eli5: how does it work that for every pound of weight you lose, there is a 4 pound stress reduction in your knees?
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Knees, ankles and hips take a shock load when walking. I did some training where they had a force pad at the bottom of some stairs. You could do 3 to 4 times your actual body weight on that last step if you are rushing down the stairs.
Force not weight. If you weigh a pound of fat, it shows 1lb. If you throw that same piece of fat it will generate more than a pound of force. Knees take forces.
Your body weight in pounds is the so-called ‘static load’; I.E. your weight while still and unmoving. When you walk, each heel-strike has considerably more force than your weight in pounds: your body is under the strain of the ‘dynamic load’ of your mass moving under the effect of gravity.
As a simple analogy, compare a rock sitting on a surface to picking that same rock up to your center of gravity and dropping it. Obviously the dropped rock has a much greater force, as its kinetic energy multiplies the weight. In the same fashion, walking and running multiplies the forces acting on your joints.
Earths gravity pulls us constantly *and exponentially.* the longer you fall towards the earth from above, the more you accelerate and thus, the more force your impact carries when you land.
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Place a rock on your foot. That’s how much it weighs. Now drop the rock on your foot. Different experience?
Walking or especially running requires your knees to not just support your body weight but stopping your falling body weight.
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A really good analogy for how weight on a lever works, from one of my favourite movies.
Pick up something really light. Say, your phone. Hold it out at arm’s length. Hold it perfectly still and see how long you can hold it.
Weight applied to one end of an object applies more pressure on the other end.
This is one of the many reasons the metric system rocks and the imperial system blows. The gram is a unit of mass. Pounds is a unit of mass, force, and currency. So someone says “pounds” and you think mass, but they mean force, and everyone gets confused.
Long story short though, the forces going through your body are directly related to the weight of your body. Reduce the weight and you reduce the forces. How much are the forces reduced compared to how much the weight is reduced? There will be some function that varies with body mass, but its going to be complex.
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