eli5: how does it work that for every pound of weight you lose, there is a 4 pound stress reduction in your knees?

Question

eli5: how does it work that for every pound of weight you lose, there is a 4 pound stress reduction in your knees?

In: 1542

Answer 1

It’s not unusual to see forces in the 3 times body weight range going on the knee. If you jump from a roof and had to roll you feel the force that is applied to your knees.

Similar to what people are saying regarding the lever arm. In the body the fulcrum is the spine. This is why it’s common for pregnant woman to have lower back pain, as the weight of a child plus fluids is ~20lbs, but since it’s centered about 4 times the distance from the spine as the back muscles are, they need to apply a force of 80lbs to stay balanced.