How sitting kills you(r knees)

Posted on August 22, 2014


Preface: I decided to make this a non-technical post; I think that will be more impactful for this topic. For the biochemistry / biophysics inclined, I’m sure there are some details you’ll wish were in here. I included some references and words at the end to hopefully satisfy that desire.

There’s been a nice wave of media the last few years detailing how detrimental prolonged sitting is on one’s health. Oddly, explaining a mechanism for why this is has proven elusive. Here’s what I mean, from a little WebMD article:

WebMD Sitting Article

By the way, why does WebMD seem to get so much hate? It’s one of the best general health sites out there. Their reporting is much better than most media outlets. See: Their lack of sensationalism.

Anyways, I feel I can present a good framework for why sitting is so bad, specifically for the knees. Cardiovascularly is where much of the attention has been placed when it comes to sitting (as it should be), but a clear understanding for why sitting is bad hasn’t seemed to be presented. Hopefully in doing so for the knee joint I can persuade people to get off their ass more.

In other words, a lot of people want to know the rationale behind why they should do something. It’s not always enough to say, “Do this…because I’m a doctor / nutritionist / whatever professional.” Let’s provide some rationale.

Basics on how the knee works

The cartilage gets the most attention with the knee. It really shouldn’t be so obsessed over, but it is. So I’m going to run with that.

As many with some remedial knowledge of the knee know, the cartilage doesn’t have a great blood supply from itself. It’s not like bone where there are capillaries flowing through it. That said, the cartilage does get its nutrients. If it didn’t, it would die. It just does it in a different manner.

A good analogy I read many years ago in my undergrad anatomy and physiology book was to think of a sponge. When the knee joint is compressed, such as when you step on the ground, the cartilage -the sponge- is compressed.

Disc as sponge ball getting squeezed

When you release that compression, e.g. you don’t have the leg in contact with the ground, the sponge is able to deform back to its original shape.

Sponge ball reversion KNEE

Think of that sponge as if it’s in a tank surrounded by water. Initially, it has a bunch of water around it, and inside of it.

Knee Sponge ANALOGY

Say we squeeze that sponge. The water inside of it will seep out:

Sponge water tank GIF 2

When we release the sponge though, when we stop squeezing it, water will seep back into it:

Sponge tank water back in GIF 2

(If you’re thinking, “That really looks like sperm swimming into an egg.” Why yes, it does.)

The tank would be our leg and the sponge is the stuff inside our knee. (Cartilage and such.) The squeezing is done by the leg bones; the water is everything that flows through our knee. Blood, water, cells. Basically, it’s nutrition for our knee joint. The cartilage doesn’t get its nutrition directly like bone or muscle does, but it still gets it indirectly. Like the sponge in the tank.

Let’s make this a little more anatomy oriented. If we take our femur and tibia, the sponge (cartilage between), surround it with nutrition (“water”), and encase it in our skin (our tank), then compress it all together:

Knee Compression GIFWe can see how the nutrition is going to get squeezed out of the cartilage given enough compression. No big deal. If we want to get some nutrition back into the joint, we just release that compression:

Knee Compression REVERSAL GIF

A disclaimer

Compression is not bad. I know I have red lines (“bad”) for compression and green lines (“good”) for decompression, but that’s for the context of this post, which I’ll get to. This compression – decompression cycle is actually very healthy. It’s how our certain anatomical structures -like cartilage- thrive. Nutrition comes into the joint, the cartilage uses it and generates waste products, this gets squeezed out of the joint, and in comes rushing the next batch of nutrition. It’s like a sped up version of our digestive process. Eat, generate waste, poop, eat again.

We do this when we walk. As our foot lands on the ground -> Leg bones compress -> Foot starts to come off ground -> Leg bones decompress. So on and so forth. This cycle is necessary. It’s excessive compression that’s the problem.

Oh sitting, how I loathe thee

What happens when we sit?

Sitting normal hip positioning


We compress our knee joint. (Amongst other things.)

Sitting knee compression lines

We’re at this stage:

Knee compression analogy Last

And based on how long we sit, we’re staying at that stage. This presents a problem because how is our joint supposed to get any nutrition in this state? If someone compresses your throat then releases, it may not feel great, but there’s not any threat to your body getting oxygen. If someone compresses your throat for a few minutes, then we have a problem.

Where’s that threshold for the knees? I’m not sure. The research I’ve looked at isn’t exactly applicable to a real life scenario. The research I’ve seen excises cartilage and performs the study with the cartilage outside of the body. Insightful, but tough to extract hard recommendations from this.

Suffice to say, sitting for 17 of the 18 hours you’re awake is not a smart move. Nor is sitting for eight hours straight. Even if you exercise after work, that’s still a very long period of compression. The body can’t get nutrition into the joint. It’s trying to squirt some oil onto the hinge, but the door is closed. Just like if the throat is compressed long enough the body starts to die, when you compress your knee enough you are literally killing structures within the joint.

All this “marvelous technology” aimed at cartilage preservation, rebuilding new cartilage, is nothing compared to the Get-Off Your-Ass-And-Walk-More technology. The body already has mechanisms in place for these sorts of things. They need to be used!

“Everyone needs to squat”

Sitting is awfully similar to a squat position:

Parallel squat side view


If you’re someone who sits all day -someone who is in a squat position all day- do you really need to spend a lot of time and energy squatting as an exercise? If you’re compressing your knees all day, it might not be the best idea to do the same thing –loaded- in your spare time.

I love squatting. I have nearly every client do it at some point. However, squatting is not going to agree with everyone. No matter how good your form is, some people with beat up knees need to not do it for a while.

It’s like runners. “I’m foam rolling, getting massages, doing my stretches, I don’t understand why my knees are still bothering me after doing 60 miles per week for the last three years.” Having good form, doing corrective exercises, can go a long way, but there are limits to what the body can handle. If you’re doing one activity for upwards of 16 HOURS a day, that’s probably a threshold you shouldn’t be trying to push even further. You’re better off performing an exercise program where the knees get a break, where you go through the compress / decompress cycle, or don’t compress at all for the time being. Mat work is great here. So is walking, maybe some light jogging, perhaps walking lunges.

At the same time, you want to modify how you go about your day. Getting up more often is one obvious, and the best, solution. I recognize this isn’t always plausible though. So, I put together a small resource detailing how to make sitting more knee friendly. It covers:

  • A few positions you want to avoid. Such as when sitting in a work chair or a stool.
  • Positions and actions you want to embrace.
  • Lessening the amount of compression in a seated position.
  • How to decompress your knees while sitting.

It’s five bucks, called “Making Sitting More Knee Friendly,” and you can get it here: Add to Cart

After purchasing you’ll be redirected to a page containing a password and link. Use the password on the linked page and you’re good to go.

References and comments

This was my undergrad A&P book:

Anatomy & Physiology: The Unity of Form and Function

One study:


Another study:

Cartilage viability after repetitive loading: a preliminary report

Some quotes and notes for the second study:

“Chondrocyte viability and matrix integrity were assessed in cartilage explants that were unloaded (controls), statically loaded (0.5 MPa, 1–3 h), and cyclically loaded (1 MPa, 1–72 h, 0.5 Hz)”

MPa (megapascal) is a measure of force. Oddly, the statically loaded cartilage was compressed at a lesser force than the cyclically. That said,

“In the cyclically loaded explants, chondrocyte viability was significantly affected (i.e., cell death was observed) after 6 h of continuous, repetitive stress application. A similar trend of cell death was observed in the statically loaded explants by 3 h.”

Here are a couple cool pictures from the paper depicting their results:

Knee compression histology 1

The above is only the control and cyclically loaded groups. It doesn’t contain the statically loaded. In the cyclic group, after three hours you can see remnants of cell death (red cells at the top), and at six hours you can see where, according to the paper, it becomes significant. Also, note that at 24 hours the cell death, if anything, looks less.

“More interesting, however, was that cell viability in the repetitively loaded specimens did not decrease for loading times greater than 6 h. The chondrocytes in the middle and deep zones of the cartilage remained viable for up to 72 h of cyclic loading,”

Meaning after a certain level of cell death, somewhere around 6-15 hours of loading, it’s almost as if the cartilage samples turned this process off. At the very least maintaining the cartilage’s structure. We don’t know if the same thing happened in the statically loaded group (I doubt it would), because the study oddly didn’t load the static group for more than 3 hours. (Even though it loaded the cyclic group upto 72 hours.)

Here’s another picture depicting the cell death. A and B are control samples (not loaded), C and D were loaded. Notice particularly in D the “shredding” taking place.

Knee compression histology 2

Final point on this paper: This was not an in vivo study. So it’s tough to extrapolate too much as to how this transpires in the body. In the statically loaded group, I doubt much would change. Even in a body where there is nutrition and cells which can come to the rescue, you can’t rescue something if you can’t get to it. It’s like the trying to inject oil into a joint you can’t get to analogy I used earlier.

In the cyclic group though, things would probably change. The body would be able to inject some nutrition into the joint during the parts of the cycle where decompression took place. And the authors mentioned due to some of their methods, the type of compression that took place in their study is probably excessive compared to what you see in the body.

That said, the numbers of three hours and six hours in the static and cyclic groups probably aren’t too far off. Said another way: You probably want to avoid sitting for three or more hours at a time, and you probably want to avoid moving in the same manner for six hours straight. Even for walking, that’s likely a bit much.

Here’s that purchase button again: Add to Cart

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Posted in: Knee Pain, Pain