Working with Army Biometrics. 30% of one's own Ideal weight is the point where penalties to fatigue and agility (mobility) kick in the most. Use the Weight of the person if their BMI was 18.5-25 (average 21).
When you ask a regular person, regardless of size or weight, to carry 48lbs the difference of their performance with or without the weight is rather small. Its too small for many game systems to value. Where the difference between a fit /trained person really are more pronounced is over time.
The Roman 60lbs. Is the Roman 60lbs weight the same as the 30% Carrying Capacity rule of thumb? If you look up the height and weight of Roman Soldiers, (averaging 5'2"-5'4") their fit BMI is roughly at 105-141lbs, 60lbs roughly the 45% the roman soldier's body weight.
How long is a soldier expected to carry 45% their fit body weight? In the SWM, and the Roman marching method 4-5 hours is typically allocated for travel.
Does this rule of thumb work for Animals?
A Pack mule or pony, can carry about 20% their body weight. In the Example of the Pack Horse in the SWM (small wars manual) a pack horse with a philips pack saddle and a m1919 machine guns carries 192lbs total. The animal is probably 900lbs. In SWM this is a load animals are expected to carry a whole march (4-5 hours).
200lbs is barely enough to carry a rider, equestrian gear and the rider's gear. 250-300lbs is roughly the typical load capacity of a riding horse. Thats roughly 50% more their pack weight. In Fief, it mentions that warhorses only have the endurance to carry a rider for 2hours. I would start with the assumption that at 50% more than the safe weight (20% for horses, 30% for humans) any more than 4 hours would be detrimental.
Assuming succeeding increments (hours) of burden is damaging the animal or human in the same way, strained muscles (cramping), or potentially torn muscles (Strength loss/weakening) that will take weeks to nurture back, depending on the degree). Further weakening can cripple a human or an animal, when it weakens to the point that any sudden slip will put a lot of force to fall on the weakened joint or limb.
So Riders who work their animals more than 4-5 hours, their 30% their weight capacity will cripple their animal. The odds of this happening sharply increases by the factors of weight and every Over-Time hour. A game system should be able to reflect this.
Working back with the 30% rule, at what increments are differences of comfort and performance noticeable?
In my experience, I can't tell much of the difference between 24-32lbs when I'm in a mil sim. If I were to ad-hoc it from Anecdotal evidence.
When would run with my camelpack (2liters at 4lbs, 2lbs hiking boots, and running clothes at 2lbs) that would be around 5% my ideal fit weight. When I was carrying my backpack and walking around 8-10 hours in Singapore it really hurt my back, i was at around 20lbs encumbrance. I took out my laptop the next day and had a much easier time. Thats roughly 10% or 16lbs of my ideal weight.
2%, 5%, 10% and 20%
after the 30% and 45%
there is 70% and 90%
Also there are weight penalties when there is no Load Bearing Gear. Poorly distributed gear doesn't affect the total weight or acts as though there is more weight. Instead they affect the character by added penalties in their Fatigue Saves (their Fatigue/Endurance Condition).
Fatigue penalties and Substance Dualism.
Because not everyone subscribes that in science, substance dualism is a fallacy, the effect of the physical/neurological state of a person on their judgment, perceptions, thinking and attention tends to have no bearing in some game systems. In fact, substance dualism goes against all the effects and evidence of psychotropic and neuropharma drugs, which works on the evidence that the mind and the body is not divorced. Qualia Soups video on the matter explains a lot and does it best.
Its not much of a rant as an observation of some logical inconsistencies with the assumption of substance dualism as it applies to everyday life, and not just special circumstances. Fatigue penalties affect judgement, perception, attention and awareness so much that it is implied in many safety laws (automotive, maritime, operation of construction equipment, etc. etc.) .