“Doesn’t all that pounding ruin your knees?”
Who’s heard this question before? Better yet, who’s been asking themselves this question. If you’ve felt that stiffness in your knees the morning after a long run, I’ll bet you have. I hear it all the time in the clinic. My runners sheepishly confess, “I know running is bad for my knees, but oh, well”. As if running Is some shameful secret indulgence, like having potato chips for breakfast.
I’m a Physiotherapist and I love to run. Having reviewed the best available evidence I am entirely convinced that it is not bad for my knees. I run every day with the confidence that I am making my entire body more strong and resilient, including my knees. By the end of this article, I hope to have instilled in you that same confidence.
“But what about all that pounding?”
Pounding. I hate that word. I strongly believe that the words we use when discussing health are extremely important and “pounding” has a particularly negative sound to it. That being said, it’s the word that people use when voicing their concerns over the long term health of their knees. So I don’t want to ignore the word but I don’t think it is the best choice. Impact would be a far better choice in my opinion. Anyway, let’s dig into it.
The “Fear of Pounding”
Runners are worried about the effect of “pounding” on their knees for a number of reasons.
- Medical Professionals – Unfortunately, the medical profession as a whole has a big share of the blame. It is still not that uncommon for runners to tell me that a surgeon, doctor, physio or some other health professional has told them to quit running because it’s “bad for your knees”. Honestly, this infuriates me, but I don’t want to open that can of worms right now.
- Friends and Relatives – When your friend says “I heard running gives you knee arthritis,” they likely saw or read something about this and are just worried about your health.
- Common Sense – One reason that runners have a fear of pounding is just plain old common sense. As your car gets older, the suspension wears out because you used it too much. So the knees must be like that too. This is a reasonable logic but it is actually not correct and we will dig into that a bit later.
- Knee Pain – This is a sticky one. Knee pain is the most common running injury. Most of us runners get it at some point or we at least have friends who have trouble with knee pain. So it’s only natural to assume that if runners get knee pain fairly often, running must be bad for your knees right? Well, read on to find out.
Forces experienced by the knee joint
When we land each foot as we run we exert a force into the ground. As we learn in high school physics, the ground will exert an equal and opposite force on our leg. That’s called the ground reaction force. As the ground reaction force travels back up through our leg we absorb the load with our tissues (muscles, tendons, ligaments, bones, joints). As this happens the knee joint experiences loads in the region of 5 times body weight (Willy 2016). As a 157 lbs runner my knees will experience a load of roughly 5 x 157 = 785 lbs every step. If I take roughly 10,000 steps an hour as I run that will be 5000 steps per leg. That’s 5000 x 785 for a whopping total of 392,000 lbs of cumulative load experienced by each of my knees for every hour I run!
One way to look at this is “Oh crap, 392,000 lbs per hour! That will ruin my knees, I should quit running”. Another way to look at this is “Wow, my knees can tolerate 392,000 lbs every hour? That’s amazing!”. I like the second one better.
As interesting as this little bit of maths is, it doesn’t really matter. The question is not “how much load is going through my knee?”. The question is “can I adapt to that load?”.
As much as I hate the word pounding I love the phrase adaptive capacity. I use this phrase more than any other in my clinical practice and it is literally the North Star of running injury prevention and rehabilitation. Adaptive capacity refers to the natural ability of our body’s tissues to adapt to the loads placed upon them provided that the load is within the adaptive capacity of that tissue.
Every living tissue in the body has the capacity to adapt to load. Muscles, tendons, bones, joint cartilage. They all adapt. Let’s take the knee joint for example. When you stand, walk, run, jump etc. you put different loads through your knees. If the load that you apply to the tissues of the knee is too high you will develop an injury.
When the load is way higher than the capacity of the knee you will get an immediate and acute injury like a muscle tear, ligament sprain, bone fracture or meniscal tear. When the load is just a little bit too high you will develop a repetitive stress injury like patellar tendonopathy (tendonitis), stress fracture or patellofemoral pain (runner’s knee). In the very long term, loads that are just a little bit too high for the knee can lead to Osteoarthritis (OA). Conversely, if you do not load your knees regularly the tissues will get weaker. This is deconditioning. The body is not going to waste energy making the tissues of your knee stronger if they do not need to be.
In between the two zones is the sweet spot. The adaptive zone of the knee. When you regularly place loads on your knee that are within the adaptive zone of the knee you will strengthen the tissues of the knee. All of the muscles, ligaments, tendons and bones will become stronger in response to the load. Because of this strengthening they will be able to tolerate more load in the future. We call that increasing load tolerance.
Does running place too high of a load on the knees?
So what have we learned so far?
- Running places a very high load on the knees
- Too much load on the knees will lead to injury
- Too little load on the knees will lead to deconditioning
- Loads within the adaptive zone of the knee will lead to increased load tolerance
The question then becomes:
“Is the load that running places on the knees above the adaptive capacity of knees?”
An easy cop out here would be to say “it depends”, but that wouldn’t make for a very interesting article, would it? So let’s look at it a different way. If we look at a lot of runners, over a long period of time, can most of them tolerate the load that running places on their knees? If they can, then we can conclude running is not bad your knees. If they can’t, we can conclude that running is bad for your knees.
So, what does the research say?
Knee Osteoarthritis in Runners
Looking at knee osteoarthritis is the best way we can answer the question of whether running is bad for your knees. That’s because developing knee osteoarthritis is a good example of the knees being unable to tolerate the loads placed on them over a long period of time. If a runner develops symptomatic* knee osteoarthritis, we know that their knees did not adapt to the load placed upon them.
*There is a difference between normal change with age and symptomatic osteoarthritis. I’ll be unpacking that difference in a future post so be sure to subscribe at the bottom.
Quite a number of studies have been done over the last few years to determine if running increases the likelihood of developing knee osteoarthritis. In 2017 Lo and colleagues analysed 2,637 people. About one third of the participants were runners and after a 10 year follow up Lo and friends concluded that “There is no increased risk of symptomatic knee osteoarthritis among self-selected runners compared with non-runners” (Lo 2017).
From 1984 to 2002 Chakravarty and colleagues followed 45 long distance runners and 53 non-runners. When designing the study back in 1984 Chakravarty and pals had “originally hypothesized that long-distance running may be associated with increased incidence and severity of OA”. However, when reviewing the x-rays almost 20 years later they were surprised to find that “long-distance running was not associated with accelerated incidence or severity of radiographic OA” (Chakravarty 2008)
In 2016 Timmins and colleagues did a big meta-analysis of all the research on running and knee osteoarthritis. That means they pooled all the data from all the trials on record and analysed it so they could draw some more firm conclusions. Unfortunately, they found mixed results. They concluded that “moderate- to low-quality evidence suggests no association with OA diagnosis” (Timmins 2016). Interesting, their results suggested that running may offer some protection against needing a knee replacement later in life.
In 1995, Kujala and colleagues compared the knees of former professional runners with former professional soccer players, weightlifters and shooters. Knee OA was found in only 14% of the runners compared with 3% in shooters, 29% in soccer players, and 31% in weight lifters (Kujala 1995).
What does all this mean?
Looking at the studies above, we should feel a lot more able to answer the question “is running bad for your knees?”. It would seem that in the long-term, runners are no more likely to develop knee osteoarthritis than non-runners. If you read the studies above, they suggest that osteoarthritis seems to be more strongly linked with things like a high BMI or previous traumatic injury.
So it would seem that, in the long-term, the knees of most runners are able to adapt to the high loads placed on them during running. Most runners seem to be able to naturally find a volume of running that places loads on the knees within the adaptive capacity of their knees. Sure, runners will get knee pain sometimes. However, so do non-runners.
In the long-term, running is not bad for your knees and it is extremely good for your health. So, my advice? Keep running.
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