The problem came up on how to model fatigue in the game AGINCOURT, 1415: DARK GREW THE SKY WITH ARROWS.
The French foot armored units (dismounted knights and men-at-arm), weighed down by many pounds of metal armor, had to trudge across a muddy field then melee in the muck. Fatigue had to be an important factor in the battle as the exhausted French were overwhelmed or captured.
Creating a fatigue factor for each unit and increasing it with every turn or movement or melee in the mud was considered.
However, a unit morale factor already exists which is used to determine if a unit routs or is captured. In battle, a unit's morale may decrease due to fatigue (and other factors). A unit suffering a hit and fatigue even more probably might definitely take a morale loss.
The rulel chosen to simulate fatigue in the game was the following. After a fire or melee combat hit, a unit automatically undergoes a morale check where a d6 morale roll is compared to the current unit morale (1 - 4). If the morale roll is greater than the current morale, then the unit takes a morale step loss (eg; goes from good to fair). If an armored foot unit (infantry) is in plowed field (mud) terrain and takes a fire or melee hit, then the d6 morale roll for that unit is modified by +1 to simulate the effect of fatigue in the sometimes knee-deep muck.
In addition, any infantry (armored foot) units meleeing in plowed field terrain take a - 1 melee roll modifier. This also simulates fatigue and the difficulty of effective use of weapons in the terrain by fighters weighted down by armor in the slop.
While not perfect, this model will increase French infantry losses in plowed field terrain. The lowered unit morale will lead to greater probablity of rout and capture.
But the rule is for armored foot units. What about English armored units? The same rule applies to them; they had to fight in the same muddy terrain so when an English infantry unit takes a hit in plowed field terrain, that unit also gets the + 1 morale roll modifier. However, the English knights, in the historical scenario, do not slog for several turns across a muddy field, taking fire (arrow) hits and having to climb over the bodies of fallen men and horses.