Tuesday, October 3, 2023

Everything you want to know about running and knee osteoarthritis

Running is said to cause knee osteoarthritis. But the results from dozens of studies show that it does not.

I have been running marathons (42 km or 26 miles) for more than 30 years. I have also trained many to do so. However, when I meet a non–runner, I often get a piece of friendly advice, “running will damage your knees”. Orthopedic surgeons swear by it; ordinary mortals live by it: “Running causes, and worsens, osteoarthritis”

But what is the evidence in the medical literature? Zero. Zilch. Zip. Nil. Nada. Nulla. Nothing. So why do people believe that running damages knees?

To begin, I will assume that you know how the knee joint works. You probably also know that there is something called cartilage, and osteoarthritis is a sign of damage to that cartilage. If not, do read some wonderful articles on the internet to which I have given links throughout this blog.


Cartilage is a rubber-like elastic tissue that covers the ends of bones at the joints. It looks as smooth as porcelain. But, it is flexible as well as firm.

Cartilage cushions the relative movement of two bones forming a knee joint, viz., femur and tibia. Cartilage is a living tissue. It is not just an inert material lining the space between two bones that form the knee.

If the cartilage erodes, the two bones start rubbing against each other and cause pain. This is the start of osteoarthritis. Let us understand this a bit better.

There are three types of cartilages in our body. Articular cartilage is the type found in a knee joint.

Articular cartilage covers the ends of the femur and tibia in a knee joint. It absorbs the shock of any impact by compressing. It distributes loads evenly on the joint to avoid very high pressure at any single point of contact between the two bones. It glides smoothly against the cartilage of the opposing bone in that knee joint.

Articular cartilage is made up of certain types of proteins called collagens. Collagens have strong, rope-like fibers, which form a grid, or a matrix, inside the knee cartilage. The collagen grid becomes a tough gel when it is saturated with water. This gel does not wear out very easily.

Unfortunately, the knee joint collagen does not have any blood supply. So, there is no quick way to get any nutrients to the cartilage. That is why if you get a knee cartilage injury, it takes a long time to heal.

A type of knee cartilage damage is a meniscus tear, which often happens in sports injuries. Such tears are very difficult to heal, for the same reason.

Surrounding the knee joint is a clear, viscous fluid, called synovial fluid. This lubricating liquid provides nutrients to the cartilage and removes waste products from it. In other words, the synovial fluid is the lifeline of your knee cartilage.

If you would love reading arcane, medical stuff about cartilage, read: The basic science of cartilage.

Cartilage Remodeling

You must have heard that your bones strengthen on impact. A similar thing happens with your cartilage.

The body rebuilds bones to make them stronger to withstand an impact. In medical terms, there are certain cells called osteocytes, in the bone mass, or matrix. They act as mechanical sensors for impact on the bone. Based on their sensing, they tell the body to strengthen or weaken the bone. This process is called bone remodeling, or bone restructuring.

Read on this website: Everything you wanted to know about bones. Warning: The article is really long but by the end of it, you will know everything you need to know about your bones. And you will be surprised to learn that a few things that you knew for sure are not so.

In cartilage, there are certain cells called chondrocytes, that sense loading and impact on the cartilage. Just like osteocytes constitute 5% of the bone, chondrocytes are only about 1% of the cartilage. But, they are the living part of your cartilage. They give the cartilage the ability to regenerate. And you were told that cartilage cannot be regenerated! The right statement should have been:

“The cartilage can grow back. But once it is injured or damaged, certain chemical processes happen inside the cartilage that prevent it from regrowing. If we can control those processes, the cartilage can regrow!”

Read on Stanford Medicine: Researchers find method to regrow cartilage in the joints.

When you put extra weight on your knees, the feedback mechanism of chondrocytes tells the body to repair or rebuild the cartilage. Through an intricate mix of chemicals, it signals the body to produce components of the cartilage grid or matrix. This makes the cartilage stronger.

Thus, exercise strengthens the joint cartilage, in general. Just as it is incorrect to assume that a bone will break on impact (it actually strengthens), it is unscientific to expect the cartilage to degrade with impact (it will strengthen, too).

Of course, there is a physiological limit to which a bone can handle an impact on it. If the stress on the bone is too high or abnormal (at an angle or perpendicular to the bone), it can fracture.

Similarly, the cartilage also has a physiological limit to which it can handle an impact. If you exercise in an excessive manner or put forces that are outside of the cartilage’s handling capacity, it can break down, too. Otherwise, it should strengthen on routine impact.

Friedrich Nietzsche, the German philosopher, had famously said: “That which does not kill us makes us stronger.” That is also true about your bones and knee cartilage.

Forces on Knee Joints

So, how much is the impact when you run? Is it within the tolerance limits of your knee cartilage?

When you walk on flat ground, the force on your knees is about 2–3 times your body weight. So, an 80 kg person will put 200 kg of force on his knees.

This force does not get divided over the two knees. When you are walking, most of your weight falls on the leading, or landing, leg as you step forward. The force of your body weight will be divided in half only when you are standing.

If you climb stairs, depending on their steepness, the pressure on the leading knee increases to 3–4 times your body weight. This is since the knee joint also has to lift your body weight up.

When you squat down to pick up something you dropped on the floor, this pressure can go up to 4–5 times your body weight. Incidentally, squatting down, or bending at the knees, is the right way to pick things up. Bending at your back is the wrong way to lift them.

When you run, during every step, the impact pressure on your knee is about 4 times your body weight. Thus, running is one of the highest impact activities as far as the knee cartilage is concerned.

Another ‘problem’ with running is that it causes repeated impacts. As you run, you take about 85–90 steps a minute with each leg. So, even in a short run, those steps add up many hits on your knee cartilage.


When the cartilage starts breaking down, it can lead to osteoarthritis. With wear and tear, the surface of knee cartilage becomes rough. The cartilage develops fissures. This is the beginning of osteoarthritis.

As time progresses, these cracks reach deep into the cartilage, down to the underlying bone. The fragments of the cracked cartilage start coming off. The bones slowly become devoid of their cartilage cover.

When such bones rub against each other, there is very little lubrication. So, the friction between the bone ends increases.

This forces the cells in the bones, osteocytes, to respond in the only way they know of: Overproducing the bone. Tiny stubs or fingers of bone, called osteophytes or bone spurs, are formed. They protrude into the joint space, causing more damage and pain. This is seen in advanced osteoarthritis.

A paper published in 2018 in the journal Scientific Reports showed that “after an injury in the knee if there is pressure put on the cartilage, osteoarthritis is caused by easy leakage of proteoglycans through the injury surface by high fluid outflow.” Proteoglycans are compounds in the cartilage that are bound to proteins. Proteoglycans allow the cartilage to withstand compression forces.

About 80% of cartilage is made of water. When the cartilage is compressed, up to 70% of the water in the cartilage squeezes out. Thus, if an injured knee gets pressured by any activity, some proteoglycans leak out of cartilage. This weakens the cartilage further, eventually leading to osteoarthritis.

Sports participation

Many sports can stress the knee cartilage. They may involve high impacts and twisting forces. If a player has a joint injury, an unstable joint, or a muscle weakness, it can lead to abnormal peak forces on the cartilage of the joint involved. These can increase the risk of osteoarthritis.

Impact needed to damage knee cartilage

For physics mavens: did you know that Newton and Pascal were related?
Well, 1 pascal is 1 newton per square meter. :-))

Pressure is the force applied over a unit of area. A newton is a unit to measure force. A pascal is a unit used to measure pressure. One pascal pressure is equal to one newton force applied over an area of a square meter.

If you remember the school physics, force = mass x acceleration.

Mass is measured in kg. For calculating the force it would exert, the acceleration is g, the gravitational constant on earth. So 1 kg mass will weigh 9.81 newtons on earth.

Note that weight is actually a force and not mass. So, it is never measured in kg. It is measured in newtons, while mass is measured in kg.

If an 80 kg person stands on one knee, it would put 80 kg x 9.81 m/s2 weight, or 784 newtons weight on the knee. Let us round that number to 800 newtons.

In a typical knee, the cross-section of the contact area between the two bones varies between 5 to 13 cm2. It changes based on the person’s weight as well as the activity. Let us take an average of 10 cm2.

So, it would exert 800 newton/10 cm2 or 80 newton/cm2 pressure on the knee. This is also equal to 80 x 10,000 newton/m2 pressure, or 800,000 pascal pressure.

1 megapascal or MPa is 1000,000 pascals. So, an 80 kg person would put 0.8 MPa pressure on the knee, if he stands on one knee.

The scientific name for this pressure on the knee cartilage is von Mises stress. The cartilage will fracture or give way if the von Mises stress on it exceeds its capacity.

The knee surface is not uniform. So the pressure on the cartilage will be different at different locations in the knee joint. So, one should be careful in differentiating between average pressure and peak pressure. Unfortunately, medical literature seems to use both phrases interchangeably.

We will assume peak pressures to be 3 times the average pressures. So, in an 80 kg person, von Mises stress (peak pressure) should be 2.5 MPa.

The physiological limit for cartilage breakdown turns out to be about 17–25 MPa. The technical word for breakdown limit is yield limit. If peak pressure of more than the cartilage yield limit is put on the cartilage, it can crack or fracture. This can lead to the beginning of osteoarthritis, as we saw above. The exact fracturing limit depends on the direction of the force and the quality of the cartilage.

If we take the cartilage yield limit as 17 MPa, for an 80 kg person, it would be roughly 6 bodyweights–equivalent. In other words, if any activity puts 6 times the bodyweight of the person on the cartilage, it will crack.

Table 1 below shows the peak forces encountered by our knee cartilage in some sports activities. The peak force is given as a multiple of body weight. This is since a heavier person will have a higher impact on the knee, in proportion to his weight.

This data was taken from real people by inserting sensors into their bodies. Since no healthy person can bear a sensor inserted in his knee joint, it was done in patients who had undergone knee replacement surgery. Thus, the data is not for a normal person; but it gives a rough idea of the impacts we are discussing.

Table 1. Peak forces encountered by knee cartilage in various sports activities, as a multiple of body weight of the person
ActivityPeak forces (times body wt)
Walking (2–5 km/hr)2.1
Jogging (8 km/hr)4.2
Stationary cycling1.0 — 1.5
StairMaster level 33.1
Elliptical machine level 112.2
Leg press2.8
Knee extension exercise1.5
Rowing machine0.9

As you can see, most sports activities do not increase the peak forces on the knee joint to beyond 4.5 times the body weight. I don’t have the data for rope skipping and plyometric jumps. But they would be right up there with tennis and jogging.

Thus, unless you overdo any sports activity, or get injured doing it incorrectly, it is unlikely that it would cause osteoarthritis by damaging the cartilage.


A lot of research in both animals and humans has shown (we will see below) that moderate levels of running, even over a lifetime, do not cause any joint damage, or osteoarthritis. In fact, some studies have shown that even beginner runners and die-hard marathoners do not have an increased incidence of osteoarthritis.

A point to note is, occasionally, there is a misdiagnosis of a running-related joint injury. For example, The most common cause of knee pain in runners is not osteoarthritis, but rather an abnormal tracking of the kneecap. This condition is called Runner’s Knee, or patellofemoral pain syndrome. Runner’s knee is a reversible joint injury. It causes pain behind and around the knee cap. Osteoarthritic pain should be located a bit below the knee cap.

There are other sources of knee pain, too. For example, Iliotibial band syndrome (ITBS) causes pain on the outside of the knee (right side of the right knee, left side of the left knee).

In fact, any component of the knee — muscles, tendons, ligaments, bones, and joints — can cause pain and injury. A runner should not suspect osteoarthritis just because there is knee pain.


People used to think that inactivity, or rest, would prevent the worsening of arthritis. They felt it would protect the fragile knee joint from further damage.

However, the latest research shows the opposite. The maxim goes, “If you don’t use it, you lose it.” Thus, if you do not use the joints, they become less flexible. Their cartilage matrix gets gradually modified and weakens.

Studies find that joints are improved by exercise. The muscles, ligaments, and tendons surrounding the joint are strengthened by the stresses of athletic activity, which improves the joint mechanics. Some exercises also improve the flexibility, or range of motion (ROM), of the attached muscles.

Thus, physical exercises help in preventing and perhaps treating, osteoarthritis. They also increase the ability of joints to withstand years of activity without any arthritic damage.

The evidence

Let us now look at the evidence for, or against, running, when it comes to osteoarthritis.

We will take 8–9 logical questions to cover the whole gamut of running — walking, beginner runners, moderate activity, marathoners, elite runners, 30–minute runs, decades of running, 3–6 months of training, pain, x–rays (radiographs), MRI scans of knees, inflammation markers in the joint fluid, etc.

We will look at the research over the last 30 years. I chose 30 years because I have been running marathons for the last 30 years. No, just kidding! I actually chose 20 years because modern techniques to study the knee joint have come about only in the last 20 years. I added another 10 years to include a couple of studies, just to show how the scientific opinion has changed over the years, as more study results were published.

1. Do runners get knee osteoarthritis?

Let us look at the evidence garnered over the decades.

Animal studies

Animal studies have shown that excessive exercise or internal injury leads to osteoarthritis. Other studies have shown that moderate exercise either does not lead to joint degeneration or is beneficial in reducing the risk of osteoarthritis.

Human studies

Almost all studies published before the year 2000 showed that long-distance running increased osteoarthritis.

Only one study published in 1988 did not show that osteoarthritis worsened with running. It showed that moderate long-distance running (40 km per week, on average) was not associated with a higher incidence of osteoarthritis of hip or knee. This was a large study with a 25–year follow-up.

Here is the summary of various studies done till 2006. Read: Does long-distance running cause osteoarthritis?

Interestingly, all these studies tracked elite athletes and the incidence of osteoarthritis in that group. Since the number of top athletes is always limited, the studies had only a few participants. The methodology for confirmation of osteoarthritis was also limited, since the new techniques, such as an MRI scan of knee cartilage, were not developed.

Modern studies

A Stanford University study published in 2009 in the American Journal of Preventive Medicine, showed that long-distance running among healthy older individuals was not associated with accelerated osteoarthritis (as measured by x-ray).

The researchers took 45 long-distance runners and 53 non-runners with an average age of 58 years. At the beginning of the study, 7% of long-distance runners and 0% of non–runners had mild arthritic knees. The researchers followed their knee conditions for 18 years.

After 18 years, 20% of the runners and 32% of the non-runners had knee arthritis. 2.2% of the runners and 9.7% of the non-runners ended up with severe knee arthritis.

This study raised the possibility that severe osteoarthritis may not be more common among runners.

In simple English: Will running ruin my knees?

In conclusion

Long-distance runners have lesser chances of developing knee osteoarthritis than non-runners.

2. If running is safe, walking and gym exercises should be safer

The studies on non–elite runners, who don’t do a lot of running, have shown that running lowers the risk of osteoarthritis.

However, runners have higher fitness levels. They are usually not obese or overweight. Maybe, that could be the reason for the lower incidence of osteoarthritis in them? In other words, runners may be better off not because of their running, but because of their fitness.

In that case, if you take equally fit people (in statistics, called matched pairs) who do gentler exercises, it would be even better. For example, those who did walking and other (non-running) forms of exercise should have an even lesser incidence of knee osteoarthritis.

A paper published in 2014 in the journal Medicine and Science in Sports and Exercise studied how running, walking, and other exercises affect osteoarthritis risk.

The researchers at the Lawrence Berkeley National Laboratory followed 74,752 runners for up to 7.1 years and 14,625 walkers for up to 5.7 years. It was the largest study till then about knee osteoarthritis and running/walking, by an order of magnitude.

They grouped people based on their weekly mileage as:

  • No (minimal) activity: less than 12.4 km/week
  • Low activity: 12.4 — 24.8 km/week
  • Moderate activity: 24.8 — 37.2 km/week
  • High activity: more than 37.2 km/week.

They found that regardless of the weekly mileage run, running and walking both reduced the risk of osteoarthritis. Running reduced the risk more than walking. These risks were calculated compared to the risk in those who did minimal or no activity.

Similar levels of rigorous exercise, of non-running, non-walking type, actually increased the risk of osteoporosis.

In other words, a lot of running or walking reduced the risk of osteoarthritis compared to that in people doing no activity. Rigorous exercises of other types (such as gym resistance training) worsened the risk.

Runners had lower BMI, a measure of excess body weight. However, only 45% of the reduction in the knee osteoarthritis risk in runners could be attributed to their lower BMI. This meant that if you were lean because of any reason other than running, your risk of osteoarthritis would decrease only by 45%. Thus, there was some protective effect of running itself.

Neither marathon participation frequency, marathon intensity (speed), nor 10–km intensity showed an increase in the risk of osteoarthritis, among the runners.

Arthritis–protective effects of running or walking appeared even at the low mileage of 12.4 km/week. That is, the benefits were seen in runners and walkers who were doing a low level of activity, as compared to the no–activity group. But how can so little activity show benefit?

This suggested that the benefit was not fully due to the activity. Being inactive actually caused harm (increased chances of osteoarthritis). So, people who were doing even a little bit of activity found themselves ahead of the inactive people, when it came to preventing osteoarthritis.

Thus, there was some benefit from walking, even more benefit from running, and some harm from doing very little activity. In more descriptive English, read: The Explainer: Can Running Actually Be Good for Your Knees?

In conclusion

Running more than 12.4 km/week reduced the risk of knee arthritis.

Walking more than 12.4 km/week also reduced the risk of knee osteoarthritis but not as much as running did.

Other non-running forms of rigorous exercises, which were equivalent in exertion aspect to running more than 12.4 km/week, increased the risk of osteoarthritis.

The benefits of running and walking appeared even for low weekly mileage. That indicated that it was not purely the activity that reduced the risk, but rather the inactivity increased the risk.

3. Walking is not better than running. But why?

We saw that running was better than walking for knee osteoarthritis risk. But why? Both activities involve the same kind of action. Thus, there was nothing inherently different between running and walking. And, running puts much more load on the knee cartilage compared to walking.

A paper published in 2014 in the journal Medicine and Science in Sports and Exercise discussed the reasons for the difference between running and walking.

The paper showed that compared to walking, running involves a shorter duration of ground contact and a longer length of strides. As a result, the load per unit of distance run is not higher than that in walking. Also, while the peak load increases with running speed, the total load on the knee cartilage per unit of distance decreased with increasing speed.

Read in simpler English: Why runners don’t get knee arthritis?

In conclusion

Running has shorter ground contact and longer stride length. So, the load per unit distance covered in running is not higher than that in walking.

While the peak load increases with running speed, the total load on the knee cartilage per unit distance decreases with increasing speed.

4. Runners, who get osteoarthritis, stop running. Maybe, that is why runners have less osteoarthritis

We saw that running is better than walking and other exercises for osteoarthritis.

But, biostatisticians call this a self–selection bias: if a runner gets osteoarthritis, he will not run anymore. So runners, by self–selection, would be people, who didn’t get osteoarthritis. No wonder osteoarthritis is lower in runners.

To avoid this bias, the researchers at Baylor College of Medicine decided to select average individuals who had no particular affiliation with running. They were not runners. You can call them leisure runners. A few of them were ex–runners.

The scientists followed these people for 11 years to see if they develop osteoarthritis and figure out if there are any factors that lead to it. They published their findings in 2017 in the journal Arthritis Care and Research.

The researchers tracked 2,637 individuals, of an average age of 64 years, over 11 years. 30% of these had run at some point in their life. Some of them had run earlier in their life but did not run during the 11–year study period. Such people were called ‘former runners’.

Compared to non–runners, former runners were 18% less likely to report pain, while current runners were 24% less likely to report pain. This was when the data was adjusted for age, BMI, and other factors.

Adjustment for obesity

BMI stands for body mass index, a measure of how overweight the person is. Adjusting for BMI means, you look at the two groups and their average BMIs. If you find that the runner group has a lower average BMI, you say that “well, the groups are not similar. The runner group is lighter, less overweight. So the results we see may not be only because of running, but also because of lesser weight they lug around.” So, we adjust the results, or osteoarthritis outcomes, for BMI differences, using certain statistical techniques.

But, that comparison is unfair. Current runners are lighter than non-runners, on average. To fully account for the difference caused by running, BMI adjustment should not be done. Without BMI adjustment, the current runners were 29% less likely to report pain, instead of 24%.

Running did not make people immune to knee pain. 33% of runners reported frequent pain. It was just that 41% of non-runners also reported frequent pain.

Everyone knows someone who has tried running and given up because of knee pain. But, if one would have looked at non-runners, one would have found even more percentage of non-runners with knee pain.

In simple English, read: Here’s more evidence that running does not ruin your knees.

In conclusion

Ordinary people, who were former runners, complained less of knee pain compared to people who had never run.

Runners complained of knee pain. But non–runners complained even more.

When it comes to osteoarthritis, having run at some point in life was better than having never run. Current running was the best of all.

5. Running for many years may be helpful. But, what about knees of new, beginner runners?

We saw that ex–runners were less likely to report knee pain than non–runners. But, that was an observational study. It analysed people at a single point in time. It grouped people into current–runners, ex–runners, and non–runners. And, then it compared the groups.

But, the three groups may also be different because of reasons other than their running habits.

It would be better if a study would compare the same person, before and after? That would be a longitudinal study. And, it would remove the inherent limitations of a study that compares different groups at the same time.

Also, it would be more akin to a real-life situation. After all, you simply want to know what will happen to your knees, before and after taking up running. You are not bothered about which category of running group you fall into.

A study published in 2014 in the journal Knee Surgery, Sports Traumatology, Arthroscopy did exactly that. They took 10 beginner runners, put them on a 6–month marathon training program, and made them run a marathon at the end of it. They checked their knee cartilage volume and thickness, before and after.

These marathon beginners ended up running between 25 to 60 km per week. So this was a very small study, but an extreme test.

The researchers found that high-impact forces during long-distance running are well tolerated even in marathon beginners and do not lead to cartilage loss.

More details in simple English: Marathon training does not hurt knees, even in beginners.

In conclusion

Beginner marathon runners who underwent a 6–month training program had no damage to their knee cartilage.

People who have never run before don’t get cartilage problems, even if they undergo extreme training, such as marathon training.

6. What does running do inside your knees?

Many running trials showed that runners complained of less knee pain than non–runners. Occasionally, even x–rays showed less damage inside the knees of runners. But, mostly, the source was the participants and their complaints about pain.

What if the people who run, happen to have a higher tolerance for pain? In that case, the runners will complain of less pain than non–runners. Or, is there something actually happening inside the runners’ knees that leads to less pain?

The studies done before the year 2000 used mailed questionnaires, memory recalls, and subjective pain perception by individuals. This added various biases into the results, reducing their value. Recent papers use newer techniques to judge the actual cartilage health, removing subjectivity.

Modern techniques

dGEMRIC scores

Delayed Gadolinium-Enhanced MRI of Cartilage (dGEMRIC) is a new imaging technique to estimate something called Glycosaminoglycan (GAG) content of the knee cartilage, which indicates its health.

Cartilage GAG content is an important aspect of the biomechanical properties of cartilage. Increasing GAG content means improving knee cartilage health.

A paper published in 2004 in the journal Magnetic Resonance in Medicine showed that human knee cartilage adapts to exercise by increasing its GAG content in exercising individuals with healthy knees.

Another paper published in 2005 in the journal Arthritis and Rheumatism showed that moderate exercise increased the GAG content in the knee cartilage of individuals whose knees were at a high risk of osteoarthritis.

There were 45 people with an average 46 years of age. They had undergone partial medial meniscus resection (a part of their knee cartilage was removed for various reasons) 3–5 years prior to the start of the trial. So, they were at a high risk of developing osteoarthritis.

They were not exercising before the trial. But, they underwent supervised exercise 3 times weekly for 4 months. Their dGEMRIC scores, and cartilage GAG content, were checked before and after the 4–month exercise period.

The results showed that the adult human articular cartilage has the potential to adapt to loading change. Moderate exercise may be a good treatment not only to improve joint symptoms and function but also to improve the knee cartilage GAG content in patients at a high risk of developing osteoarthritis.

While both these papers did not specifically look at running as an exercise, they showed that knee cartilage can become healthier with exercise. This was not just a pain perception of an individual, but rather the actual condition of cartilage.

Inflammatory markers

Advanced laboratory tests of blood and synovial fluid, the liquid around knee cartilage, can tell us about the condition of the cartilage.

A cytokine is a signaling molecule. A pro-inflammatory cytokine is a type of signaling molecule that is excreted by our immune cells, such as helper T cells and macrophages, and certain other cell types that promote inflammation (swelling). A pro-inflammatory cytokine called Interleukin–15, is linked to increased progression and severity of osteoarthritis.

GM–CSF is a marker, or indicator, for cartilage turnover. High levels of GM-CSF are found in joint fluids in rheumatoid arthritis. Reduction in GM-CSF helps in reducing inflammation and damage.

Measuring the levels of these chemicals tells us about the health of cartilage.

Scientists at Brigham Young University took 6 recreational runners. They checked their blood and synovial fluid inside their knees before and after a 30–minute run. They compared the levels of GM–CSF and IL–15 with the levels when they did not run. The findings were published in 2016 in the European Journal of Applied Physiology.

No runner would be excited about a syringe extracting fluids from his knee multiple times. So the trial was small. Also, very little synovial fluid could be extracted from the healthy knees of runners, limiting the test.

IL–15 levels in the knee decreased with increasing foot-strikes during the run. GM–CSF levels in the knee decreased after running.

While the study was small due to its invasive methods, it hints at the mechanism by which running could be protective of the knee.

It was also a pilot study, which means it was designed to find out what was improving (to decide on what to study in detail in a later trial), instead of how much was the improvement.

In simple English: Can running help prevent osteoarthritis?

In conclusion

The knee cartilage has the potential to adapt to changes in the applied pressure.

Moderate exercise may improve the knee cartilage GAG content in patients at high risk of developing osteoarthritis.

Running reduces the inflammatory markers in the knee joint, reducing the pathway to pain and damage. Modern techniques make newer results more reliable.

7. Running improved structural damage in knee osteoarthritis

We saw that running is protective of healthy knee cartilage, in all variations: runners, ex–runners, beginner runners, short runs, marathons, heavy training, years of running, etc.

However, what if a knee is already degenerating with osteoarthritis? Would running with such knee damage it further, since the structural integrity of the cartilage is compromised in an osteoarthritic knee?

Researchers used the data from a large, multi–centre study called the Osteoarthritis Initiative, which tracked nearly 5,000 people over a decade. Their findings were published in 2018 in the journal Clinical Rheumatology.

The study had 1,203 people over 50 years of age, who had osteoarthritis in at least one knee. 138 of these were runners during this study period.

23.6% of the non-runners got worse (joint space narrowed) during the study period, while 19.5% of the runners got worse.

29.0% of the non-runners reported new frequent knee pain, compared to 26.8% of the runners.

39.1% of the non-runners reported improvements in their knee pain, compared to 50% of the runners.

Adjusting for characteristics, such as age and BMI, the runners were about 70% more likely to see improvements in their symptoms.

In simpler English: Running with bad knees might be OK after all.

In conclusion

Running improved the symptoms as well as the cartilage structural damage in people with OA.

8. What about those crazy runners – the marathoners?

We learned that beginner runners were not affected by marathon training. Now, let us look at the other extreme. What about those runners, who run lots and lots of marathons? A marathon is a running race of about 42 km or 26 miles.

Researchers at the Department of Orthopedic Surgery, at Thomas Jefferson University, USA looked at 675 marathon runners, of an average age of 48 years. These runners had completed an average of 76 marathons over an average of 19 years. Looks like these nutcases were my kind (I have finished 182 marathons in 30 years :-))

The researchers published a paper in 2018 in The Journal of Bone and Joint Surgery, American Edition about their findings. They found that there was no statistically significant OA risk associated with running duration, intensity, mileage, or the number of marathons completed (p > 0.05). On the other hand, they found age, family history, and surgical history were statistically significant independent risk factors for OA.

Arthritis prevalence was 8.8% for marathoners. It was lower than the prevalence in the matched normal population (17.9%) in a statistically significant manner (p < 0.001).

Statistical significance

Now, a word on statistical significance. I find that most people interpret these p–values wrongly, drawing faulty conclusions.

In fact, the entire field of biostatistics needs you to understand that all conclusions of medical trials are just guesses. You conclude something and specify how confident you are in your claim.

So, you may have exactly the same result, with different confidence in your claim. And the two would mean completely different things. For example,

  1. Marathoners had 8.8% risk of OA, while non–runners had 17.9% risk of OA, p > 0.05
  2. Marathoners had 8.8% risk of OA, while non–runners had 17.9% risk of OA, p < 0.01

The latter means there is a statistically significant lower OA risk in marathoners. The former means there is no statistically significant lower OA risk in marathoners. But, it does not mean that there is no lower OA risk in marathoners. In fact, the data tells us that there is a lower OA risk in marathoners, but it is not statistically significant.

If all this sounds confusing, welcome to the jungle. The medical research world is fraught with such convoluted stuff. That is why I have written a comprehensive article on this website: Understanding statistical significance: simple wordings, confusing meanings.

And, I have used exactly the above example of marathoners versus non–runners and their OA risks to explain the concept of statistical significance and its erroneous interpretations. Do read.

In conclusion

The prevalence of osteoarthritis in people who ran a large number of marathons was nearly half (8.8% versus 17.9%) as compared to people who were not runners.

9. The real secret why knee arthritis is getting prevalent

The percentage of people having knee arthritis has doubled in the last 75 years. But why?

Some people attribute it to the modern fad of running. After all, no one ran for recreation a century ago. So, this point is worth a mention at the end of this article.

Experts say that the increase in osteoarthritis prevalence is because of increased obesity and longer lifespans. Sounds logical, right? After all, obesity has increased by leaps and bounds. And, obesity is linked to increased osteoarthritis risk. Also, people are living longer. Since osteoarthritis affects one in old age, the longer you live, the higher are your chances of getting osteoarthritis.

Researchers at Harvard University did some ingenious, and extremely macabre, work to debunk this hypothesis. They published a paper in 2017 in the journal Proceedings of the National Academy of Sciences of the USA about their findings.

The researchers traveled all over the USA, exhumed ancient and modern skeletons, and studied them for signs of osteoarthritis. They found that knee arthritis in the last 75 years has doubled, even for people of the same age and level of obesity (BMI).

Once you get over the gruesome part, you will realise how ingenious this was. After all, 75 years back, no one kept any data about osteoarthritis, BMI, and age in the same person.

But, the scientists can look at the ends of the bones in the knee joint of a skeleton and know if the person had osteoarthritis. Forensic science also allows them to estimate the weight of the dead person, and so the BMI.

All in all, they found the culprit. It was not running, it was not obesity, and it was not increased lifespan. It was, hold your breath, inactivity. Yes, the modern sedentary lifestyle doubled the incidence of knee osteoarthritis.

Read more about this study on this website: The real secret why knee arthritis is getting more prevalent.

In conclusion

Running has been blamed for knee osteoarthritis. However, physical inactivity seems to be the bigger risk for knee damage.

Knee cartilage does not seem to follow wear–and–tear model. It is a living tissue, that improves with the added workload.

If one sits at a desk all day, one ends up with thinner, lower–quality cartilage in the joints, and weakness in the muscles that would otherwise take some of the load off those joints.

The problem isn’t too much running; it is not enough running. If you don’t use it (your cartilage), you lose it.

So, why does running help prevent osteoarthritis?

The causes of osteoarthritis are not fully understood. Most likely, it is caused by a combination of factors, instead of just one reason. We already know that age, gender, obesity, family history, and prior knee injury affect the chances of your getting osteoarthritis.

So, what is helping prevent osteoarthritis in runners? Currently, it is just a bunch of conjectures.

Lesser obesity

One explanation is that runners are generally not obese. Now, obesity is an inflammatory disease, and so is osteoarthritis. Plus, obesity is known to increase osteoarthritis chances. Unfortunately, research has shown that ‘less obesity’ explains only a part of the benefits of running.

Weight bearing activity

Running involves bearing your weight on feet and knees. Could that be helping? No. Osteoarthritis is more common in people who stand for a living, such as traffic policemen or courier delivery persons. Similarly, people who kneel often, as certain religious practices involve, also have a higher incidence of osteoarthritis.

Physical exercise

Running involves a physical exercise. Could that be the key?

As we saw, the cartilage has no blood supply. It gets its nutrients through the synovial fluid surrounding it.

Nutrient exchange

This exchange of nutrients and waste products possibly happens only during a physical activity.

As the cartilage is compressed, some of its internal liquid comes out into the synovial fluid, dumping its waste products. And, as the pressure is removed, the liquid takes healthy nutrients with it back into the cartilage. Of course, this is just a hypothesis.

But, the benefit of running is not due to the physical exercise part either. Soccer players and weightlifters have a higher prevalence of osteoarthritis than runners.

One possibility is that the former two sports can cause a knee injury trauma. That can develop into osteoarthritis. But, even if we adjust for that possibility, running still seems to be better than the two.

Cartilage stiffening

Another hypothesis is that the cartilage gets stiffer during a dynamic activity, such as running than during inactivity. That might reduce the impact stress on the joints while running.

Weightlifting is a static position for the knee cartilage and so may not involve this stiffening of the cartilage. Similar is the situation for a postman and courier delivery person, who lug heavy weights all day long. They walk very little in relation to the weight they carry, in contrast to a runner. That is why they develop osteoarthritis; the runners don’t.

Extreme running

And finally, anything in excess, even running, can lead to damage. The elite runners, who run lots of kilometers in training, maybe pushing the limits. It is quite likely that their cartilages stray into overuse zone, and lose the protective benefits of running. Some of the results in the 1980s and 1990s were on world-class runners, and osteoarthritic issues were noticed in them.

Also, world-class runners have very intense training sessions. They can lead to injuries, such as meniscus or ligament tears, in the knee. About 10% of knee osteoarthritis in later life originates from such an injury.

However, it is not the challenge of the running distance. After all, ultra–marathoners don’t seem to have excess knee arthritis.

So, it is not the race distance; but, the training intensity and the mileage may be causing osteoarthritis in world-class runners.

So, finally, when does running worsen osteoarthritis?

Running can cause, or worsen, osteoarthritis in some peculiar conditions. Osteoarthritis can be caused by running with poor bio-mechanics, supporting muscle weakness (due to no cross-training), intense training schedule (causing knee injuries), and very high weekly mileage (my guess: more than 150 km/week)

Most people run for an entire lifetime. They do not get affected by arthritis, except that caused by ageing. With age, the cartilage breaks down gradually, due to changes in its underlying structure.

By the time one turns 65 years old, three out of four people show some evidence of osteoarthritis. So, take it within your stride, pun intended. And, don’t forget that if you do not run, you are even more likely to get osteoarthritis.

In conclusion

Running does not cause osteoarthritis. In fact, proper running decreases your chances of getting osteoarthritis.

To avoid running-related osteoarthritis:
• Run with a proper form.
• Cross-train the muscles around the knee.
• Avoid very intense training, such as high–impact plyometrics. Or do them knowing the injury risk.
• Be careful with trail running, which can cause traumatic knee injuries.
• Avoid more than 150 km/week running.

But, perhaps, the most important skill you will need (from my 30 years of experience) is the ability to tune out all those experts and genuine well-wishers, who will caution you to avoid running for preventing knee osteoarthritis.

First published on: 10th February 2019
Image Credit: Pixabay on Pexels.com
Last updated on: 27th March 2022



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