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
Thanks everyone. I now have a great new reason to drop some weight, as my knees aren’t perfect anymore.
There’s a lot of nuance that goes into health and weight. I recommend people look into intuitive eating and health at every size.
Gravity! Gravity is pulling us toward the earth at about 4lbs. So for every pound you lose, gravity isn’t pulling that pound down at 4lbs.
Stress is by definition a body’s internal reaction to force. Force equals mass multiplied by acceleration due to gravity.
At your knees, hypothetically let’s say, you have a mass of 1 kg. The force exerted by this mass on the knee is approximately 10N. Now let’s say the mass on knee is 1 lb ie. 0.45kg, the appropriate force exerted is 4.5N. This force creates stress equal and in opposite direction.
So approximately 1 lb of mass reduction lessens stress by a multiple of 4-4.5.
Hold something heavy in your arm close to your chest.
Now hold it far away from your chest.
You *know* the weight didn’t change, but it *feels* like it.
Not quite the same, but it’s a similar idea with your knees. Different parts have different mechanical advantages and experience different forces upon them.
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.
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.
is this true for muscle? like if you’re mainly muscle around 10 percent bf?
Latest Answers