The Friction Coefficient: Why Your Joints Are Failing (And How to Stop It)
How to make small changes that keep you moving efficiently
Your knees are 100 times slicker than ice.
This is a physics fact that most people, even some doctors, don’t appreciate.
The coefficient of friction for ice sliding on ice is about 0.1. The coefficient of friction for healthy human cartilage is approximately 0.001.
That means the bearings inside your knees and hips are nearly frictionless. They are engineering marvels designed to last for 100 years of heavy use without wearing out.
So, why do I have a 6-month waitlist of patients who need them replaced at age 55?
It isn’t because they “used them too much.” It isn’t because of “bad genetics.” It is almost always a problem of physics.
Welcome to The Human Machine. Today, we are talking about Friction.
The 4x Force Multiplier
To understand why the machine breaks, you have to understand the load it carries.
According to Harvard Health, the mechanics of walking create a massive force multiplier. For every one pound of body weight, your knees absorb four pounds of pressure. That means if you are just 10 pounds overweight, your knees aren’t carrying 10 extra pounds, they are carrying 40 extra pounds of load with every single step.
Now, apply that math to a misaligned machine. If your alignment is off by just a few millimeters, that massive compounded force isn’t being distributed evenly across the joint surface. It is being concentrated like a laser beam on one specific spot of cartilage.
That is how you grind a hole in a bearing that is supposed to be indestructible.
The Race Car Tire Analogy
I want you to stop thinking like a patient and start thinking like a mechanic.
Imagine you buy a brand new Ferrari. You drive it off the lot, but the front right wheel is misaligned by just 3 degrees. You can drive that car for 500 miles and everything will feel fine. But at mile 5,000, that tire will be bald on the inside edge.
Did the tire fail because it was “old”? No. The other three tires are brand new. Did it fail because you “drove too fast”? No. The tire is rated for 200 mph. It failed because misalignment converts rolling energy into shear force.
This is exactly what happens in the human knee. When your hip stabilizers are weak, your femur rotates internally. When your femur rotates, your knee track shifts. Now, instead of gliding on that slick 0.001 surface, you are grinding against the edge of the bearing under 4x the normal load.
Arthritis is not a disease of aging. Arthritis is a mechanical failure caused by uneven loading.
The Check Engine Light
In a car, a light pops up on the dashboard when the oil pressure drops. In the human machine, the “Check Engine Light” is subtle, and most of you are ignoring it.
It’s called Start-Up Pain.
Do you feel stiff for the first 10 steps when you get out of bed?
Do you have to “shake it out” when you stand up after a 2-hour flight?
Does your knee ache only when you go down stairs?
That is not “getting old.” That is your machine telling you that your alignment is off. That is the sound of your tires scrubbing the pavement.
If you ignore the light, you eventually blow the engine. In my world, that means you end up on my operating table.
The 5-Minute Inspection
You don’t need an MRI to check your alignment. You just need a floor. Here are the two tests I use to determine if a patient’s mechanics are compromised.
Test 1: The Terminal Knee Extension (The Lockout)
A healthy knee should not just straighten; it should hyperextend slightly (about 5 degrees). This “locking out” mechanism allows your leg to become a rigid pillar, resting on bone rather than muscle.
The Test:
Sit on the floor with your legs out straight.
Place a rolled-up towel under your ankle.
Push your knee down toward the floor.
Pass: Your knee touches the floor comfortably. Fail: There is a gap, or it feels “stuck.” The Fix: If you can’t lock out, your quad is constantly firing just to keep you upright. You are driving with the parking brake on. You need to relentlessly stretch the posterior capsule (back of the knee) until you get that range back.
Test 2: Hip Internal Rotation (The Windshield Wiper)
The hip is a ball-and-socket joint. It is designed to rotate. If it stops rotating, that torque has to go somewhere. Usually, it goes downstream to your knee or upstream to your lower back.
The Test:
Sit on a chair with your knees bent at 90 degrees.
Keep your knees touching.
Swing your feet outward as far as you can (like windshield wipers moving apart).
Pass: Your feet swing out 35–45 degrees. Fail: You can barely move them, or you have to lift your hip to do it. The Fix: If your hips are stiff, your knees are taking a beating with every step. You need to mobilize the hip capsule. (We will cover specific protocols for this in Part 4 of this series).
Maintenance vs. Replacement
I am a surgeon. I make my living replacing parts. But I am also an engineer, and I hate seeing a good machine go to waste because of poor maintenance.
If you have “bad knees,” stop blaming your age. Look at your alignment. Check your tire pressure. Fix the friction.
And if you do need the replacement? Well, we’ve engineered that process to take 55 minutes. But I’d rather you keep your original parts as long as possible.
Next week: We are looking at the engine. Why I sleep 5 hours a night, perform 1,000 surgeries a year, and have more energy now than I did at 30.
R. Michael Meneghini, MD
Call to Action: Try the “Lockout Test” right now. If you fail, you have homework. Share this with a friend who complains about their “bad knees.”