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

That’s not an exact calculation. It depends on what you are doing.

If you jump up and land down, you are putting force on your knees both launching and landing. The force is measured in Newtons, not pounds because it’s a measure of force, not weight. That stress point of force goes up substantially as you gain weight causing much more wear and tear on your connecting tissue and cartilage.

In rock climbing this is pretty apparent. A standard carabiner is rated to hold 20 kilonewtons of force. A kilonewton is about 224 pounds. Why would you need equipment that can take 4500 pounds of force when the heaviest climbers are probably more like 250 pounds. Well if someone is falling, they will apply many times the amount of force. In fact, climbing ropes are designed to not put more than about 1600 pounds of force on you so as not to injure you. They reduce the force by stretching a little bit so the force is spread out over a period of time. If it was a static line, you might take 10,000 pounds or more force and have your back broken. Because the rope can only apply 1600 pounds of force on you, this force would be nearly doubled (minus friction) when a belayer is holding the other side. So, a carabiner will undergo 3000 pounds of force on a regular basis. This is the energy it takes to reduce your speed from FAST to STOPPED.