Foggy-Hollow
Explorer
You're dealing with 2 separate "units" here, so to speak — 2 distinct yet connected aspects of your knee.
Let me put on my physics nerd glasses for a moment; I'll try to tie it back to things you can actually touch and feel.
Mass is mass, weight is mass. That's a fundamental physics property and pretty simple. Pick up a bowling ball — it has mass, and it's likely somewhat heavy.
Loading is a different beast: it's force divided by area. You may recall Einstein's F=ma => here the a stands for gravity (acceleration due to gravity). The crucial point is that m is mass, so when mass drops, force drops.
Load =
Force = mass x gravity
———. ——
Area. Area
(That's as deep as the physics goes, I promise — hang in there with me!)
Suppose you're placing a gallon of milk down: the amount of "push" — force — that the bottom surface experiences is uniform across the whole face. Now balance that same jug on a pencil, and you've got identical mass or force concentrated over a far smaller area doing the supporting. So the /loading/ shifts even though the jug itself hasn't changed. What changed is the area that "feels the weight."
Physics lesson over.
Our knees aren't so different. While walking, your weight rests on 1 knee at a time. If you're running, you're probably airborne or at minimum experiencing more vertical motion than walking. My waist measures … 30-some inches around, maybe? Perhaps 40? The bones in my knee certainly aren't that size. Milk-jug-sized waist (and weight) meets drink-coaster-sized knees.
Is that 2.2x figure accurate? I can't say for certain off the top of my head, but it sounds plausible.
With love, your friendly neighborhood enginerd who adores running even though I'm limited right now.