes, was a proper base layer and padded under armor.  These absorbed sweat and prevented chafing, and also absorbed some of the shock of being struck during battle. 

Knights started with a bass layer, including drawers and a linen shirt.  Linen is effective at wicking moisture and absorbing some BO.  Men also wore a close-fitting linen coif, sometimes quilted, over their cropped hair, to help with sweat and padding.  The main upper garment was a quilted or padded jack or gambeson, later known as an arming doublet.  Some of these had their own ties to better assist with strapping on the various pieces of upper body armor.  Hose-like garments known as chausses protected the legs.  Armor was fixed on leather laces or later with straps and buckles, and usually required one or two assistants, often squires training to be knights, in order to get ready for battle. 

Reenactors, of course, know all about this stuff.  For those of us who don't hang around Renaissance Faires or living history societies, there's help and hope.  At Warwick Castle in England, the wax artists of Madame Tussaud's have recreated Richard "the Kingmaker" Neville, 16th Earl of Warwick, and his retinue getting ready for the Battle of Barnet in April, 1471.  In the tableau, a squire helps Warwick with a piece of leg armor.  His under armor is clearly visible, including chausses, gambeson, as well as a linen undershirt.  Unfortunately, his precautions for bodily safety didn't help him.  Warwick was killed in the battle.