What Really Happens To Your Knees When You Gain 1Kg?

You might think that gaining 1 Kg of body weight should have negligible effects on your knees. But I’m here to question that line of thought. You see, I am a hobby-runner. Whenever I gain or lose body weight, I “feel” it. More importantly, my knees “feel” it.

Now I understand that a typical person might not be as sensitive to body weight change as a runner. That is why I would like for us to move away from the realm of “feeling” the change and move towards “quantifying” the change. That is the scientific way.

In this essay, I will be going through the effect of a 1 Kg body weight gain on the knees. For this, I will be using physics as the science medium. But at the same time, I will keep the language as non-technical as possible, so that anyone can just ‘get it’. I’ve given a live demo of such an analysis to friends in person before, and they were indeed surprised to learn the truth. So, I won’t be surprised if you end up being surprised!

In Physics, Weight is a Force

For those of us who don’t use physics knowledge on a day-to-day basis, this fact is very easy to forget. For those of us who never really cared about physics, this might come across as a bit of a surprise. We use Kilograms (Kgs) to measure our body weight. But what this actually denotes is the amount of force our body exerts on the floor (or whatever object) we are standing on.

Your body weight is caused by Earth’s gravity acting on your body mass; mass and weight are two separate things in physics. For instance, your body mass would stay the same on Earth and Mars. But your body weight would be different because the gravity acting on your body mass is different on Earth and Mars.

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Weight is Static Whereas Your Knees Experience Dynamic Forces

Weight is what is known as a static force. The body weight that you measure on a weighing scale is no different. But when it comes to forces that your knees experience, they are largely dynamic in nature. And dynamic body forces are always much higher than the static body force caused by body weight.

It is easier to visualize this using an example. When you get on a weighing scale, initially, as you are getting on it, the scale moves or shows a higher weight -reading before it settles down. Now, this settling down is nothing other than conversion from a dynamic to a static state.

The higher number you saw momentarily before the reading ‘settled down’ was indeed true. It was a dynamic force your body experienced at that moment. It just was not your body weight.

Your Knees Bear More Than Your Body Weight When You Move

Does this mean that your knees always experience forces higher than your body weight when you move? Almost in all cases, yes! Whenever you walk, run or jump, the forces acting are dynamic, and the load that your knee bears is your body weight multiplied by a factor.

As you might have guessed, the more dynamic the activity, the more force/load your knees experience. Considering the three activities I mentioned so far, jumping would cause the highest load on your knees, running the next highest, and walking the least. So, now you have a picture of how static and dynamic forces affect your knees. But by how much? We are still yet to quantify the effect of a 1 Kg body weight increase on your knees. Right, that’s our next challenge.

For a 1 Kg Body Weight Increase, What is the Increase in Force Your Knees Experience?

I told you that I’m going to follow a scientific approach. And that is exactly what we are trying to do here: quantify the effects using physics.

Luckily for us, some clever folks have conducted research to help answer our questions. In the results published in this scientific article, the researchers experimentally figured out that a 1 Kg increase in body weight would increase the forces experienced by your knees by a factor of 4 during daily activities.

Yes, you read that right, each kilogram you gain causes your knees to bear at least 4 Kgs more. What’s worse is that depending on the intensity of the activity, the load factor would increase further. Think about sports activities. Playing football for instance would be significantly harder on the knees than just walking. You see the challenge now.

How to Benefit From this Knowledge?

It is no wonder that as a hobby-runner, I am extremely sensitive to body weight changes. The faster I run, the more load my knees experience.

The good news is that physics works both ways. If you reduce your body weight by 1 Kg, your knees will experience a 4 Kgs lesser load during daily activities.

“A weight reduction of 9.8 N (1 kg) was associated with reductions of 40.6 N and 38.7 N in compressive and resultant forces, respectively. Thus, each weight-loss unit was associated with an approximately 4-unit reduction in knee-joint forces.”

– Messier et al.

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Further reading that might interest you: How to Tame Running Injuries? and Why Do You Feel Odd When You Oversleep?

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Disclaimer: In this article, I am not presenting any medical or life advice. For that kind of thing, you will need to contact a qualified professional (which I am not). All I am trying to present here is objective knowledge that you may benefit from. In the end, you, and you alone are responsible for your actions.